Re: [PATCH] Fix x86_64 _spin_lock_irqsave()
From: Suleiman Souhlal
Date: Thu Aug 24 2006 - 08:31:53 EST
Andi Kleen wrote:
On Thursday 24 August 2006 13:04, Suleiman Souhlal wrote:
Andi Kleen wrote:
Edward Falk <efalk@xxxxxxxxxx> writes:
Add spin_lock_string_flags and _raw_spin_lock_flags() to
asm-x86_64/spinlock.h so that _spin_lock_irqsave() has the same
semantics on x86_64 as it does on i386 and does *not* have interrupts
disabled while it is waiting for the lock.
Did it fix anything for you?
I think this was to work around the fact that some buggy drivers try to
grab spinlocks without disabling interrupts when they should, which
would cause deadlocks when trying to rendez-vous every cpu via IPIs.
That doesn't help them at all because they could then deadlock later.
If the driver uses spin_lock() when it knows that the hardware won't
generate the interrupt that would need to be masked, and
spin_lock_irqsave() elsewhere, there shouldn't be any deadlocks unless
IPIs are involved.
For example, say a driver uses spin_lock(&driver_lock) in its interrupt
handler and spin_lock_irqsave(&driver_lock) elsewhere.
Imagine CPU1 is handling a such a interrupt while CPU2 is trying to send
a packet (for example).
In a regular situation, CPU1 shouldn't be interrupted by anything
needing driver_lock, and so it should be able to complete and let CPU2
acquire the lock.
Now, if CPU3 is trying to do an IPI rendez-vous, it will interrupt CPU1
and try to interrupt CPU2 too. However, since spin_lock_irqsave() spins
with interrupts disabled, the system will deadlock.
With this patch, IPI rendez-vous shouldn't cause these problems, since
it will let the rendez-vous will be able to complete. Or am I missing
something?
-- Suleiman
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