linux-kernel - Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by WRITE

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Date:	Wed, 18 May 2016 14:29:42 -0400
From:	Waiman Long <waiman.long@....com>
To:	Jason Low <jason.low2@....com>
CC:	Davidlohr Bueso <dave@...olabs.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
	Dave Chinner <david@...morbit.com>,
	Peter Hurley <peter@...leysoftware.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>, <jason.low2@...com>
Subject: Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by
 WRITE_ONCE

On 05/18/2016 01:21 PM, Jason Low wrote:
> On Wed, 2016-05-18 at 07:04 -0700, Davidlohr Bueso wrote:
>> On Tue, 17 May 2016, Waiman Long wrote:
>>
>>> Without using WRITE_ONCE(), the compiler can potentially break a
>>> write into multiple smaller ones (store tearing). So a read from the
>>> same data by another task concurrently may return a partial result.
>>> This can result in a kernel crash if the data is a memory address
>>> that is being dereferenced.
>>>
>>> This patch changes all write to rwsem->owner to use WRITE_ONCE()
>>> to make sure that store tearing will not happen. READ_ONCE() may
>>> not be needed for rwsem->owner as long as the value is only used for
>>> comparison and not dereferencing.
> It might be okay to leave out READ_ONCE() for reading rwsem->owner, but
> couldn't we include it to at least document that we're performing a
> "special" lockless read?
>

Using READ_ONCE() does have a bit of cost as it limits compiler 
optimization. If we changes all access to rwsem->owner to READ_ONCE() 
and WRITE_ONCE(), we may as well change its type to volatile and be done 
with. I am not against doing that, but it feels a bit over-reach for me. 
On the other hand, we may define a do-nothing macro that designates the 
owner as a special variable for documentation purpose, but don't need 
protection at that particular call site.

Cheers,
Longman