linux-kernel - Re: [PATCH] qrwlock: Fix bug in interrupt handling code

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Date:	Thu, 09 Apr 2015 18:09:55 -0400
From:	Waiman Long <waiman.long@...com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...nel.org>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [PATCH] qrwlock: Fix bug in interrupt handling code

On 04/09/2015 04:14 PM, Peter Zijlstra wrote:
> On Thu, Apr 09, 2015 at 04:07:55PM -0400, Waiman Long wrote:
>> The qrwlock is fair in the process context, but becoming unfair when
>> in the interrupt context to support use cases like the tasklist_lock.
>> However, the unfair code in the interrupt context has problem that
>> may cause deadlock.
>>
>> The fast path increments the reader count. In the interrupt context,
>> the reader in the slowpath will wait until the writer release the
>> lock. However, if other readers have the lock and the writer is just
>> in the waiting mode. It will never get the write lock because the
>> that interrupt context reader has increment the count. This will
>> cause deadlock.
>>
>> This patch fixes this problem by checking the state of the
>> reader/writer count retrieved at the fast path. If the writer
>> is in waiting mode, the reader will get the lock immediately and
>> return. Otherwise, it will wait until the writer release the lock
>> like before.
> A little word on how you found this issue would be nice.

It is not found by testing. I didn't see any problem with a running 
Linux kernel so far.

I am in the process of trying to make the qrwlock lock unfair in virt. 
When I inspect the code, I found out that the interrupt code didn't look 
right. That is why I send out a patch to fix that.

Regards,
Longman
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