linux-kernel - Re: frequent lockups in 3.18rc4

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Message-ID: <CA+55aFzuRCJDWQcBBET6x1OHThLrtJmnn=hS40tE7hNXfmfm6g@mail.gmail.com>
Date:	Sat, 13 Dec 2014 15:09:59 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Dave Jones <davej@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Chris Mason <clm@...com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Dâniel Fraga <fragabr@...il.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: frequent lockups in 3.18rc4

On Sat, Dec 13, 2014 at 2:59 PM, Linus Torvalds
<torvalds@...ux-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
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