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From: Andi Kleen <ak@suse.de>
To: "Satyam Sharma" <satyam.sharma@gmail.com>
Cc: "Jan Glauber" <jan.glauber@de.ibm.com>,
	"Heiko Carstens" <heiko.carstens@de.ibm.com>,
	"David Miller" <davem@davemloft.net>,
	akpm@osdl.org, mingo@elte.hu, schwidefsky@de.ibm.com,
	linux-kernel@vger.kernel.org, "Alan Cox" <alan@redhat.com>
Subject: Re: [patch] i386/x86_64: smp_call_function locking inconsistency
Date: Fri, 8 Jun 2007 21:42:21 +0200	[thread overview]
Message-ID: <200706082142.21449.ak@suse.de> (raw)
In-Reply-To: <a781481a0706070707j301ba886h10f4607fd6a6e8de@mail.gmail.com>

On Thursday 07 June 2007 16:07:04 Satyam Sharma wrote:
> Hi,
> 
> I'm about six months late here(!), but I noticed this bug in
> arch/x86_64/kernel/smp.c while preparing another related
> patch today and then found this thread during Googling ...
> 
> On 2/9/07, Heiko Carstens <heiko.carstens@de.ibm.com> wrote:
> > On i386/x86_64 smp_call_function_single() takes call_lock with
> > spin_lock_bh(). To me this would imply that it is legal to call
> > smp_call_function_single() from softirq context.
> > It's not since smp_call_function() takes call_lock with just
> > spin_lock(). We can easily deadlock:
> >
> > -> [process context]
> > -> smp_call_function()
> > -> spin_lock(&call_lock)
> > -> IRQ -> do_softirq -> tasklet
> > -> [softirq context]
> > -> smp_call_function_single()
> > -> spin_lock_bh(&call_lock)
> > -> dead
> 
> You're absolutely right, and this bug still exists in the latest -git.

bug is definitely too strong a word. It might be unnecessary to disable bhs, 
but I don't see any bug in here as long as you can't show a case where
the smp_call_function() is called from BHs.

There was a patch floating around to use it from sysrq to display state
of all CPUs (and sysrq is softirq), but I don't think that ever
made it mainline.

And smp_call_function() can be called from panic which can violate
quite some assumptions, but some deadlock possibility there is ok.

I also don't like making it soft/hard irq save because that would
make it much more intrusive to the machine for no good reason
(e.g. slab can call it quite often in some cases)

The _bh should be probably just removed and possibly a WARN_ON added.

-Andi

  parent reply	other threads:[~2007-06-08 19:44 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2007-02-08 20:32 Heiko Carstens
2007-02-08 20:43 ` David Miller
2007-02-09  8:42   ` Heiko Carstens
2007-02-09 12:57     ` Jan Glauber
2007-06-07 14:07       ` Satyam Sharma
2007-06-07 16:27         ` Heiko Carstens
2007-06-07 16:54           ` Satyam Sharma
2007-06-07 17:18             ` Satyam Sharma
2007-06-07 17:22               ` Avi Kivity
2007-06-07 17:33                 ` Satyam Sharma
2007-06-10  7:38                   ` Avi Kivity
2007-06-08 19:43             ` Andi Kleen
2007-06-08 19:42         ` Andi Kleen [this message]
2007-02-09  7:40 ` Andi Kleen

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