linux-kernel - Re: [PATCH 1/4] locking/rwsem: Avoid stale ->count for rwsem_down_write

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Message-ID: <20160509142923.GA29630@linux-uzut.site>
Date:	Mon, 9 May 2016 07:29:23 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	mingo@...nel.org, peterz@...radead.org, tglx@...utronix.de,
	Waiman.Long@....com, jason.low2@...com,
	linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 1/4] locking/rwsem: Avoid stale ->count for
 rwsem_down_write_failed()

On Sun, 08 May 2016, Peter Hurley wrote:

>On 05/08/2016 09:56 PM, Davidlohr Bueso wrote:
>> The field is obviously updated w.o the lock and needs a READ_ONCE
>> while waiting for lock holder(s) to go away, just like we do with
>> all other ->count accesses.
>
>This isn't actually fixing a bug because it's passed through
>several full barriers which will force reloading from sem->count.

Yes.

>
>I think the patch is ok if you want it just for consistency anyway,
>but please change $subject and changelog.

Yeah, I wasn't actually concerned about a specific bug, it was more
just for documentation and consistency. This code has been like this
for ever, but it would still be good to have the READ_ONCE. It is
slightly suboptimal to use, but I do not see any real impact either.

Thanks,
Davidlohr