linux-kernel - Re: [PATCH] infiniband: fix a subtle race condition

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Message-ID: <20180614070108.GD18426@mtr-leonro.mtl.com>
Date:   Thu, 14 Jun 2018 10:01:08 +0300
From:   Leon Romanovsky <leon@...nel.org>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-rdma@...r.kernel.org,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...lanox.com>
Subject: Re: [PATCH] infiniband: fix a subtle race condition

On Wed, Jun 13, 2018 at 11:21:54PM -0700, Cong Wang wrote:
> On Wed, Jun 13, 2018 at 10:34 PM, Leon Romanovsky <leon@...nel.org> wrote:
> >
> > Hi Cong,
> >
> > If the compiler optimizes the first line (mutex_lock) as you wrote,
> > it will reuse "f" for the second line (mutex_unlock) too.
>
> Nope, check the assembly if you don't trust me, at least
> my compiler always fetches ctx->file without this patch.
>
> I can show you the assembly code tomorrow (too late to
> access my dev machine now).

I trust you, so don't need to check it however wanted to emphasize
that your solution is compiler specific and not universally true.

>
>
> >
> > You need to ensure that ucma_modify_id() doesn't run in parallel to
> > anything that uses "ctx->file" directly and indirectly.
> >
>
> Talk is easy, show me the code. :) I knew there is probably
> some other race with this code even after my patch, possibly with
> ->close() for example, but for this specific unlock warning, this patch
> is sufficient. I can't solve all the races in one patch.

We do prefer complete solution once the problem is fully understood.

It looks like you are one step away from final patch. It will be conversion
of mutex to be rwlock and separating between read (almost in all places)
and write (in ucma_migrate_id) paths.

Thanks

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