linux-kernel - Re: frequent lockups in 3.18rc4

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20141213233508.GN22149@ZenIV.linux.org.uk>
Date:	Sat, 13 Dec 2014 23:35:09 +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 03:09:59PM -0800, Linus Torvalds wrote:
> On Sat, Dec 13, 2014 at 2:59 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> > The generic code does that mnt_want_write/mnt_drop_write
> > dance adound the call to setxattr, and that in turn does
> >
> >         while (ACCESS_ONCE(mnt->mnt.mnt_flags) & MNT_WRITE_HOLD)
> >                 cpu_relax();
> >
> > with preemption explicitly disabled.
> 
> Btw, I see no reason why mnt_want_write/mnt_drop_write disables
> preemption. They don't care, they just care about the ordering of the
> write counts and the MNT_WRITE_HOLD bit. It's the code that sets the
> bit that should care, afaik. But maybe I'm missing something.

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.
On UP we have mnt->mnt_writers--, with no locks held.  On SMP we have
this_cpu_dec() instead, also without any locks.  You really don't want to
lose the timeslice in the middle of either...
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/