On Mon, Dec 16, 2013 at 04:26:36PM +0100, Peter Zijlstra wrote: > 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? It takes care of one problem, but there are others, including rcu_read_unlock() inovking the scheduler to deboost itself. So for the moment, we still need the below patch. Thanx, Paul > --- > 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