Re: [tip:core/rcu] rcu: Break call_rcu() deadlock involving scheduler and perf

[Peter Zijlstra <peterz@infradead.org>]

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@infradead.org>
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@redhat.com>
    Helped-out-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: <stable@kernel.org>
    Signed-off-by: Peter Zijlstra <peterz@infradead.org>
    Signed-off-by: Ingo Molnar <mingo@kernel.org>

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;
 }