linux-kernel - Re: frequent lockups in 3.18rc4

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Message-ID: <20141213233918.GO22149@ZenIV.linux.org.uk>
Date:	Sat, 13 Dec 2014 23:39:18 +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 02:59:43PM -0800, Linus Torvalds wrote:
> Side note: I think I've found a real potential lockup bug in
> fs/namespace.c, but afaik it could only trigger with the RT patches.

> Except it won't with the RT patches, I guess. So it looks like you could have:\
> 
>  - mnt_make_readonly() sets that bit
>  - gets preempted with the RT patches
>  - we run mnt_want_write() on all CPU's, which disables preemption and
> waits for the bit to be cleared
>  - nothing happens.
> 
> This is clearly not what happens in your lockup, but it does seem to
> be a potential issue for the RT kernel.
> 
> Added Al and Thomas to the cc, for fs/namespace.c and RT kernel
> respectively. Maybe the RT patches already fix this, I didn't actually
> check.

I agree that it's a thing to keep in mind on the RT side of things, but
IMO it belongs in RT patches...
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