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Diffstat (limited to '')
| -rw-r--r-- | debian/patches-rt/0118-tasklet-Prevent-tasklets-from-going-into-infinite-sp.patch | 303 |
1 files changed, 303 insertions, 0 deletions
diff --git a/debian/patches-rt/0118-tasklet-Prevent-tasklets-from-going-into-infinite-sp.patch b/debian/patches-rt/0118-tasklet-Prevent-tasklets-from-going-into-infinite-sp.patch new file mode 100644 index 000000000..2885b4101 --- /dev/null +++ b/debian/patches-rt/0118-tasklet-Prevent-tasklets-from-going-into-infinite-sp.patch @@ -0,0 +1,303 @@ +From ff4b7bd98df1b1100237e3ac2f11c687a486c18b Mon Sep 17 00:00:00 2001 +From: Ingo Molnar <mingo@elte.hu> +Date: Tue, 29 Nov 2011 20:18:22 -0500 +Subject: [PATCH 118/347] tasklet: Prevent tasklets from going into infinite + spin in RT +Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/4.19/older/patches-4.19.246-rt110.tar.xz + +When CONFIG_PREEMPT_RT_FULL is enabled, tasklets run as threads, +and spinlocks turn are mutexes. But this can cause issues with +tasks disabling tasklets. A tasklet runs under ksoftirqd, and +if a tasklets are disabled with tasklet_disable(), the tasklet +count is increased. When a tasklet runs, it checks this counter +and if it is set, it adds itself back on the softirq queue and +returns. + +The problem arises in RT because ksoftirq will see that a softirq +is ready to run (the tasklet softirq just re-armed itself), and will +not sleep, but instead run the softirqs again. The tasklet softirq +will still see that the count is non-zero and will not execute +the tasklet and requeue itself on the softirq again, which will +cause ksoftirqd to run it again and again and again. + +It gets worse because ksoftirqd runs as a real-time thread. +If it preempted the task that disabled tasklets, and that task +has migration disabled, or can't run for other reasons, the tasklet +softirq will never run because the count will never be zero, and +ksoftirqd will go into an infinite loop. As an RT task, it this +becomes a big problem. + +This is a hack solution to have tasklet_disable stop tasklets, and +when a tasklet runs, instead of requeueing the tasklet softirqd +it delays it. When tasklet_enable() is called, and tasklets are +waiting, then the tasklet_enable() will kick the tasklets to continue. +This prevents the lock up from ksoftirq going into an infinite loop. + +[ rostedt@goodmis.org: ported to 3.0-rt ] + +Signed-off-by: Ingo Molnar <mingo@elte.hu> +Signed-off-by: Steven Rostedt <rostedt@goodmis.org> +Signed-off-by: Thomas Gleixner <tglx@linutronix.de> +--- + include/linux/interrupt.h | 33 +++++----- + kernel/softirq.c | 126 ++++++++++++++++++++++++++++++++------ + 2 files changed, 125 insertions(+), 34 deletions(-) + +diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h +index 315f852b4981..35fa335c475b 100644 +--- a/include/linux/interrupt.h ++++ b/include/linux/interrupt.h +@@ -536,8 +536,9 @@ static inline struct task_struct *this_cpu_ksoftirqd(void) + to be executed on some cpu at least once after this. + * If the tasklet is already scheduled, but its execution is still not + started, it will be executed only once. +- * If this tasklet is already running on another CPU (or schedule is called +- from tasklet itself), it is rescheduled for later. ++ * If this tasklet is already running on another CPU, it is rescheduled ++ for later. ++ * Schedule must not be called from the tasklet itself (a lockup occurs) + * Tasklet is strictly serialized wrt itself, but not + wrt another tasklets. If client needs some intertask synchronization, + he makes it with spinlocks. +@@ -562,27 +563,36 @@ struct tasklet_struct name = { NULL, 0, ATOMIC_INIT(1), func, data } + enum + { + TASKLET_STATE_SCHED, /* Tasklet is scheduled for execution */ +- TASKLET_STATE_RUN /* Tasklet is running (SMP only) */ ++ TASKLET_STATE_RUN, /* Tasklet is running (SMP only) */ ++ TASKLET_STATE_PENDING /* Tasklet is pending */ + }; + +-#ifdef CONFIG_SMP ++#define TASKLET_STATEF_SCHED (1 << TASKLET_STATE_SCHED) ++#define TASKLET_STATEF_RUN (1 << TASKLET_STATE_RUN) ++#define TASKLET_STATEF_PENDING (1 << TASKLET_STATE_PENDING) ++ ++#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT_FULL) + static inline int tasklet_trylock(struct tasklet_struct *t) + { + return !test_and_set_bit(TASKLET_STATE_RUN, &(t)->state); + } + ++static inline int tasklet_tryunlock(struct tasklet_struct *t) ++{ ++ return cmpxchg(&t->state, TASKLET_STATEF_RUN, 0) == TASKLET_STATEF_RUN; ++} ++ + static inline void tasklet_unlock(struct tasklet_struct *t) + { + smp_mb__before_atomic(); + clear_bit(TASKLET_STATE_RUN, &(t)->state); + } + +-static inline void tasklet_unlock_wait(struct tasklet_struct *t) +-{ +- while (test_bit(TASKLET_STATE_RUN, &(t)->state)) { barrier(); } +-} ++extern void tasklet_unlock_wait(struct tasklet_struct *t); ++ + #else + #define tasklet_trylock(t) 1 ++#define tasklet_tryunlock(t) 1 + #define tasklet_unlock_wait(t) do { } while (0) + #define tasklet_unlock(t) do { } while (0) + #endif +@@ -616,12 +626,7 @@ static inline void tasklet_disable(struct tasklet_struct *t) + smp_mb(); + } + +-static inline void tasklet_enable(struct tasklet_struct *t) +-{ +- smp_mb__before_atomic(); +- atomic_dec(&t->count); +-} +- ++extern void tasklet_enable(struct tasklet_struct *t); + extern void tasklet_kill(struct tasklet_struct *t); + extern void tasklet_kill_immediate(struct tasklet_struct *t, unsigned int cpu); + extern void tasklet_init(struct tasklet_struct *t, +diff --git a/kernel/softirq.c b/kernel/softirq.c +index 6f584861d329..1d3a482246cc 100644 +--- a/kernel/softirq.c ++++ b/kernel/softirq.c +@@ -21,6 +21,7 @@ + #include <linux/freezer.h> + #include <linux/kthread.h> + #include <linux/rcupdate.h> ++#include <linux/delay.h> + #include <linux/ftrace.h> + #include <linux/smp.h> + #include <linux/smpboot.h> +@@ -475,11 +476,38 @@ static void __tasklet_schedule_common(struct tasklet_struct *t, + unsigned long flags; + + local_irq_save(flags); ++ if (!tasklet_trylock(t)) { ++ local_irq_restore(flags); ++ return; ++ } ++ + head = this_cpu_ptr(headp); +- t->next = NULL; +- *head->tail = t; +- head->tail = &(t->next); +- raise_softirq_irqoff(softirq_nr); ++again: ++ /* We may have been preempted before tasklet_trylock ++ * and __tasklet_action may have already run. ++ * So double check the sched bit while the takslet ++ * is locked before adding it to the list. ++ */ ++ if (test_bit(TASKLET_STATE_SCHED, &t->state)) { ++ t->next = NULL; ++ *head->tail = t; ++ head->tail = &(t->next); ++ raise_softirq_irqoff(softirq_nr); ++ tasklet_unlock(t); ++ } else { ++ /* This is subtle. If we hit the corner case above ++ * It is possible that we get preempted right here, ++ * and another task has successfully called ++ * tasklet_schedule(), then this function, and ++ * failed on the trylock. Thus we must be sure ++ * before releasing the tasklet lock, that the ++ * SCHED_BIT is clear. Otherwise the tasklet ++ * may get its SCHED_BIT set, but not added to the ++ * list ++ */ ++ if (!tasklet_tryunlock(t)) ++ goto again; ++ } + local_irq_restore(flags); + } + +@@ -497,11 +525,21 @@ void __tasklet_hi_schedule(struct tasklet_struct *t) + } + EXPORT_SYMBOL(__tasklet_hi_schedule); + ++void tasklet_enable(struct tasklet_struct *t) ++{ ++ if (!atomic_dec_and_test(&t->count)) ++ return; ++ if (test_and_clear_bit(TASKLET_STATE_PENDING, &t->state)) ++ tasklet_schedule(t); ++} ++EXPORT_SYMBOL(tasklet_enable); ++ + static void tasklet_action_common(struct softirq_action *a, + struct tasklet_head *tl_head, + unsigned int softirq_nr) + { + struct tasklet_struct *list; ++ int loops = 1000000; + + local_irq_disable(); + list = tl_head->head; +@@ -513,25 +551,56 @@ static void tasklet_action_common(struct softirq_action *a, + struct tasklet_struct *t = list; + + list = list->next; ++ /* ++ * Should always succeed - after a tasklist got on the ++ * list (after getting the SCHED bit set from 0 to 1), ++ * nothing but the tasklet softirq it got queued to can ++ * lock it: ++ */ ++ if (!tasklet_trylock(t)) { ++ WARN_ON(1); ++ continue; ++ } + +- if (tasklet_trylock(t)) { +- if (!atomic_read(&t->count)) { +- if (!test_and_clear_bit(TASKLET_STATE_SCHED, +- &t->state)) +- BUG(); +- t->func(t->data); ++ t->next = NULL; ++ ++ if (unlikely(atomic_read(&t->count))) { ++out_disabled: ++ /* implicit unlock: */ ++ wmb(); ++ t->state = TASKLET_STATEF_PENDING; ++ continue; ++ } ++ /* ++ * After this point on the tasklet might be rescheduled ++ * on another CPU, but it can only be added to another ++ * CPU's tasklet list if we unlock the tasklet (which we ++ * dont do yet). ++ */ ++ if (!test_and_clear_bit(TASKLET_STATE_SCHED, &t->state)) ++ WARN_ON(1); ++again: ++ t->func(t->data); ++ ++ while (!tasklet_tryunlock(t)) { ++ /* ++ * If it got disabled meanwhile, bail out: ++ */ ++ if (atomic_read(&t->count)) ++ goto out_disabled; ++ /* ++ * If it got scheduled meanwhile, re-execute ++ * the tasklet function: ++ */ ++ if (test_and_clear_bit(TASKLET_STATE_SCHED, &t->state)) ++ goto again; ++ if (!--loops) { ++ printk("hm, tasklet state: %08lx\n", t->state); ++ WARN_ON(1); + tasklet_unlock(t); +- continue; ++ break; + } +- tasklet_unlock(t); + } +- +- local_irq_disable(); +- t->next = NULL; +- *tl_head->tail = t; +- tl_head->tail = &t->next; +- __raise_softirq_irqoff(softirq_nr); +- local_irq_enable(); + } + } + +@@ -563,7 +632,7 @@ void tasklet_kill(struct tasklet_struct *t) + + while (test_and_set_bit(TASKLET_STATE_SCHED, &t->state)) { + do { +- yield(); ++ msleep(1); + } while (test_bit(TASKLET_STATE_SCHED, &t->state)); + } + tasklet_unlock_wait(t); +@@ -637,6 +706,23 @@ void __init softirq_init(void) + open_softirq(HI_SOFTIRQ, tasklet_hi_action); + } + ++#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT_FULL) ++void tasklet_unlock_wait(struct tasklet_struct *t) ++{ ++ while (test_bit(TASKLET_STATE_RUN, &(t)->state)) { ++ /* ++ * Hack for now to avoid this busy-loop: ++ */ ++#ifdef CONFIG_PREEMPT_RT_FULL ++ msleep(1); ++#else ++ barrier(); ++#endif ++ } ++} ++EXPORT_SYMBOL(tasklet_unlock_wait); ++#endif ++ + static int ksoftirqd_should_run(unsigned int cpu) + { + return local_softirq_pending(); +-- +2.36.1 + |