linux-kernel - Re: frequent lockups in 3.18rc4

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Message-ID: <CA+55aFw5pb7g+k1EOGj6t2xqibGGUBmZMuA_YAgtyejiv76omg@mail.gmail.com>
Date:	Sat, 13 Dec 2014 15:38:57 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Dave Jones <davej@...hat.com>, 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>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: frequent lockups in 3.18rc4

On Sat, Dec 13, 2014 at 3:35 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> Er...  There's much more direct reason - suppose we get a timer interrupt
> right in the middle of mnt_drop_write().  And lost the timeslice.

So?

You didn't have preemption disabled in *between* the mnt_want_write()
and mnt_drop_write(), there's absolutely no reason to have it inside
of them.

Nobody cares if you get preempted and go away for a while. It's
exactly equivalent to sleeping while doing the write that the pair was
protecting.

Seriously, the preemption disable looks like just voodoo code. It
doesn't protect anything, it doesn't fix anything, it doesn't change
anything. All it does is disable preemption over a random sequence of
code.

                      Linus
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