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From: Andrea Parri <andrea.parri@amarulasolutions.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org,
	"Paul E. McKenney" <paulmck@linux.ibm.com>,
	Rob Clark <robdclark@gmail.com>, Sean Paul <sean@poorly.run>,
	David Airlie <airlied@linux.ie>, Daniel Vetter <daniel@ffwll.ch>,
	Jordan Crouse <jcrouse@codeaurora.org>,
	Jens Axboe <axboe@kernel.dk>, Omar Sandoval <osandov@fb.com>,
	"Yan, Zheng" <zyan@redhat.com>, Sage Weil <sage@redhat.com>,
	Ilya Dryomov <idryomov@gmail.com>,
	Dennis Dalessandro <dennis.dalessandro@intel.com>,
	Mike Marciniszyn <mike.marciniszyn@intel.com>,
	Doug Ledford <dledford@redhat.com>,
	Jason Gunthorpe <jgg@ziepe.ca>
Subject: Re: [PATCH 0/5] Fix improper uses of smp_mb__{before,after}_atomic()
Date: Tue, 30 Apr 2019 18:44:04 +0200	[thread overview]
Message-ID: <20190430164404.GA2874@andrea> (raw)
In-Reply-To: <20190430083409.GD2677@hirez.programming.kicks-ass.net>

On Tue, Apr 30, 2019 at 10:34:09AM +0200, Peter Zijlstra wrote:
> On Mon, Apr 29, 2019 at 10:14:56PM +0200, Andrea Parri wrote:
> > Hello!
> > 
> > A relatively common misuse of these barriers is to apply these to
> > operations which are not read-modify-write operations, such as
> > atomic_set() and atomic_read(); examples were discussed in [1].
> > 
> > This series attempts to fix those uses by (conservatively) replacing
> > the smp_mb__{before,after}_atomic() barriers with full memory barriers.
> 
> I don't think blindly doing this replacement makes the code any better;
> much of the code you found is just straight up dodgy to begin with.
> 
> I think the people should mostly just consider this a bug report.

Bug, misuse, patch, and rfc seem all appropriate to me in this context.


> Also, remember a memory barrier without a coherent comment is most
> likely a bug anyway.

Right.  Hopefully, the people in Cc: will want to shed some light about
this: I know what these smp_mb__{before,after}_atomic() can not do, but
I can only guess (I won't!) what they are supposed to accomplish (e.g.,
which mem. accesses are being ordered, what are the matching barriers);
maybe this can also justify the "conservative" approach presented here.

  Andrea

      reply	other threads:[~2019-04-30 16:44 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-29 20:14 Andrea Parri
2019-04-29 20:14 ` [PATCH 1/5] drm/msm: " Andrea Parri
2019-05-09 20:19   ` Andrea Parri
2019-04-29 20:14 ` [PATCH 2/5] bio: fix improper use of smp_mb__before_atomic() Andrea Parri
2019-04-30  8:21   ` Peter Zijlstra
2019-05-09 20:23   ` Andrea Parri
2019-05-10  3:40   ` Ming Lei
2019-04-29 20:14 ` [PATCH 3/5] sbitmap: " Andrea Parri
2019-05-09 20:26   ` Andrea Parri
2019-05-10  3:41   ` Ming Lei
2019-05-10  6:27     ` Andrea Parri
2019-04-29 20:15 ` [PATCH 4/5] ceph: " Andrea Parri
2019-04-30  8:23   ` Peter Zijlstra
2019-04-30  9:08     ` Yan, Zheng
2019-05-09 20:55       ` Andrea Parri
2019-05-13 13:04         ` Yan, Zheng
2019-05-13 22:47           ` Andrea Parri
2019-04-29 20:15 ` [PATCH 5/5] IB/hfi1: Fix improper uses " Andrea Parri
2019-04-29 21:24   ` Ruhl, Michael J
2019-04-29 23:16     ` Andrea Parri
2019-05-09 21:12       ` Andrea Parri
2019-05-14 12:32         ` Dennis Dalessandro
2019-05-14 14:48           ` Andrea Parri
2019-04-30  8:28   ` Peter Zijlstra
2019-04-30  8:34 ` [PATCH 0/5] Fix improper uses of smp_mb__{before,after}_atomic() Peter Zijlstra
2019-04-30 16:44   ` Andrea Parri [this message]

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