From: Waiman Long <longman@redhat.com>
To: Christoph Hellwig <hch@lst.de>
Cc: Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.com>,
linux-kernel@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [PATCH] locking/rwsem: Fix kernel crash when spinning on RWSEM_OWNER_UNKNOWN
Date: Wed, 15 Jan 2020 09:27:16 -0500 [thread overview]
Message-ID: <021830af-fd89-50e5-ad26-6061e5abdce1@redhat.com> (raw)
In-Reply-To: <20200115065055.GA21219@lst.de>
On 1/15/20 1:50 AM, Christoph Hellwig wrote:
> On Tue, Jan 14, 2020 at 02:03:03PM -0500, Waiman Long wrote:
>> The commit 91d2a812dfb9 ("locking/rwsem: Make handoff writer
>> optimistically spin on owner") will allow a recently woken up waiting
>> writer to spin on the owner. Unfortunately, if the owner happens to be
>> RWSEM_OWNER_UNKNOWN, the code will incorrectly spin on it leading to a
>> kernel crash. This is fixed by passing the proper non-spinnable bits
>> to rwsem_spin_on_owner() so that RWSEM_OWNER_UNKNOWN will be treated
>> as a non-spinnable target.
>>
>> Fixes: 91d2a812dfb9 ("locking/rwsem: Make handoff writer optimistically spin on owner")
>>
>> Reported-by: Christoph Hellwig <hch@lst.de>
>> Signed-off-by: Waiman Long <longman@redhat.com>
> This survives all the tests that showed the problems with the original
> code:
>
> Tested-by: Christoph Hellwig <hch@lst.de>
>
>> if ((wstate == WRITER_HANDOFF) &&
>> - (rwsem_spin_on_owner(sem, 0) == OWNER_NULL))
>> + rwsem_spin_on_owner(sem, RWSEM_NONSPINNABLE) == OWNER_NULL)
> Nit: the inner braces in the first half of the conditional aren't required
> either.
I typically over-parenthesize the code to make it easier to read as we
don't need to think too much about operator precedence to see if it is
doing the right thing. I remove the 2nd parentheses to avoid breaking
the 80-colnum limit.
Cheers,
Longman
next prev parent reply other threads:[~2020-01-15 14:27 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-14 19:03 [PATCH] locking/rwsem: Fix kernel crash when spinning on RWSEM_OWNER_UNKNOWN Waiman Long
2020-01-15 6:50 ` Christoph Hellwig
2020-01-15 14:27 ` Waiman Long [this message]
2020-01-15 15:16 ` David Laight
2020-01-15 15:47 ` Waiman Long
2020-01-15 16:01 ` David Laight
2020-01-15 15:28 ` Waiman Long
2020-01-15 15:37 ` Peter Zijlstra
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