From: Hugh Dickins <hughd@google.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: linuxppc-dev@lists.ozlabs.org,
Christoph Lameter <cl@linux-foundation.org>,
linux-kernel@vger.kernel.org,
"Paul E. McKenney" <paul.mckenney@linaro.org>
Subject: Re: linux-next ppc64: RCU mods cause __might_sleep BUGs
Date: Mon, 7 May 2012 14:38:24 -0700 (PDT) [thread overview]
Message-ID: <alpine.LSU.2.00.1205071421130.1920@eggly.anvils> (raw)
In-Reply-To: <20120507185017.GA21152@linux.vnet.ibm.com>
On Mon, 7 May 2012, Paul E. McKenney wrote:
> On Mon, May 07, 2012 at 09:21:54AM -0700, Hugh Dickins wrote:
> >
> > In 70 hours I got six isolated messages like the below (but from
> > different __might_sleep callsites) - where before I'd have flurries
> > of hundreds(?) and freeze within the hour.
> >
> > And the "rcu_nesting" debug line I'd added to the message was different:
> > where before it was showing ffffffff on some tasks and 1 on others i.e.
> > increment or decrement had been applied to the wrong task, these messages
> > now all showed 0s throughout i.e. by the time the message was printed,
> > there was no longer any justification for the message.
> >
> > As if a memory barrier were missing somewhere, perhaps.
>
> These fields should be updated only by the corresponding CPU, so
> if memory barriers are needed, it seems to me that the cross-CPU
> access is the bug, not the lack of a memory barrier.
Yes: the code you added appeared to be using local CPU accesses only
(very much intentionally), and the context switch should already have
provided all the memory barriers needed there.
>
> Ah... Is preemption disabled across the access to RCU's nesting level
> when printing out the message? If not, a preeemption at that point
> could result in the value printed being inaccurate.
Preemption was enabled in the cases I saw. So you're pointing out that
#define rcu_preempt_depth() (__this_cpu_read(rcu_read_lock_nesting))
should have been
#define rcu_preempt_depth() (this_cpu_read(rcu_read_lock_nesting))
to avoid the danger of spurious __might_sleep() warnings.
Yes, I believe you've got it - thanks.
Hugh
next prev parent reply other threads:[~2012-05-07 21:38 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-04-30 22:37 linux-next ppc64: RCU mods cause __might_sleep BUGs Hugh Dickins
2012-04-30 23:14 ` Paul E. McKenney
2012-05-01 0:33 ` Benjamin Herrenschmidt
2012-05-01 5:10 ` Hugh Dickins
2012-05-01 14:22 ` Paul E. McKenney
2012-05-01 21:42 ` Hugh Dickins
2012-05-01 23:25 ` Paul E. McKenney
2012-05-02 20:25 ` Hugh Dickins
2012-05-02 20:49 ` Paul E. McKenney
2012-05-02 21:32 ` Paul E. McKenney
2012-05-02 21:36 ` Paul E. McKenney
2012-05-02 21:20 ` Benjamin Herrenschmidt
2012-05-02 21:54 ` Paul E. McKenney
2012-05-02 22:54 ` Hugh Dickins
2012-05-03 0:14 ` Paul E. McKenney
2012-05-03 0:24 ` Hugh Dickins
2012-05-07 16:21 ` Hugh Dickins
2012-05-07 18:50 ` Paul E. McKenney
2012-05-07 21:38 ` Hugh Dickins [this message]
2012-05-01 13:39 ` Paul E. McKenney
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