Merging upstream version 1.35.0.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 481 insertions, 98 deletions

diff --git a/libnetdata/locks/locks.c b/libnetdata/locks/locks.c
index ca9a5aee9..8b5348678 100644
--- a/libnetdata/locks/locks.c
+++ b/libnetdata/locks/locks.c
@@ -2,11 +2,32 @@
 
 #include "../libnetdata.h"
 
+#ifdef NETDATA_TRACE_RWLOCKS
+
+#ifndef NETDATA_TRACE_RWLOCKS_WAIT_TIME_TO_IGNORE_USEC
+#define NETDATA_TRACE_RWLOCKS_WAIT_TIME_TO_IGNORE_USEC 10
+#endif
+
+#ifndef NETDATA_TRACE_RWLOCKS_HOLD_TIME_TO_IGNORE_USEC
+#define NETDATA_TRACE_RWLOCKS_HOLD_TIME_TO_IGNORE_USEC 10000
+#endif
+
+#ifndef NETDATA_THREAD_LOCKS_ARRAY_SIZE
+#define NETDATA_THREAD_LOCKS_ARRAY_SIZE 10
+#endif
+static __thread netdata_rwlock_t *netdata_thread_locks[NETDATA_THREAD_LOCKS_ARRAY_SIZE];
+
+
+#endif // NETDATA_TRACE_RWLOCKS
+
 // ----------------------------------------------------------------------------
 // automatic thread cancelability management, based on locks
 
 static __thread int netdata_thread_first_cancelability = 0;
-static __thread int netdata_thread_lock_cancelability = 0;
+static __thread int netdata_thread_nested_disables = 0;
+
+static __thread size_t netdata_locks_acquired_rwlocks = 0;
+static __thread size_t netdata_locks_acquired_mutexes = 0;
 
 inline void netdata_thread_disable_cancelability(void) {
     int old;
@@ -14,18 +35,19 @@ inline void netdata_thread_disable_cancelability(void) {
     if(ret != 0)
         error("THREAD_CANCELABILITY: pthread_setcancelstate() on thread %s returned error %d", netdata_thread_tag(), ret);
     else {
-        if(!netdata_thread_lock_cancelability)
+        if(!netdata_thread_nested_disables)
             netdata_thread_first_cancelability = old;
 
-        netdata_thread_lock_cancelability++;
+        netdata_thread_nested_disables++;
     }
 }
 
 inline void netdata_thread_enable_cancelability(void) {
-    if(netdata_thread_lock_cancelability < 1) {
-        error("THREAD_CANCELABILITY: netdata_thread_enable_cancelability(): invalid thread cancelability count %d on thread %s - results will be undefined - please report this!", netdata_thread_lock_cancelability, netdata_thread_tag());
+    if(netdata_thread_nested_disables < 1) {
+        error("THREAD_CANCELABILITY: netdata_thread_enable_cancelability(): invalid thread cancelability count %d on thread %s - results will be undefined - please report this!",
+            netdata_thread_nested_disables, netdata_thread_tag());
     }
-    else if(netdata_thread_lock_cancelability == 1) {
+    else if(netdata_thread_nested_disables == 1) {
         int old = 1;
         int ret = pthread_setcancelstate(netdata_thread_first_cancelability, &old);
         if(ret != 0)
@@ -35,10 +57,10 @@ inline void netdata_thread_enable_cancelability(void) {
                 error("THREAD_CANCELABILITY: netdata_thread_enable_cancelability(): old thread cancelability on thread %s was changed, expected DISABLED (%d), found %s (%d) - please report this!", netdata_thread_tag(), PTHREAD_CANCEL_DISABLE, (old == PTHREAD_CANCEL_ENABLE)?"ENABLED":"UNKNOWN", old);
         }
 
-        netdata_thread_lock_cancelability = 0;
+        netdata_thread_nested_disables = 0;
     }
     else
-        netdata_thread_lock_cancelability--;
+        netdata_thread_nested_disables--;
 }
 
 // ----------------------------------------------------------------------------
@@ -51,6 +73,13 @@ int __netdata_mutex_init(netdata_mutex_t *mutex) {
     return ret;
 }
 
+int __netdata_mutex_destroy(netdata_mutex_t *mutex) {
+    int ret = pthread_mutex_destroy(mutex);
+    if(unlikely(ret != 0))
+        error("MUTEX_LOCK: failed to destroy (code %d).", ret);
+    return ret;
+}
+
 int __netdata_mutex_lock(netdata_mutex_t *mutex) {
     netdata_thread_disable_cancelability();
 
@@ -59,6 +88,9 @@ int __netdata_mutex_lock(netdata_mutex_t *mutex) {
         netdata_thread_enable_cancelability();
         error("MUTEX_LOCK: failed to get lock (code %d)", ret);
     }
+    else
+        netdata_locks_acquired_mutexes++;
+
     return ret;
 }
 
@@ -68,6 +100,8 @@ int __netdata_mutex_trylock(netdata_mutex_t *mutex) {
     int ret = pthread_mutex_trylock(mutex);
     if(ret != 0)
         netdata_thread_enable_cancelability();
+    else
+        netdata_locks_acquired_mutexes++;
 
     return ret;
 }
@@ -76,93 +110,105 @@ int __netdata_mutex_unlock(netdata_mutex_t *mutex) {
     int ret = pthread_mutex_unlock(mutex);
     if(unlikely(ret != 0))
         error("MUTEX_LOCK: failed to unlock (code %d).", ret);
-    else
+    else {
+        netdata_locks_acquired_mutexes--;
         netdata_thread_enable_cancelability();
+    }
 
     return ret;
 }
 
+#ifdef NETDATA_TRACE_RWLOCKS
+
+#warning NETDATA_TRACE_RWLOCKS ENABLED - EXPECT A LOT OF OUTPUT
+
 int netdata_mutex_init_debug(const char *file __maybe_unused, const char *function __maybe_unused,
                              const unsigned long line __maybe_unused, netdata_mutex_t *mutex) {
-    usec_t start = 0;
-    (void)start;
-
-    if(unlikely(debug_flags & D_LOCKS)) {
-        start = now_boottime_usec();
-        debug(D_LOCKS, "MUTEX_LOCK: netdata_mutex_init(0x%p) from %lu@%s, %s()", mutex, line, file, function);
-    }
+    debug(D_LOCKS, "MUTEX_LOCK: netdata_mutex_init(%p) from %lu@%s, %s()", mutex, line, file, function);
 
     int ret = __netdata_mutex_init(mutex);
 
-    debug(D_LOCKS, "MUTEX_LOCK: netdata_mutex_init(0x%p) = %d in %llu usec, from %lu@%s, %s()", mutex, ret, now_boottime_usec() - start, line, file, function);
+    debug(D_LOCKS, "MUTEX_LOCK: netdata_mutex_init(%p) = %d, from %lu@%s, %s()", mutex, ret, line, file, function);
 
     return ret;
 }
 
-int netdata_mutex_lock_debug(const char *file __maybe_unused, const char *function __maybe_unused,
+int netdata_mutex_destroy_debug(const char *file __maybe_unused, const char *function __maybe_unused,
                              const unsigned long line __maybe_unused, netdata_mutex_t *mutex) {
-    usec_t start = 0;
-    (void)start;
+    debug(D_LOCKS, "MUTEX_LOCK: netdata_mutex_destroy(%p) from %lu@%s, %s()", mutex, line, file, function);
 
-    if(unlikely(debug_flags & D_LOCKS)) {
-        start = now_boottime_usec();
-        debug(D_LOCKS, "MUTEX_LOCK: netdata_mutex_lock(0x%p) from %lu@%s, %s()", mutex, line, file, function);
-    }
+    int ret = __netdata_mutex_destroy(mutex);
+
+    debug(D_LOCKS, "MUTEX_LOCK: netdata_mutex_destroy(%p) = %d, from %lu@%s, %s()", mutex, ret, line, file, function);
+
+    return ret;
+}
+
+int netdata_mutex_lock_debug(const char *file __maybe_unused, const char *function __maybe_unused,
+                             const unsigned long line __maybe_unused, netdata_mutex_t *mutex) {
+    debug(D_LOCKS, "MUTEX_LOCK: netdata_mutex_lock(%p) from %lu@%s, %s()", mutex, line, file, function);
 
+    usec_t start_s = now_monotonic_high_precision_usec();
     int ret = __netdata_mutex_lock(mutex);
+    usec_t end_s = now_monotonic_high_precision_usec();
 
-    debug(D_LOCKS, "MUTEX_LOCK: netdata_mutex_lock(0x%p) = %d in %llu usec, from %lu@%s, %s()", mutex, ret, now_boottime_usec() - start, line, file, function);
+    // remove compiler unused variables warning
+    (void)start_s;
+    (void)end_s;
+
+    debug(D_LOCKS, "MUTEX_LOCK: netdata_mutex_lock(%p) = %d in %llu usec, from %lu@%s, %s()", mutex, ret, end_s - start_s, line, file, function);
 
     return ret;
 }
 
 int netdata_mutex_trylock_debug(const char *file __maybe_unused, const char *function __maybe_unused,
                                 const unsigned long line __maybe_unused, netdata_mutex_t *mutex) {
-    usec_t start = 0;
-    (void)start;
-
-    if(unlikely(debug_flags & D_LOCKS)) {
-        start = now_boottime_usec();
-        debug(D_LOCKS, "MUTEX_LOCK: netdata_mutex_trylock(0x%p) from %lu@%s, %s()", mutex, line, file, function);
-    }
+    debug(D_LOCKS, "MUTEX_LOCK: netdata_mutex_trylock(%p) from %lu@%s, %s()", mutex, line, file, function);
 
+    usec_t start_s = now_monotonic_high_precision_usec();
     int ret = __netdata_mutex_trylock(mutex);
+    usec_t end_s = now_monotonic_high_precision_usec();
+
+    // remove compiler unused variables warning
+    (void)start_s;
+    (void)end_s;
 
-    debug(D_LOCKS, "MUTEX_LOCK: netdata_mutex_trylock(0x%p) = %d in %llu usec, from %lu@%s, %s()", mutex, ret, now_boottime_usec() - start, line, file, function);
+    debug(D_LOCKS, "MUTEX_LOCK: netdata_mutex_trylock(%p) = %d in %llu usec, from %lu@%s, %s()", mutex, ret, end_s - start_s, line, file, function);
 
     return ret;
 }
 
 int netdata_mutex_unlock_debug(const char *file __maybe_unused, const char *function __maybe_unused,
                                const unsigned long line __maybe_unused, netdata_mutex_t *mutex) {
-    usec_t start = 0;
-    (void)start;
-
-    if(unlikely(debug_flags & D_LOCKS)) {
-        start = now_boottime_usec();
-        debug(D_LOCKS, "MUTEX_LOCK: netdata_mutex_unlock(0x%p) from %lu@%s, %s()", mutex, line, file, function);
-    }
+    debug(D_LOCKS, "MUTEX_LOCK: netdata_mutex_unlock(%p) from %lu@%s, %s()", mutex, line, file, function);
 
+    usec_t start_s = now_monotonic_high_precision_usec();
     int ret = __netdata_mutex_unlock(mutex);
+    usec_t end_s = now_monotonic_high_precision_usec();
+
+    // remove compiler unused variables warning
+    (void)start_s;
+    (void)end_s;
 
-    debug(D_LOCKS, "MUTEX_LOCK: netdata_mutex_unlock(0x%p) = %d in %llu usec, from %lu@%s, %s()", mutex, ret, now_boottime_usec() - start, line, file, function);
+    debug(D_LOCKS, "MUTEX_LOCK: netdata_mutex_unlock(%p) = %d in %llu usec, from %lu@%s, %s()", mutex, ret, end_s - start_s, line, file, function);
 
     return ret;
 }
 
+#endif // NETDATA_TRACE_RWLOCKS
 
 // ----------------------------------------------------------------------------
-// r/w lock
+// rwlock
 
 int __netdata_rwlock_destroy(netdata_rwlock_t *rwlock) {
-    int ret = pthread_rwlock_destroy(rwlock);
+    int ret = pthread_rwlock_destroy(&rwlock->rwlock_t);
     if(unlikely(ret != 0))
         error("RW_LOCK: failed to destroy lock (code %d)", ret);
     return ret;
 }
 
 int __netdata_rwlock_init(netdata_rwlock_t *rwlock) {
-    int ret = pthread_rwlock_init(rwlock, NULL);
+    int ret = pthread_rwlock_init(&rwlock->rwlock_t, NULL);
     if(unlikely(ret != 0))
         error("RW_LOCK: failed to initialize lock (code %d)", ret);
     return ret;
@@ -171,11 +217,13 @@ int __netdata_rwlock_init(netdata_rwlock_t *rwlock) {
 int __netdata_rwlock_rdlock(netdata_rwlock_t *rwlock) {
     netdata_thread_disable_cancelability();
 
-    int ret = pthread_rwlock_rdlock(rwlock);
+    int ret = pthread_rwlock_rdlock(&rwlock->rwlock_t);
     if(unlikely(ret != 0)) {
         netdata_thread_enable_cancelability();
         error("RW_LOCK: failed to obtain read lock (code %d)", ret);
     }
+    else
+        netdata_locks_acquired_rwlocks++;
 
     return ret;
 }
@@ -183,21 +231,25 @@ int __netdata_rwlock_rdlock(netdata_rwlock_t *rwlock) {
 int __netdata_rwlock_wrlock(netdata_rwlock_t *rwlock) {
     netdata_thread_disable_cancelability();
 
-    int ret = pthread_rwlock_wrlock(rwlock);
+    int ret = pthread_rwlock_wrlock(&rwlock->rwlock_t);
     if(unlikely(ret != 0)) {
         error("RW_LOCK: failed to obtain write lock (code %d)", ret);
         netdata_thread_enable_cancelability();
     }
+    else
+        netdata_locks_acquired_rwlocks++;
 
     return ret;
 }
 
 int __netdata_rwlock_unlock(netdata_rwlock_t *rwlock) {
-    int ret = pthread_rwlock_unlock(rwlock);
+    int ret = pthread_rwlock_unlock(&rwlock->rwlock_t);
     if(unlikely(ret != 0))
         error("RW_LOCK: failed to release lock (code %d)", ret);
-    else
+    else {
         netdata_thread_enable_cancelability();
+        netdata_locks_acquired_rwlocks--;
+    }
 
     return ret;
 }
@@ -205,9 +257,11 @@ int __netdata_rwlock_unlock(netdata_rwlock_t *rwlock) {
 int __netdata_rwlock_tryrdlock(netdata_rwlock_t *rwlock) {
     netdata_thread_disable_cancelability();
 
-    int ret = pthread_rwlock_tryrdlock(rwlock);
+    int ret = pthread_rwlock_tryrdlock(&rwlock->rwlock_t);
     if(ret != 0)
         netdata_thread_enable_cancelability();
+    else
+        netdata_locks_acquired_rwlocks++;
 
     return ret;
 }
@@ -215,129 +269,458 @@ int __netdata_rwlock_tryrdlock(netdata_rwlock_t *rwlock) {
 int __netdata_rwlock_trywrlock(netdata_rwlock_t *rwlock) {
     netdata_thread_disable_cancelability();
 
-    int ret = pthread_rwlock_trywrlock(rwlock);
+    int ret = pthread_rwlock_trywrlock(&rwlock->rwlock_t);
     if(ret != 0)
         netdata_thread_enable_cancelability();
+    else
+        netdata_locks_acquired_rwlocks++;
 
     return ret;
 }
 
+#ifdef NETDATA_TRACE_RWLOCKS
+
+// ----------------------------------------------------------------------------
+// lockers list
+
+void not_supported_by_posix_rwlocks(const char *file, const char *function, const unsigned long line, netdata_rwlock_t *rwlock, char locktype, const char *reason) {
+    __netdata_mutex_lock(&rwlock->lockers_mutex);
+    fprintf(stderr,
+            "RW_LOCK FATAL ON LOCK %p: %d '%s' (function %s() %lu@%s) attempts to acquire a '%c' lock, but it is not supported by POSIX because: %s. At this attempt, the task is holding %zu rwlocks and %zu mutexes. There are %zu readers and %zu writers holding this lock:\n",
+            rwlock,
+            gettid(), netdata_thread_tag(),
+            function, line, file,
+            locktype,
+            reason,
+            netdata_locks_acquired_rwlocks, netdata_locks_acquired_mutexes,
+            rwlock->readers, rwlock->writers);
+
+    int i;
+    usec_t now = now_monotonic_high_precision_usec();
+    netdata_rwlock_locker *p;
+    for(i = 1, p = rwlock->lockers; p ;p = p->next, i++) {
+        fprintf(stderr,
+                "     => %i: RW_LOCK %p: process %d '%s' (function %s() %lu@%s) is having %zu '%c' lock for %llu usec.\n",
+                i, rwlock,
+                p->pid, p->tag,
+                p->function, p->line, p->file,
+                p->callers, p->lock,
+                (now - p->start_s));
+    }
+    __netdata_mutex_unlock(&rwlock->lockers_mutex);
+}
+
+static void log_rwlock_lockers(const char *file, const char *function, const unsigned long line, netdata_rwlock_t *rwlock, const char *reason, char locktype) {
+
+    // this function can only be used by one thread at a time
+    // because otherwise, the threads may deadlock waiting for each other
+    static netdata_mutex_t log_lockers_mutex = NETDATA_MUTEX_INITIALIZER;
+    __netdata_mutex_lock(&log_lockers_mutex);
+
+    // now work on this locker
+    __netdata_mutex_lock(&rwlock->lockers_mutex);
+    fprintf(stderr,
+            "RW_LOCK ON LOCK %p: %d '%s' (function %s() %lu@%s) %s a '%c' lock (while holding %zu rwlocks and %zu mutexes). There are %zu readers and %zu writers holding this lock:\n",
+            rwlock,
+            gettid(), netdata_thread_tag(),
+            function, line, file,
+            reason, locktype,
+            netdata_locks_acquired_rwlocks, netdata_locks_acquired_mutexes,
+            rwlock->readers, rwlock->writers);
+
+    int i;
+    usec_t now = now_monotonic_high_precision_usec();
+    netdata_rwlock_locker *p;
+    for(i = 1, p = rwlock->lockers; p ;p = p->next, i++) {
+        fprintf(stderr,
+                "     => %i: RW_LOCK %p: process %d '%s' (function %s() %lu@%s) is having %zu '%c' lock for %llu usec.\n",
+                i, rwlock,
+                p->pid, p->tag,
+                p->function, p->line, p->file,
+                p->callers, p->lock,
+                (now - p->start_s));
+
+        if(p->all_caller_locks) {
+            // find the lock in the netdata_thread_locks[]
+            // and remove it
+            int k;
+            for(k = 0; k < NETDATA_THREAD_LOCKS_ARRAY_SIZE ;k++) {
+                if (p->all_caller_locks[k] && p->all_caller_locks[k] != rwlock) {
+
+                    // lock the other lock lockers list
+                    __netdata_mutex_lock(&p->all_caller_locks[k]->lockers_mutex);
+
+                    // print the list of lockers of the other lock
+                    netdata_rwlock_locker *r;
+                    int j;
+                    for(j = 1, r = p->all_caller_locks[k]->lockers; r ;r = r->next, j++) {
+                        fprintf(
+                            stderr,
+                            "     ~~~> %i: RW_LOCK %p: process %d '%s' (function %s() %lu@%s) is having %zu '%c' lock for %llu usec.\n",
+                            j,
+                            p->all_caller_locks[k],
+                            r->pid,
+                            r->tag,
+                            r->function,
+                            r->line,
+                            r->file,
+                            r->callers,
+                            r->lock,
+                            (now - r->start_s));
+                    }
+
+                    // unlock the other lock lockers list
+                    __netdata_mutex_unlock(&p->all_caller_locks[k]->lockers_mutex);
+                }
+            }
+        }
+
+    }
+    __netdata_mutex_unlock(&rwlock->lockers_mutex);
+
+    // unlock this function for other threads
+    __netdata_mutex_unlock(&log_lockers_mutex);
+}
+
+static netdata_rwlock_locker *add_rwlock_locker(const char *file, const char *function, const unsigned long line, netdata_rwlock_t *rwlock, char lock_type) {
+    netdata_rwlock_locker *p = mallocz(sizeof(netdata_rwlock_locker));
+    p->pid = gettid();
+    p->tag = netdata_thread_tag();
+    p->lock = lock_type;
+    p->file = file;
+    p->function = function;
+    p->line = line;
+    p->callers = 1;
+    p->all_caller_locks = netdata_thread_locks;
+    p->start_s = now_monotonic_high_precision_usec();
+
+    // find a slot in the netdata_thread_locks[]
+    int i;
+    for(i = 0; i < NETDATA_THREAD_LOCKS_ARRAY_SIZE ;i++) {
+        if (!netdata_thread_locks[i]) {
+            netdata_thread_locks[i] = rwlock;
+            break;
+        }
+    }
+
+    __netdata_mutex_lock(&rwlock->lockers_mutex);
+    p->next = rwlock->lockers;
+    rwlock->lockers = p;
+    if(lock_type == 'R') rwlock->readers++;
+    if(lock_type == 'W') rwlock->writers++;
+    __netdata_mutex_unlock(&rwlock->lockers_mutex);
+
+    return p;
+}
+
+static void remove_rwlock_locker(const char *file __maybe_unused, const char *function __maybe_unused, const unsigned long line __maybe_unused, netdata_rwlock_t *rwlock, netdata_rwlock_locker *locker) {
+    usec_t end_s = now_monotonic_high_precision_usec();
+
+    if(locker->callers == 0)
+        fprintf(stderr,
+                "RW_LOCK ON LOCK %p: %d, '%s' (function %s() %lu@%s) callers should be positive but it is zero\n",
+                rwlock,
+                locker->pid, locker->tag,
+                locker->function, locker->line, locker->file);
+
+    if(locker->callers > 1 && locker->lock != 'R')
+        fprintf(stderr,
+                "RW_LOCK ON LOCK %p: %d, '%s' (function %s() %lu@%s) only 'R' locks support multiple holders, but here we have %zu callers holding a '%c' lock.\n",
+                rwlock,
+                locker->pid, locker->tag,
+                locker->function, locker->line, locker->file,
+                locker->callers, locker->lock);
+
+    __netdata_mutex_lock(&rwlock->lockers_mutex);
+    locker->callers--;
+
+    if(!locker->callers) {
+        int doit = 0;
+
+        if (rwlock->lockers == locker) {
+            rwlock->lockers = locker->next;
+            doit = 1;
+        } else {
+            netdata_rwlock_locker *p;
+            for (p = rwlock->lockers; p && p->next != locker; p = p->next)
+                ;
+            if (p && p->next == locker) {
+                p->next = locker->next;
+                doit = 1;
+            }
+        }
+        if(doit) {
+            if(locker->lock == 'R') rwlock->readers--;
+            if(locker->lock == 'W') rwlock->writers--;
+        }
+
+        if(!doit) {
+            fprintf(stderr,
+                    "RW_LOCK ON LOCK %p: %d, '%s' (function %s() %lu@%s) with %zu x '%c' lock is not found.\n",
+                    rwlock,
+                    locker->pid, locker->tag,
+                    locker->function, locker->line, locker->file,
+                    locker->callers, locker->lock);
+        }
+        else {
+            // find the lock in the netdata_thread_locks[]
+            // and remove it
+            int i;
+            for(i = 0; i < NETDATA_THREAD_LOCKS_ARRAY_SIZE ;i++) {
+                if (netdata_thread_locks[i] == rwlock)
+                    netdata_thread_locks[i] = NULL;
+            }
+
+            if(end_s - locker->start_s >= NETDATA_TRACE_RWLOCKS_HOLD_TIME_TO_IGNORE_USEC)
+                fprintf(stderr,
+                        "RW_LOCK ON LOCK %p: %d, '%s' (function %s() %lu@%s) holded a '%c' for %llu usec.\n",
+                        rwlock,
+                        locker->pid, locker->tag,
+                        locker->function, locker->line, locker->file,
+                        locker->lock, end_s - locker->start_s);
+
+            freez(locker);
+        }
+    }
+
+    __netdata_mutex_unlock(&rwlock->lockers_mutex);
+}
+
+static netdata_rwlock_locker *find_rwlock_locker(const char *file __maybe_unused, const char *function __maybe_unused, const unsigned long line __maybe_unused, netdata_rwlock_t *rwlock) {
+    pid_t pid = gettid();
+    netdata_rwlock_locker *p;
+
+    __netdata_mutex_lock(&rwlock->lockers_mutex);
+    for(p = rwlock->lockers; p ;p = p->next) {
+        if(p->pid == pid) break;
+    }
+    __netdata_mutex_unlock(&rwlock->lockers_mutex);
+
+    return p;
+}
+
+static netdata_rwlock_locker *update_or_add_rwlock_locker(const char *file, const char *function, const unsigned long line, netdata_rwlock_t *rwlock, netdata_rwlock_locker *locker, char locktype) {
+    if(!locker) {
+        return add_rwlock_locker(file, function, line, rwlock, locktype);
+    }
+    else if(locker->lock == 'R' && locktype == 'R') {
+        __netdata_mutex_lock(&rwlock->lockers_mutex);
+        locker->callers++;
+        __netdata_mutex_unlock(&rwlock->lockers_mutex);
+        return locker;
+    }
+    else {
+        not_supported_by_posix_rwlocks(file, function, line, rwlock, locktype, "DEADLOCK - WANTS TO CHANGE LOCK TYPE BUT ALREADY HAS THIS LOCKED");
+        return locker;
+    }
+}
+
+// ----------------------------------------------------------------------------
+// debug versions of rwlock
 
 int netdata_rwlock_destroy_debug(const char *file __maybe_unused, const char *function __maybe_unused,
                                  const unsigned long line __maybe_unused, netdata_rwlock_t *rwlock) {
-    usec_t start = 0;
-    (void)start;
+    debug(D_LOCKS, "RW_LOCK: netdata_rwlock_destroy(%p) from %lu@%s, %s()", rwlock, line, file, function);
 
-    if(unlikely(debug_flags & D_LOCKS)) {
-        start = now_boottime_usec();
-        debug(D_LOCKS, "RW_LOCK: netdata_rwlock_destroy(0x%p) from %lu@%s, %s()", rwlock, line, file, function);
-    }
+    if(rwlock->readers)
+        error("RW_LOCK: destroying a rwlock with %zu readers in it", rwlock->readers);
+    if(rwlock->writers)
+        error("RW_LOCK: destroying a rwlock with %zu writers in it", rwlock->writers);
 
     int ret = __netdata_rwlock_destroy(rwlock);
+    if(!ret) {
+        while (rwlock->lockers)
+            remove_rwlock_locker(file, function, line, rwlock, rwlock->lockers);
+
+        if (rwlock->readers)
+            error("RW_LOCK: internal error - empty rwlock with %zu readers in it", rwlock->readers);
+        if (rwlock->writers)
+            error("RW_LOCK: internal error - empty rwlock with %zu writers in it", rwlock->writers);
+    }
 
-    debug(D_LOCKS, "RW_LOCK: netdata_rwlock_destroy(0x%p) = %d in %llu usec, from %lu@%s, %s()", rwlock, ret, now_boottime_usec() - start, line, file, function);
+    debug(D_LOCKS, "RW_LOCK: netdata_rwlock_destroy(%p) = %d, from %lu@%s, %s()", rwlock, ret, line, file, function);
 
     return ret;
 }
 
 int netdata_rwlock_init_debug(const char *file __maybe_unused, const char *function __maybe_unused,
                               const unsigned long line __maybe_unused, netdata_rwlock_t *rwlock) {
-    usec_t start = 0;
-    (void)start;
-
-    if(unlikely(debug_flags & D_LOCKS)) {
-        start = now_boottime_usec();
-        debug(D_LOCKS, "RW_LOCK: netdata_rwlock_init(0x%p) from %lu@%s, %s()", rwlock, line, file, function);
-    }
+    debug(D_LOCKS, "RW_LOCK: netdata_rwlock_init(%p) from %lu@%s, %s()", rwlock, line, file, function);
 
     int ret = __netdata_rwlock_init(rwlock);
+    if(!ret) {
+        __netdata_mutex_init(&rwlock->lockers_mutex);
+        rwlock->lockers = NULL;
+        rwlock->readers = 0;
+        rwlock->writers = 0;
+    }
 
-    debug(D_LOCKS, "RW_LOCK: netdata_rwlock_init(0x%p) = %d in %llu usec, from %lu@%s, %s()", rwlock, ret, now_boottime_usec() - start, line, file, function);
+    debug(D_LOCKS, "RW_LOCK: netdata_rwlock_init(%p) = %d, from %lu@%s, %s()", rwlock, ret, line, file, function);
 
     return ret;
 }
 
 int netdata_rwlock_rdlock_debug(const char *file __maybe_unused, const char *function __maybe_unused,
                                 const unsigned long line __maybe_unused, netdata_rwlock_t *rwlock) {
-    usec_t start = 0;
-    (void)start;
 
-    if(unlikely(debug_flags & D_LOCKS)) {
-        start = now_boottime_usec();
-        debug(D_LOCKS, "RW_LOCK: netdata_rwlock_rdlock(0x%p) from %lu@%s, %s()", rwlock, line, file, function);
+    debug(D_LOCKS, "RW_LOCK: netdata_rwlock_rdlock(%p) from %lu@%s, %s()", rwlock, line, file, function);
+
+    netdata_rwlock_locker *locker = find_rwlock_locker(file, function, line, rwlock);
+
+#ifdef NETDATA_TRACE_RWLOCKS_LOG_NESTED
+    if(locker && locker->lock == 'R') {
+        log_rwlock_lockers(file, function, line, rwlock, "NESTED READ LOCK REQUEST", 'R');
+    }
+#endif // NETDATA_TRACE_RWLOCKS_LOG_NESTED
+
+    int log = 0;
+    if(rwlock->writers) {
+        log_rwlock_lockers(file, function, line, rwlock, "WANTS", 'R');
+        log = 1;
     }
 
+    usec_t start_s = now_monotonic_high_precision_usec();
     int ret = __netdata_rwlock_rdlock(rwlock);
+    usec_t end_s = now_monotonic_high_precision_usec();
+
+    if(!ret) {
+        locker = update_or_add_rwlock_locker(file, function, line, rwlock, locker, 'R');
+        if(log) log_rwlock_lockers(file, function, line, rwlock, "GOT", 'R');
+
+    }
 
-    debug(D_LOCKS, "RW_LOCK: netdata_rwlock_rdlock(0x%p) = %d in %llu usec, from %lu@%s, %s()", rwlock, ret, now_boottime_usec() - start, line, file, function);
+    if(end_s - start_s >= NETDATA_TRACE_RWLOCKS_WAIT_TIME_TO_IGNORE_USEC)
+        fprintf(stderr,
+                "RW_LOCK ON LOCK %p: %d, '%s' (function %s() %lu@%s) WAITED for a READ lock for %llu usec.\n",
+                rwlock,
+                gettid(), netdata_thread_tag(),
+                function, line, file,
+                end_s - start_s);
+
+    debug(D_LOCKS, "RW_LOCK: netdata_rwlock_rdlock(%p) = %d in %llu usec, from %lu@%s, %s()", rwlock, ret, end_s - start_s, line, file, function);
 
     return ret;
 }
 
 int netdata_rwlock_wrlock_debug(const char *file __maybe_unused, const char *function __maybe_unused,
                                 const unsigned long line __maybe_unused, netdata_rwlock_t *rwlock) {
-    usec_t start = 0;
-    (void)start;
 
-    if(unlikely(debug_flags & D_LOCKS)) {
-        start = now_boottime_usec();
-        debug(D_LOCKS, "RW_LOCK: netdata_rwlock_wrlock(0x%p) from %lu@%s, %s()", rwlock, line, file, function);
+    debug(D_LOCKS, "RW_LOCK: netdata_rwlock_wrlock(%p) from %lu@%s, %s()", rwlock, line, file, function);
+
+    netdata_rwlock_locker *locker = find_rwlock_locker(file, function, line, rwlock);
+    if(locker)
+        not_supported_by_posix_rwlocks(file, function, line, rwlock, 'W', "DEADLOCK - WANTS A WRITE LOCK BUT ALREADY HAVE THIS LOCKED");
+
+    int log = 0;
+    if(rwlock->readers) {
+        log_rwlock_lockers(file, function, line, rwlock, "WANTS", 'W');
+        log = 1;
     }
 
+    usec_t start_s = now_monotonic_high_precision_usec();
     int ret = __netdata_rwlock_wrlock(rwlock);
+    usec_t end_s = now_monotonic_high_precision_usec();
 
-    debug(D_LOCKS, "RW_LOCK: netdata_rwlock_wrlock(0x%p) = %d in %llu usec, from %lu@%s, %s()", rwlock, ret, now_boottime_usec() - start, line, file, function);
+    if(!ret){
+        locker = update_or_add_rwlock_locker(file, function, line, rwlock, locker, 'W');
+        if(log) log_rwlock_lockers(file, function, line, rwlock, "GOT", 'W');
+    }
+
+    if(end_s - start_s >= NETDATA_TRACE_RWLOCKS_WAIT_TIME_TO_IGNORE_USEC)
+        fprintf(stderr,
+                "RW_LOCK ON LOCK %p: %d, '%s' (function %s() %lu@%s) WAITED for a WRITE lock for %llu usec.\n",
+                rwlock,
+                gettid(), netdata_thread_tag(),
+                function, line, file,
+                end_s - start_s);
+
+    debug(D_LOCKS, "RW_LOCK: netdata_rwlock_wrlock(%p) = %d in %llu usec, from %lu@%s, %s()", rwlock, ret, end_s - start_s, line, file, function);
 
     return ret;
 }
 
 int netdata_rwlock_unlock_debug(const char *file __maybe_unused, const char *function __maybe_unused,
                                 const unsigned long line __maybe_unused, netdata_rwlock_t *rwlock) {
-    usec_t start = 0;
-    (void)start;
 
-    if(unlikely(debug_flags & D_LOCKS)) {
-        start = now_boottime_usec();
-        debug(D_LOCKS, "RW_LOCK: netdata_rwlock_unlock(0x%p) from %lu@%s, %s()", rwlock, line, file, function);
-    }
+    debug(D_LOCKS, "RW_LOCK: netdata_rwlock_unlock(%p) from %lu@%s, %s()", rwlock, line, file, function);
 
+    netdata_rwlock_locker *locker = find_rwlock_locker(file, function, line, rwlock);
+    if(unlikely(!locker))
+        not_supported_by_posix_rwlocks(file, function, line, rwlock, 'U', "UNLOCK WITHOUT LOCK");
+
+    usec_t start_s = now_monotonic_high_precision_usec();
     int ret = __netdata_rwlock_unlock(rwlock);
+    usec_t end_s = now_monotonic_high_precision_usec();
+
+    if(end_s - start_s >= NETDATA_TRACE_RWLOCKS_WAIT_TIME_TO_IGNORE_USEC)
+        fprintf(stderr,
+                "RW_LOCK ON LOCK %p: %d, '%s' (function %s() %lu@%s) WAITED to UNLOCK for %llu usec.\n",
+                rwlock,
+                gettid(), netdata_thread_tag(),
+                function, line, file,
+                end_s - start_s);
+
+    if(likely(!ret && locker)) remove_rwlock_locker(file, function, line, rwlock, locker);
 
-    debug(D_LOCKS, "RW_LOCK: netdata_rwlock_unlock(0x%p) = %d in %llu usec, from %lu@%s, %s()", rwlock, ret, now_boottime_usec() - start, line, file, function);
+    debug(D_LOCKS, "RW_LOCK: netdata_rwlock_unlock(%p) = %d in %llu usec, from %lu@%s, %s()", rwlock, ret, end_s - start_s, line, file, function);
 
     return ret;
 }
 
 int netdata_rwlock_tryrdlock_debug(const char *file __maybe_unused, const char *function __maybe_unused,
                                    const unsigned long line __maybe_unused, netdata_rwlock_t *rwlock) {
-    usec_t start = 0;
-    (void)start;
+    debug(D_LOCKS, "RW_LOCK: netdata_rwlock_tryrdlock(%p) from %lu@%s, %s()", rwlock, line, file, function);
 
-    if(unlikely(debug_flags & D_LOCKS)) {
-        start = now_boottime_usec();
-        debug(D_LOCKS, "RW_LOCK: netdata_rwlock_tryrdlock(0x%p) from %lu@%s, %s()", rwlock, line, file, function);
-    }
+    netdata_rwlock_locker *locker = find_rwlock_locker(file, function, line, rwlock);
+    if(locker && locker->lock == 'W')
+        not_supported_by_posix_rwlocks(file, function, line, rwlock, 'R', "DEADLOCK - WANTS A READ LOCK BUT IT HAS A WRITE LOCK ALREADY");
 
+    usec_t start_s = now_monotonic_high_precision_usec();
     int ret = __netdata_rwlock_tryrdlock(rwlock);
+    usec_t end_s = now_monotonic_high_precision_usec();
+
+    if(!ret)
+        locker = update_or_add_rwlock_locker(file, function, line, rwlock, locker, 'R');
 
-    debug(D_LOCKS, "RW_LOCK: netdata_rwlock_tryrdlock(0x%p) = %d in %llu usec, from %lu@%s, %s()", rwlock, ret, now_boottime_usec() - start, line, file, function);
+    if(end_s - start_s >= NETDATA_TRACE_RWLOCKS_WAIT_TIME_TO_IGNORE_USEC)
+        fprintf(stderr,
+                "RW_LOCK ON LOCK %p: %d, '%s' (function %s() %lu@%s) WAITED to TRYREAD for %llu usec.\n",
+                rwlock,
+                gettid(), netdata_thread_tag(),
+                function, line, file,
+                end_s - start_s);
+
+    debug(D_LOCKS, "RW_LOCK: netdata_rwlock_tryrdlock(%p) = %d in %llu usec, from %lu@%s, %s()", rwlock, ret, end_s - start_s, line, file, function);
 
     return ret;
 }
 
 int netdata_rwlock_trywrlock_debug(const char *file __maybe_unused, const char *function __maybe_unused,
                                    const unsigned long line __maybe_unused, netdata_rwlock_t *rwlock) {
-    usec_t start = 0;
-    (void)start;
+    debug(D_LOCKS, "RW_LOCK: netdata_rwlock_trywrlock(%p) from %lu@%s, %s()", rwlock, line, file, function);
 
-    if(unlikely(debug_flags & D_LOCKS)) {
-        start = now_boottime_usec();
-        debug(D_LOCKS, "RW_LOCK: netdata_rwlock_trywrlock(0x%p) from %lu@%s, %s()", rwlock, line, file, function);
-    }
+    netdata_rwlock_locker *locker = find_rwlock_locker(file, function, line, rwlock);
+    if(locker)
+        not_supported_by_posix_rwlocks(file, function, line, rwlock, 'W', "ALREADY HAS THIS LOCK");
 
+    usec_t start_s = now_monotonic_high_precision_usec();
     int ret = __netdata_rwlock_trywrlock(rwlock);
+    usec_t end_s = now_monotonic_high_precision_usec();
 
-    debug(D_LOCKS, "RW_LOCK: netdata_rwlock_trywrlock(0x%p) = %d in %llu usec, from %lu@%s, %s()", rwlock, ret, now_boottime_usec() - start, line, file, function);
+    if(!ret)
+        locker = update_or_add_rwlock_locker(file, function, line, rwlock, locker, 'W');
+
+    if(end_s - start_s >= NETDATA_TRACE_RWLOCKS_WAIT_TIME_TO_IGNORE_USEC)
+        fprintf(stderr,
+                "RW_LOCK ON LOCK %p: %d, '%s' (function %s() %lu@%s) WAITED to TRYWRITE for %llu usec.\n",
+                rwlock,
+                gettid(), netdata_thread_tag(),
+                function, line, file,
+                end_s - start_s);
+
+    debug(D_LOCKS, "RW_LOCK: netdata_rwlock_trywrlock(%p) = %d in %llu usec, from %lu@%s, %s()", rwlock, ret, end_s - start_s, line, file, function);
 
     return ret;
 }
+
+#endif // NETDATA_TRACE_RWLOCKS