On Mon, Dec 16, 2013 at 07:19:22AM -0800, tip-bot for Paul E. McKenney wrote: > The underlying problem is that perf is invoking call_rcu() with the > scheduler locks held, but in NOCB mode, call_rcu() will with high > probability invoke the scheduler -- which just might want to use its > locks. The reason that call_rcu() needs to invoke the scheduler is > to wake up the corresponding rcuo callback-offload kthread, which > does the job of starting up a grace period and invoking the callbacks > afterwards. > > One solution (championed on a related problem by Lai Jiangshan) is to > simply defer the wakeup to some point where scheduler locks are no longer > held. Since we don't want to unnecessarily incur the cost of such > deferral, the task before us is threefold: > > 1. Determine when it is likely that a relevant scheduler lock is held. > > 2. Defer the wakeup in such cases. > > 3. Ensure that all deferred wakeups eventually happen, preferably > sooner rather than later. > > We use irqs_disabled_flags() as a proxy for relevant scheduler locks > being held. This works because the relevant locks are always acquired > with interrupts disabled. We may defer more often than needed, but that > is at least safe. This would also allow us to do away with things like the below patch, right? --- commit 058ebd0eba3aff16b144eabf4510ed9510e1416e Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Date: Fri Jul 12 11:08:33 2013 +0200 perf: Fix perf_lock_task_context() vs RCU Jiri managed to trigger this warning: [] ====================================================== [] [ INFO: possible circular locking dependency detected ] [] 3.10.0+ #228 Tainted: G W [] ------------------------------------------------------- [] p/6613 is trying to acquire lock: [] (rcu_node_0){..-...}, at: [<ffffffff810ca797>] rcu_read_unlock_special+0xa7/0x250 [] [] but task is already holding lock: [] (&ctx->lock){-.-...}, at: [<ffffffff810f2879>] perf_lock_task_context+0xd9/0x2c0 [] [] which lock already depends on the new lock. [] [] the existing dependency chain (in reverse order) is: [] [] -> #4 (&ctx->lock){-.-...}: [] -> #3 (&rq->lock){-.-.-.}: [] -> #2 (&p->pi_lock){-.-.-.}: [] -> #1 (&rnp->nocb_gp_wq[1]){......}: [] -> #0 (rcu_node_0){..-...}: Paul was quick to explain that due to preemptible RCU we cannot call rcu_read_unlock() while holding scheduler (or nested) locks when part of the read side critical section was preemptible. Therefore solve it by making the entire RCU read side non-preemptible. Also pull out the retry from under the non-preempt to play nice with RT. Reported-by: Jiri Olsa <jolsa@xxxxxxxxxx> Helped-out-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> Cc: <stable@xxxxxxxxxx> Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> diff --git a/kernel/events/core.c b/kernel/events/core.c index ef5e7cc686e3..eba8fb5834ae 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -947,8 +947,18 @@ perf_lock_task_context(struct task_struct *task, int ctxn, unsigned long *flags) { struct perf_event_context *ctx; - rcu_read_lock(); retry: + /* + * One of the few rules of preemptible RCU is that one cannot do + * rcu_read_unlock() while holding a scheduler (or nested) lock when + * part of the read side critical section was preemptible -- see + * rcu_read_unlock_special(). + * + * Since ctx->lock nests under rq->lock we must ensure the entire read + * side critical section is non-preemptible. + */ + preempt_disable(); + rcu_read_lock(); ctx = rcu_dereference(task->perf_event_ctxp[ctxn]); if (ctx) { /* @@ -964,6 +974,8 @@ perf_lock_task_context(struct task_struct *task, int ctxn, unsigned long *flags) raw_spin_lock_irqsave(&ctx->lock, *flags); if (ctx != rcu_dereference(task->perf_event_ctxp[ctxn])) { raw_spin_unlock_irqrestore(&ctx->lock, *flags); + rcu_read_unlock(); + preempt_enable(); goto retry; } @@ -973,6 +985,7 @@ perf_lock_task_context(struct task_struct *task, int ctxn, unsigned long *flags) } } rcu_read_unlock(); + preempt_enable(); return ctx; } -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html