linux-kernel - Re: frequent lockups in 3.18rc4

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Message-ID: <20141214031429.GR22149@ZenIV.linux.org.uk>
Date:	Sun, 14 Dec 2014 03:14:29 +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 05:35:17PM -0800, Linus Torvalds wrote:
> On Sat, Dec 13, 2014 at 4:33 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > So does SMP - this_cpu_dec() relies on preemption being disabled.
> 
> No. really. It very much does not. Not on x86, not elsewhere. It's
> part of the whole point of "this_cpu_p()". They are preemption and
> interrupt safe.
> 
> It's the "__this_cpu_op()" ones that need external protection.

Right you are - I really need to get some coffee...  Sorry...

FWIW, do we need to disable interrupts there?  After all, mnt_want_write()
and mnt_drop_write() shouldn't be done from interrupt context - they can
happen via schedule_delayed_work(), but that's it...
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