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Date: Sun, 24 Mar 2002 15:24:05 -0800
From: Andrew Morton <akpm@zip.com.au>
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To: Neil Brown <neilb@cse.unsw.edu.au>
CC: Paul Clements <Paul.Clements@SteelEye.com>, marcelo@conectiva.com.br,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] 2.4.18 raid1 - fix SMP locking/interrupt errors, fix resync 
 counter errors
In-Reply-To: message from Paul Clements on Friday March 22 <15518.22081.287786.88466@notabene.cse.unsw.edu.au>
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Neil Brown wrote:
> 
> ...
> The save/restore versions are only needed if the code might be called
> from interrupt context.

Or if the caller may wish to keep interrupts disabled.

> However the routines where you made this
> change: raid1_grow_buffers, raid1_shrink_buffers, close_sync,
> are only ever called from process context, with interrupts enabled.
> Or am I missing something?

If those functions are always called with interrupts enabled then
no, you're not missing anything ;)

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.

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.

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