linux-kernel - Re: [PATCH 1/10] Add generic helpers for arch IPI function calls

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Date:	Tue, 10 Jun 2008 17:04:47 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	Jens Axboe <jens.axboe@...cle.com>, linux-kernel@...r.kernel.org,
	peterz@...radead.org, npiggin@...e.de, linux-arch@...r.kernel.org,
	jeremy@...p.org, mingo@...e.hu, paulmck@...ux.vnet.ibm.com,
	Russell King <rmk+lkml@....linux.org.uk>
Subject: Re: [PATCH 1/10] Add generic helpers for arch IPI function calls

On Tue, 2008-06-10 at 10:44 -0500, James Bottomley wrote:
> On Tue, 2008-06-10 at 15:51 +0100, Catalin Marinas wrote:
> > I was thinking whether this condition can be removed and allow the
> > smp_call_function*() to be called with IRQs disabled. At a quick look,
> > it seems to be possible if the csd_flag_wait() function calls the IPI
> > handlers directly when the IRQs are disabled (see the patch below).
[...]
> > Please let me know what you think or whether deadlocks are still
> > possible (or any other solution apart from hardware fixes :-)). Thanks.
> 
> I don't see how your proposal fixes the deadlocks.  The problem is that
> on a lot of arch's IPIs are normal interrupts.  If interrupts are
> disabled, you don't see them.

ARM uses normal interrupts for IPIs as well.

> The deadlock scenario is CPU1 enters smp_call_function() with IRQ's
> disabled as CPU2 does the same thing and spins on the call_lock.  Now
> CPU1 is waiting for an ack for its IPI to CPU2, but CPU2 will never see
> the IPI until it enables interrupts.

I can see in the generic IPI patches that the call_function_lock is only
held for list_add_tail_rcu and not while waiting for the other CPU to
complete (both arch_send_call_function_ipi and csd_flag_wait are outside
the locking region).

The patch I posted polls for an incoming IPI in the csd_flag_wait()
function if the interrupts are disabled so that it clears the wait flag
even if it doesn't get the IPI. The disadvantage might be a spurious IPI
(but I can leave with this). If interrupts are enabled, there is no
drawback, apart from a call to irq_disabled().

> One way to mitigate the effects of this is to enable interrupts if the
> architecture code finds the call_lock (x86 implementation) held against
> it, then re-disable before trying to get the lock again.  But really, in
> order to make smp_call_function work in interrupt disabled sections, the
> interrupt handler has to be modified to bar all non-IPI interrupts until
> the critical section is over (otherwise there's no point allowing it
> with disabled interrupts because an smp_call_function becomes a de facto
> interrupt enable again).  If you really want to see how something like
> this works, the voyager code has it (because interrupt disabling in the
> VIC is expensive).  But it's quite a lot of code ...

I still think it can be less complicated that this. I'll look at Paul's
post to get some ideas. As I said, I need this functionality on current
ARM SMP systems, even if it means implementing it separately.

Thanks.

-- 
Catalin

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