linux-kernel - Re: [PATCH v5 2/9] IB/core: Replace semaphore sm

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Date:   Mon, 21 Nov 2016 17:52:27 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Binoy Jayan <binoy.jayan@...aro.org>, linux-rdma@...r.kernel.org,
        Mustafa Ismail <mustafa.ismail@...el.com>,
        Lijun Ou <oulijun@...wei.com>,
        Nicholas Bellinger <nab@...ux-iscsi.org>,
        Leon Romanovsky <leonro@...lanox.com>,
        target-devel@...r.kernel.org,
        Tatyana E Nikolova <tatyana.e.nikolova@...el.com>,
        Doug Ledford <dledford@...hat.com>,
        Jenny Derzhavetz <jennyf@...lanox.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Sean Hefty <sean.hefty@...el.com>,
        Faisal Latif <faisal.latif@...el.com>,
        Hal Rosenstock <hal.rosenstock@...il.com>,
        linux-kernel@...r.kernel.org,
        "Wei Hu(Xavier)" <xavier.huwei@...wei.com>,
        Mark Brown <broonie@...nel.org>,
        Mark Bloch <markb@...lanox.com>,
        Steve Wise <swise@...ngridcomputing.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Bart Van Assche <bart.vanassche@...disk.com>,
        Matan Barak <matanb@...lanox.com>,
        Sagi Grimberg <sagig@...lanox.com>
Subject: Re: [PATCH v5 2/9] IB/core: Replace semaphore sm_sem with an atomic wait

On Monday, November 21, 2016 7:57:53 AM CET Linus Torvalds wrote:
> Don't do this.
> 
> Never ever do your own locking primitives. You will get the memory ordering
> wrong. And even if you get it right, why do it?
> 
> If you want to get rid of semaphores, and replace them with a mutex, that's
> OK. But don't replace them with something more complex like an open coded
> waiting model.

I think a mutex would't work here, since fops->open() and fops->close()
are not called from the same context and lockdep will complain
about that.

Version of the series had replaced the semaphore with a completion
here, which worked correctly, but one reviewer suggested using
the wait_event() instead since it's confusing to have a completion
starting out in 'completed' state.

	Arnd