Re: [PATCH] 2.4.18 raid1 - fix SMP locking/interrupt

[Manfred Spraul]

>
> However a bare spin_unlock_irq() in a function means that
> callers which wish to keep interrupts disabled are subtly
> subverted.   We've had bugs from this before.
>
It is trivial to catch such bugs at runtime. I tried it a year ago, and
immediately run into sleep_on() users that legitimately call
spin_lock_irq() with disabled interrupts. Perhaps they are gone now,
I'll retest my patch.

> So the irqrestore functions are much more robust.  I believe
> that they should be the default choice.  The non-restore
> versions should be viewed as a micro-optimised version,
> to be used with caution.  The additional expense of the save/restore
> is quite tiny - 20-30 cycles, perhaps.

OTHO, if a function doesn't work correctly if it's called with disabled
interrupts, then it should not use spin_lock_irqsave() - it's
misleading.
e.g. if it calls kmalloc(GFP_KERNEL), down(), schedule(), etc.

--
    Manfred