linux-kernel - Re: [PATCH 0/5] Fix improper uses of smp_mb__{before,after}

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Message-ID: <20190430164404.GA2874@andrea>
Date:   Tue, 30 Apr 2019 18:44:04 +0200
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Rob Clark <robdclark@...il.com>, Sean Paul <sean@...rly.run>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Jordan Crouse <jcrouse@...eaurora.org>,
        Jens Axboe <axboe@...nel.dk>, Omar Sandoval <osandov@...com>,
        "Yan, Zheng" <zyan@...hat.com>, Sage Weil <sage@...hat.com>,
        Ilya Dryomov <idryomov@...il.com>,
        Dennis Dalessandro <dennis.dalessandro@...el.com>,
        Mike Marciniszyn <mike.marciniszyn@...el.com>,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>
Subject: Re: [PATCH 0/5] Fix improper uses of smp_mb__{before,after}_atomic()

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