3.4.97-rt121-rc1 stable review patch. If anyone has any objections, please let me know. ------------------ From: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Austin reported a XFS deadlock/stall on RT where scheduled work gets never exececuted and tasks are waiting for each other for ever. The underlying problem is the modification of the RT code to the handling of workers which are about to go to sleep. In mainline a worker thread which goes to sleep wakes an idle worker if there is more work to do. This happens from the guts of the schedule() function. On RT this must be outside and the accessed data structures are not protected against scheduling due to the spinlock to rtmutex conversion. So the naive solution to this was to move the code outside of the scheduler and protect the data structures by the pool lock. That approach turned out to be a little naive as we cannot call into that code when the thread blocks on a lock, as it is not allowed to block on two locks in parallel. So we dont call into the worker wakeup magic when the worker is blocked on a lock, which causes the deadlock/stall observed by Austin and Mike. Looking deeper into that worker code it turns out that the only relevant data structure which needs to be protected is the list of idle workers which can be woken up. So the solution is to protect the list manipulation operations with preempt_enable/disable pairs on RT and call unconditionally into the worker code even when the worker is blocked on a lock. The preemption protection is safe as there is nothing which can fiddle with the list outside of thread context. Reported-and_tested-by: Austin Schuh <austin@xxxxxxxxxxxxxxxx> Reported-and_tested-by: Mike Galbraith <umgwanakikbuti@xxxxxxxxx> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Link: http://vger.kernel.org/r/alpine.DEB.2.10.1406271249510.5170@nanos Cc: Richard Weinberger <richard.weinberger@xxxxxxxxx> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx> Cc: stable-rt@xxxxxxxxxxxxxxx Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx> --- kernel/sched/core.c | 7 +++++-- kernel/workqueue.c | 41 +++++++++++++++++++++++++++++++++++------ 2 files changed, 40 insertions(+), 8 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 2b2863972dd1..5ba55a8b26e0 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -3574,9 +3574,8 @@ need_resched: static inline void sched_submit_work(struct task_struct *tsk) { - if (!tsk->state || tsk_is_pi_blocked(tsk)) + if (!tsk->state) return; - /* * If a worker went to sleep, notify and ask workqueue whether * it wants to wake up a task to maintain concurrency. @@ -3586,6 +3585,10 @@ static inline void sched_submit_work(struct task_struct *tsk) if (tsk->flags & PF_WQ_WORKER && !tsk->saved_state) wq_worker_sleeping(tsk); + + if (tsk_is_pi_blocked(tsk)) + return; + /* * If we are going to sleep and we have plugged IO queued, * make sure to submit it to avoid deadlocks. diff --git a/kernel/workqueue.c b/kernel/workqueue.c index 4d21bfdc1637..653d7fccb762 100644 --- a/kernel/workqueue.c +++ b/kernel/workqueue.c @@ -319,6 +319,31 @@ static inline int __next_wq_cpu(int cpu, const struct cpumask *mask, (cpu) < WORK_CPU_NONE; \ (cpu) = __next_wq_cpu((cpu), cpu_possible_mask, (wq))) +#ifdef CONFIG_PREEMPT_RT_BASE +static inline void rt_lock_idle_list(struct global_cwq *gcwq) +{ + preempt_disable(); +} +static inline void rt_unlock_idle_list(struct global_cwq *gcwq) +{ + preempt_enable(); +} +static inline void sched_lock_idle_list(struct global_cwq *gcwq) { } +static inline void sched_unlock_idle_list(struct global_cwq *gcwq) { } +#else +static inline void rt_lock_idle_list(struct global_cwq *gcwq) { } +static inline void rt_unlock_idle_list(struct global_cwq *gcwq) { } +static inline void sched_lock_idle_list(struct global_cwq *gcwq) +{ + spin_lock_irq(&gcwq->lock); +} +static inline void sched_unlock_idle_list(struct global_cwq *gcwq) +{ + spin_unlock_irq(&gcwq->lock); +} +#endif + + #ifdef CONFIG_DEBUG_OBJECTS_WORK static struct debug_obj_descr work_debug_descr; @@ -650,10 +675,16 @@ static struct worker *first_worker(struct global_cwq *gcwq) */ static void wake_up_worker(struct global_cwq *gcwq) { - struct worker *worker = first_worker(gcwq); + struct worker *worker; + + rt_lock_idle_list(gcwq); + + worker = first_worker(gcwq); if (likely(worker)) wake_up_process(worker->task); + + rt_unlock_idle_list(gcwq); } /** @@ -696,7 +727,6 @@ void wq_worker_sleeping(struct task_struct *task) cpu = smp_processor_id(); gcwq = get_gcwq(cpu); - spin_lock_irq(&gcwq->lock); /* * The counterpart of the following dec_and_test, implied mb, * worklist not empty test sequence is in insert_work(). @@ -704,11 +734,10 @@ void wq_worker_sleeping(struct task_struct *task) */ if (atomic_dec_and_test(get_gcwq_nr_running(cpu)) && !list_empty(&gcwq->worklist)) { - worker = first_worker(gcwq); - if (worker) - wake_up_process(worker->task); + sched_lock_idle_list(gcwq); + wake_up_worker(gcwq); + sched_unlock_idle_list(gcwq); } - spin_unlock_irq(&gcwq->lock); } /** -- 2.0.0 -- To unsubscribe from this list: send the line "unsubscribe stable-rt" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
