linux-kernel - Re: frequent lockups in 3.18rc4

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Date:	Sun, 14 Dec 2014 00:33:32 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
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 04:14:58PM -0800, Linus Torvalds wrote:
> 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.

So does SMP - this_cpu_dec() relies on preemption being disabled.  On x86
we might get away with that, what with having it compiled into decl %gs:const,
but on generic it turns into
	*raw_cpu_ptr(&pcp) -= 1;
and compiler has every right to turn it into
	p = raw_cpu_ptr(&pcp);
	(*p)--;
again, with no locking.  Lose the timeslice in the middle of that and you
are risking to get a different CPU when you are scheduled again, with
another process doing this_cpu_dec() on your old CPU.  Have fun - two
non-atomic decrements of the same variable by different CPUs in parallel...

We really need preemtion disabled there, UP or no UP.
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