linux-kernel - Re: [PATCH v2] blktrace: re-write setting q->blk

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Message-ID: <x49twpallne.fsf@segfault.boston.devel.redhat.com>
Date:	Wed, 28 Oct 2015 17:37:41 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Davidlohr Bueso <dave@...olabs.net>
Cc:	Jens Axboe <axboe@...nel.dk>, Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] blktrace: re-write setting q->blk_trace

Davidlohr Bueso <dave@...olabs.net> writes:

> This is really about simplifying the double xchg patterns into
> a single cmpxchg, with the same logic. Other than the immediate
> cleanup, there are some subtleties this change deals with:
>
> (i) While the load of the old bt is fully ordered wrt everything,
> ie:
>
>       old_bt = xchg(&q->blk_trace, bt);             [barrier]
>       if (old_bt)
> 	     (void) xchg(&q->blk_trace, old_bt);    [barrier]
>
> blk_trace could still be changed between the xchg and the old_bt
> load. Note that this description is merely theoretical and afaict
> very small, but doing everything in a single context with cmpxchg
> closes this potential race.
>
> (ii) Ordering guarantees are obviously kept with cmpxchg.

Hi David,

The patch itself looks ok, but it doesn't seem to apply to a recent
kernel tree.  It appears as though it is white-space damaged.  Would you
mind re-sending it?

Thanks!
Jeff
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