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Date: Sat, 13 Dec 2014 15:09:59 -0800
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Subject: Re: frequent lockups in 3.18rc4
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Dave Jones <davej@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Chris Mason <clm@fb.com>, Mike Galbraith <umgwanakikbuti@gmail.com>,
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        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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On Sat, Dec 13, 2014 at 2:59 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> The generic code does that mnt_want_write/mnt_drop_write
> dance adound the call to setxattr, and that in turn does
>
>         while (ACCESS_ONCE(mnt->mnt.mnt_flags) & MNT_WRITE_HOLD)
>                 cpu_relax();
>
> with preemption explicitly disabled.

Btw, I see no reason why mnt_want_write/mnt_drop_write disables
preemption. They don't care, they just care about the ordering of the
write counts and the MNT_WRITE_HOLD bit. It's the code that sets the
bit that should care, afaik. But maybe I'm missing something.

                 Linus