Re: [PATCH v2] blktrace: re-write setting q->blk_trace

From: Jeff Moyer <jmoyer@redhat.com>
To: Davidlohr Bueso <dave@stgolabs.net>
Cc: Jens Axboe <axboe@kernel.dk>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ingo Molnar <mingo@redhat.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] blktrace: re-write setting q->blk_trace
Date: Wed, 28 Oct 2015 17:37:41 -0400	[thread overview]
Message-ID: <x49twpallne.fsf@segfault.boston.devel.redhat.com> (raw)
In-Reply-To: <20151027031452.GI27292@linux-uzut.site> (Davidlohr Bueso's message of "Mon, 26 Oct 2015 20:14:52 -0700")

Davidlohr Bueso <dave@stgolabs.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