linux-kernel - Re: frequent lockups in 3.18rc4

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Date:	Sat, 13 Dec 2014 16:14:58 -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 3:47 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> static inline void mnt_dec_writers(struct mount *mnt)
> {
> #ifdef CONFIG_SMP
>         this_cpu_dec(mnt->mnt_pcp->mnt_writers);
> #else
>         mnt->mnt_writers--;
> #endif
> }
> 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...

Ugh, yes ok, the UP case needs it for the actual counter itself. Ugh.
What an ugly mess. I'd rather have the preemption disable where it is
actually *needed*, in that function itself for the UP case (or just
make it "atomic_t", which would likely be better still.

                     Linus
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