From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org,
jiangshanlai@gmail.com, dipankar@in.ibm.com,
akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org,
dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com,
oleg@redhat.com, joel@joelfernandes.org
Subject: Re: [PATCH tip/core/rcu 13/22] rcu: Fix grace-period hangs due to race with CPU offline
Date: Wed, 27 Jun 2018 22:13:34 -0700 [thread overview]
Message-ID: <20180628051334.GG3593@linux.vnet.ibm.com> (raw)
In-Reply-To: <20180627175134.GV2494@hirez.programming.kicks-ass.net>
On Wed, Jun 27, 2018 at 07:51:34PM +0200, Peter Zijlstra wrote:
> On Wed, Jun 27, 2018 at 08:57:21AM -0700, Paul E. McKenney wrote:
> > > Another variant, which simply skips the wakeup whever ran on an offline
> > > CPU, relying on the wakeup from rcutree_migrate_callbacks() right after
> > > the CPU really is dead.
> >
> > Cute! ;-)
> >
> > And a much smaller change.
> >
> > However, this means that if someone indirectly and erroneously causes
> > rcu_report_qs_rsp() to be invoked from an offline CPU, the result is an
> > intermittent and difficult-to-debug grace-period hang. A lockdep splat
> > whose stack trace directly implicates the culprit is much better.
>
> How so? We do an unconditional wakeup right after finding the offline
> cpu dead. There is only very limited code between offline being true and
> the CPU reporting in dead.
I am thinking more generally than this particular patch. People
sometimes invoke things from places they shouldn't, for example, the
situation leading to your patch that allows use of RCU much earlier in
the CPU-online process. It is nicer to get a splat in those situations
than a silent hang.
Thanx, Paul
next prev parent reply other threads:[~2018-06-28 5:11 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-06-26 0:20 [PATCH tip/core/rcu 0/22] Grace-period fixes for v4.19 Paul E. McKenney
2018-06-26 17:10 ` [PATCH tip/core/rcu 01/22] rcu: Clean up handling of tasks blocked across full-rcu_node offline Paul E. McKenney
2018-06-26 17:10 ` [PATCH tip/core/rcu 02/22] rcu: Fix an obsolete ->qsmaskinit comment Paul E. McKenney
2018-06-26 17:10 ` [PATCH tip/core/rcu 03/22] rcu: Make rcu_init_new_rnp() stop upon already-set bit Paul E. McKenney
2018-06-26 17:10 ` [PATCH tip/core/rcu 04/22] rcu: Make rcu_report_unblock_qs_rnp() warn on violated preconditions Paul E. McKenney
2018-06-26 17:10 ` [PATCH tip/core/rcu 05/22] rcu: Fix typo and add additional debug Paul E. McKenney
2018-06-26 17:10 ` [PATCH tip/core/rcu 06/22] rcu: Replace smp_wmb() with smp_store_release() for stall check Paul E. McKenney
2018-06-26 17:10 ` [PATCH tip/core/rcu 07/22] rcu: Prevent useless FQS scan after all CPUs have checked in Paul E. McKenney
2018-06-26 17:10 ` [PATCH tip/core/rcu 08/22] rcu: Suppress false-positive offline-CPU lockdep-RCU splat Paul E. McKenney
2018-06-26 17:10 ` [PATCH tip/core/rcu 09/22] rcu: Suppress false-positive preempted-task splats Paul E. McKenney
2018-06-26 17:10 ` [PATCH tip/core/rcu 10/22] rcu: Suppress more involved " Paul E. McKenney
2018-06-26 17:10 ` [PATCH tip/core/rcu 11/22] rcu: Suppress false-positive splats from mid-init task resume Paul E. McKenney
2018-06-26 17:10 ` [PATCH tip/core/rcu 12/22] rcu: Fix grace-period hangs " Paul E. McKenney
2018-06-26 17:10 ` [PATCH tip/core/rcu 13/22] rcu: Fix grace-period hangs due to race with CPU offline Paul E. McKenney
2018-06-26 17:44 ` Peter Zijlstra
2018-06-26 18:19 ` Paul E. McKenney
2018-06-26 19:48 ` Peter Zijlstra
2018-06-26 20:40 ` Paul E. McKenney
2018-06-26 17:51 ` Peter Zijlstra
2018-06-26 18:29 ` Paul E. McKenney
2018-06-26 20:07 ` Peter Zijlstra
2018-06-26 20:26 ` Paul E. McKenney
2018-06-26 20:32 ` Peter Zijlstra
2018-06-26 23:40 ` Paul E. McKenney
2018-06-27 8:33 ` Peter Zijlstra
2018-06-27 15:43 ` Paul E. McKenney
2018-06-27 9:11 ` Peter Zijlstra
2018-06-27 9:46 ` Peter Zijlstra
2018-06-27 15:57 ` Paul E. McKenney
2018-06-27 17:51 ` Peter Zijlstra
2018-06-28 5:13 ` Paul E. McKenney [this message]
2018-06-28 8:26 ` Peter Zijlstra
2018-06-28 12:38 ` Paul E. McKenney
2018-06-28 13:06 ` Peter Zijlstra
2018-06-29 4:30 ` Paul E. McKenney
2018-06-27 15:53 ` Paul E. McKenney
2018-06-26 17:10 ` [PATCH tip/core/rcu 14/22] rcu: Add RCU-preempt check for waiting on newly onlined CPU Paul E. McKenney
2018-06-26 17:10 ` [PATCH tip/core/rcu 15/22] rcu: Move grace-period pre-init delay after pre-init Paul E. McKenney
2018-06-26 17:10 ` [PATCH tip/core/rcu 16/22] rcu: Remove failsafe check for lost quiescent state Paul E. McKenney
2018-06-26 17:10 ` [PATCH tip/core/rcu 17/22] rcutorture: Change units of onoff_interval to jiffies Paul E. McKenney
2018-06-26 17:10 ` [PATCH tip/core/rcu 18/22] rcu: Remove CPU-hotplug failsafe from force-quiescent-state code path Paul E. McKenney
2018-06-26 17:10 ` [PATCH tip/core/rcu 19/22] rcu: Add up-tree information to dump_blkd_tasks() diagnostics Paul E. McKenney
2018-06-26 17:10 ` [PATCH tip/core/rcu 20/22] rcu: Add CPU online/offline state to dump_blkd_tasks() Paul E. McKenney
2018-06-26 17:10 ` [PATCH tip/core/rcu 21/22] rcu: Record ->gp_state for both phases of grace-period initialization Paul E. McKenney
2018-06-26 17:10 ` [PATCH tip/core/rcu 22/22] rcu: Add diagnostics for offline CPUs failing to report QS Paul E. McKenney
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