On Mon, Jul 08, 2013 at 09:17:42PM +0200, Peter Zijlstra wrote: > Subject: perf: Fix perf_lock_task_context() vs RCU > > Jiri managed to trigger: > > [] ====================================================== > [] [ 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. > > Cc: Paul E. McKenney <paul...@linux.vnet.ibm.com> > Reported-by: Jiri Olsa <jo...@redhat.com> > Signed-off-by: Peter Zijlstra <pet...@infradead.org>
Reviewed-by: Paul E. McKenney <paul...@linux.vnet.ibm.com> > --- > kernel/events/core.c | 15 ++++++++++++++- > 1 file changed, 14 insertions(+), 1 deletion(-) > > --- a/kernel/events/core.c > +++ b/kernel/events/core.c > @@ -947,8 +947,18 @@ perf_lock_task_context(struct task_struc > { > 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_struc > 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_struc > } > } > rcu_read_unlock(); > + preempt_enable(); > return ctx; > } > > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majord...@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/