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| -rw-r--r-- | m4/pthread_rwlock_rdlock.m4 | 185 |
1 files changed, 185 insertions, 0 deletions
diff --git a/m4/pthread_rwlock_rdlock.m4 b/m4/pthread_rwlock_rdlock.m4 new file mode 100644 index 0000000..66b29f8 --- /dev/null +++ b/m4/pthread_rwlock_rdlock.m4 @@ -0,0 +1,185 @@ +# pthread_rwlock_rdlock.m4 serial 5 +dnl Copyright (C) 2017-2023 Free Software Foundation, Inc. +dnl This file is free software; the Free Software Foundation +dnl gives unlimited permission to copy and/or distribute it, +dnl with or without modifications, as long as this notice is preserved. + +dnl From Bruno Haible. +dnl Inspired by +dnl https://github.com/linux-test-project/ltp/blob/master/testcases/open_posix_testsuite/conformance/interfaces/pthread_rwlock_rdlock/2-2.c +dnl by Intel Corporation. + +dnl Test whether in a situation where +dnl - an rwlock is taken by a reader and has a writer waiting, +dnl - an additional reader requests the lock, +dnl - the waiting writer and the requesting reader threads have the same +dnl priority, +dnl the requesting reader thread gets blocked, so that at some point the +dnl waiting writer can acquire the lock. +dnl Without such a guarantee, when there a N readers and each of the readers +dnl spends more than 1/Nth of the time with the lock held, there is a high +dnl probability that the waiting writer will not get the lock in a given finite +dnl time, a phenomenon called "writer starvation". +dnl Without such a guarantee, applications have a hard time avoiding writer +dnl starvation. +dnl +dnl POSIX:2017 makes this requirement only for implementations that support TPS +dnl (Thread Priority Scheduling) and only for the scheduling policies SCHED_FIFO +dnl and SCHED_RR, see +dnl https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_rwlock_rdlock.html +dnl but this test verifies the guarantee regardless of TPS and regardless of +dnl scheduling policy. +dnl Glibc does not provide this guarantee (and never will on Linux), see +dnl https://sourceware.org/bugzilla/show_bug.cgi?id=13701 +dnl https://bugzilla.redhat.com/show_bug.cgi?id=1410052 +AC_DEFUN([gl_PTHREAD_RWLOCK_RDLOCK_PREFER_WRITER], +[ + AC_REQUIRE([gl_THREADLIB_EARLY]) + AC_REQUIRE([AC_CANONICAL_HOST]) dnl for cross-compiles + AC_CACHE_CHECK([whether pthread_rwlock_rdlock prefers a writer to a reader], + [gl_cv_pthread_rwlock_rdlock_prefer_writer], + [save_LIBS="$LIBS" + LIBS="$LIBS $LIBMULTITHREAD" + AC_RUN_IFELSE( + [AC_LANG_SOURCE([[ +#include <errno.h> +#include <pthread.h> +#include <stdlib.h> +#include <unistd.h> + +#define SUCCEED() exit (0) +#define FAILURE() exit (1) +#define UNEXPECTED(n) (exit (10 + (n))) + +/* The main thread creates the waiting writer and the requesting reader threads + in the default way; this guarantees that they have the same priority. + We can reuse the main thread as first reader thread. */ + +static pthread_rwlock_t lock; +static pthread_t reader1; +static pthread_t writer; +static pthread_t reader2; +static pthread_t timer; +/* Used to pass control from writer to reader2 and from reader2 to timer, + as in a relay race. + Passing control from one running thread to another running thread + is most likely faster than to create the second thread. */ +static pthread_mutex_t baton; + +static void * +timer_func (void *ignored) +{ + /* Step 13 (can be before or after step 12): + The timer thread takes the baton, then waits a moment to make sure + it can tell whether the second reader thread is blocked at step 12. */ + if (pthread_mutex_lock (&baton)) + UNEXPECTED (13); + usleep (100000); + /* By the time we get here, it's clear that the second reader thread is + blocked at step 12. This is the desired behaviour. */ + SUCCEED (); +} + +static void * +reader2_func (void *ignored) +{ + int err; + + /* Step 8 (can be before or after step 7): + The second reader thread takes the baton, then waits a moment to make sure + the writer thread has reached step 7. */ + if (pthread_mutex_lock (&baton)) + UNEXPECTED (8); + usleep (100000); + /* Step 9: The second reader thread requests the lock. */ + err = pthread_rwlock_tryrdlock (&lock); + if (err == 0) + FAILURE (); + else if (err != EBUSY) + UNEXPECTED (9); + /* Step 10: Launch a timer, to test whether the next call blocks. */ + if (pthread_create (&timer, NULL, timer_func, NULL)) + UNEXPECTED (10); + /* Step 11: Release the baton. */ + if (pthread_mutex_unlock (&baton)) + UNEXPECTED (11); + /* Step 12: The second reader thread requests the lock. */ + err = pthread_rwlock_rdlock (&lock); + if (err == 0) + FAILURE (); + else + UNEXPECTED (12); +} + +static void * +writer_func (void *ignored) +{ + /* Step 4: Take the baton, so that the second reader thread does not go ahead + too early. */ + if (pthread_mutex_lock (&baton)) + UNEXPECTED (4); + /* Step 5: Create the second reader thread. */ + if (pthread_create (&reader2, NULL, reader2_func, NULL)) + UNEXPECTED (5); + /* Step 6: Release the baton. */ + if (pthread_mutex_unlock (&baton)) + UNEXPECTED (6); + /* Step 7: The writer thread requests the lock. */ + if (pthread_rwlock_wrlock (&lock)) + UNEXPECTED (7); + return NULL; +} + +int +main () +{ + reader1 = pthread_self (); + + /* Step 1: The main thread initializes the lock and the baton. */ + if (pthread_rwlock_init (&lock, NULL)) + UNEXPECTED (1); + if (pthread_mutex_init (&baton, NULL)) + UNEXPECTED (1); + /* Step 2: The main thread acquires the lock as a reader. */ + if (pthread_rwlock_rdlock (&lock)) + UNEXPECTED (2); + /* Step 3: Create the writer thread. */ + if (pthread_create (&writer, NULL, writer_func, NULL)) + UNEXPECTED (3); + /* Job done. Go to sleep. */ + for (;;) + { + sleep (1); + } +} +]])], + [gl_cv_pthread_rwlock_rdlock_prefer_writer=yes], + [gl_cv_pthread_rwlock_rdlock_prefer_writer=no], + [case "$host_os" in + # Guess no on glibc systems. + *-gnu* | gnu*) gl_cv_pthread_rwlock_rdlock_prefer_writer="guessing no" ;; + # Guess no on musl systems. + *-musl* | midipix*) gl_cv_pthread_rwlock_rdlock_prefer_writer="guessing no" ;; + # Guess no on bionic systems. + *-android*) gl_cv_pthread_rwlock_rdlock_prefer_writer="guessing no" ;; + # Guess yes on native Windows with the mingw-w64 winpthreads library. + # Guess no on native Windows with the gnulib windows-rwlock module. + mingw*) if test "$gl_use_threads" = yes || test "$gl_use_threads" = posix; then + gl_cv_pthread_rwlock_rdlock_prefer_writer="guessing yes" + else + gl_cv_pthread_rwlock_rdlock_prefer_writer="guessing no" + fi + ;; + # If we don't know, obey --enable-cross-guesses. + *) gl_cv_pthread_rwlock_rdlock_prefer_writer="$gl_cross_guess_normal" ;; + esac + ]) + LIBS="$save_LIBS" + ]) + case "$gl_cv_pthread_rwlock_rdlock_prefer_writer" in + *yes) + AC_DEFINE([HAVE_PTHREAD_RWLOCK_RDLOCK_PREFER_WRITER], [1], + [Define if the 'pthread_rwlock_rdlock' function prefers a writer to a reader.]) + ;; + esac +]) |