linux-kernel - Re: [PATCH] locking/rwsem: Fix kernel crash when spinning on RWSEM_OWNER

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Date:   Wed, 15 Jan 2020 09:27:16 -0500
From:   Waiman Long <longman@...hat.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] locking/rwsem: Fix kernel crash when spinning on
 RWSEM_OWNER_UNKNOWN

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@....de>
>> Signed-off-by: Waiman Long <longman@...hat.com>
> This survives all the tests that showed the problems with the original
> code:
>
> Tested-by: Christoph Hellwig <hch@....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