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

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Message-ID: <20190430083409.GD2677@hirez.programming.kicks-ass.net>
Date:   Tue, 30 Apr 2019 10:34:09 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrea Parri <andrea.parri@...rulasolutions.com>
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 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.

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