From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Qian Cai <cai@lca.pw>
Cc: peterz@infradead.org, mingo@redhat.com,
akpm@linux-foundation.org, tglx@linutronix.de,
thgarnie@google.com, tytso@mit.edu, cl@linux.com,
penberg@kernel.org, rientjes@google.com, will@kernel.org,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
keescook@chromium.org
Subject: Re: [PATCH] mm/slub: fix a deadlock in shuffle_freelist()
Date: Mon, 16 Sep 2019 21:51:15 +0200 [thread overview]
Message-ID: <20190916195115.g4hj3j3wstofpsdr@linutronix.de> (raw)
In-Reply-To: <1568642487.5576.152.camel@lca.pw>
On 2019-09-16 10:01:27 [-0400], Qian Cai wrote:
> On Mon, 2019-09-16 at 11:03 +0200, Sebastian Andrzej Siewior wrote:
> > On 2019-09-13 12:27:44 [-0400], Qian Cai wrote:
> > …
> > > Chain exists of:
> > > random_write_wait.lock --> &rq->lock --> batched_entropy_u32.lock
> > >
> > > Possible unsafe locking scenario:
> > >
> > > CPU0 CPU1
> > > ---- ----
> > > lock(batched_entropy_u32.lock);
> > > lock(&rq->lock);
> > > lock(batched_entropy_u32.lock);
> > > lock(random_write_wait.lock);
> >
> > would this deadlock still occur if lockdep knew that
> > batched_entropy_u32.lock on CPU0 could be acquired at the same time
> > as CPU1 acquired its batched_entropy_u32.lock?
>
> I suppose that might fix it too if it can teach the lockdep the trick, but it
> would be better if there is a patch if you have something in mind that could be
> tested to make sure.
get_random_bytes() is heavier than get_random_int() so I would prefer to
avoid its usage to fix what looks like a false positive report from
lockdep.
But no, I don't have a patch sitting around. A lock in per-CPU memory
could lead to the scenario mentioned above if the lock could be obtained
cross-CPU it just isn't so in that case. So I don't think it is that
simple.
Sebastian
next prev parent reply other threads:[~2019-09-16 19:51 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-13 16:27 [PATCH] mm/slub: fix a deadlock in shuffle_freelist() Qian Cai
2019-09-16 9:03 ` Sebastian Andrzej Siewior
2019-09-16 14:01 ` Qian Cai
2019-09-16 19:51 ` Sebastian Andrzej Siewior [this message]
2019-09-16 21:31 ` Qian Cai
2019-09-17 7:16 ` Sebastian Andrzej Siewior
2019-09-18 19:59 ` Qian Cai
2019-09-25 9:31 ` Peter Zijlstra
2019-09-25 15:18 ` Qian Cai
2019-09-25 16:45 ` Peter Zijlstra
2019-09-26 12:29 ` Qian Cai
2019-10-01 9:18 ` [PATCH] sched: Avoid spurious lock dependencies Peter Zijlstra
2019-10-01 10:01 ` Valentin Schneider
2019-10-01 11:22 ` Qian Cai
2019-10-01 11:36 ` Srikar Dronamraju
2019-10-01 13:44 ` Peter Zijlstra
2019-10-29 11:10 ` Qian Cai
2019-10-29 12:44 ` Peter Zijlstra
2019-11-12 0:54 ` Qian Cai
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