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Date:	Sat, 13 Dec 2014 23:47:30 +0000
From:	Al Viro <>
To:	Linus Torvalds <>
Cc:	Dave Jones <>, Chris Mason <>,
	Mike Galbraith <>,
	Ingo Molnar <>,
	Peter Zijlstra <>,
	Dâniel Fraga <>,
	Sasha Levin <>,
	"Paul E. McKenney" <>,
	Linux Kernel Mailing List <>,
	Thomas Gleixner <>
Subject: Re: frequent lockups in 3.18rc4

On Sat, Dec 13, 2014 at 03:38:57PM -0800, Linus Torvalds wrote:
> On Sat, Dec 13, 2014 at 3:35 PM, Al Viro <> 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.

Huh?  Sure, we can enable it after mnt_inc_writers() and disable just prior to
mnt_dec_writers(), but we absolutely *do* need it disabled during either.
Is that what you are talking about?  If so, yes, we can do that.

But that applies only to __mnt_want_write() - __mnt_drop_write() is pure
mnt_dec_writers() and we can't call that one with preemption enabled.
Seriously, look at the mnt_dec_writers():
static inline void mnt_dec_writers(struct mount *mnt)
It's load/modify/store, without any kind of atomicity; get preempted in the
middle of that sequence by another caller of mnt_dec_writers() and obvious bad
things will happen...

Al, really confused by now...
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