linux-kernel - Re: [PATCH v2 1/2] locking/qrwlock: Fix bug in interrupt handling code

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Message-ID: <557BA07F.8060501@hp.com>
Date:	Fri, 12 Jun 2015 23:16:15 -0400
From:	Waiman Long <waiman.long@...com>
To:	Will Deacon <will.deacon@....com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [PATCH v2 1/2] locking/qrwlock: Fix bug in interrupt handling
 code

On 06/11/2015 10:21 AM, Will Deacon wrote:
> Hi Waiman,
>
> On Tue, Jun 09, 2015 at 04:19:12PM +0100, 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.
> I'm probably just being thick here, but I'm struggling to understand the
> deadlock case.
>
> If a reader enters the slowpath in interrupt context, we spin while
> (cnts&  _QW_WMASK) == _QW_LOCKED. Consequently, if there is a writer in
> the waiting state, that won't hold up the reader and so forward progress
> is ensured. When the reader unlocks, the reader count is decremented and
> the writer can take the lock.
>
> The only problematic case I can think of is if you had a steady stream of
> readers in interrupt context, but that doesn't seem likely (and I don't
> think this patch deals with that anyway).
>
> What am I missing?
>
> Will

You are right. It was my mistake. I misread my own code. I should have a 
comment to clarify that. I will send out a revised one next week.

Cheers,
Longman
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