Re: [PATCH] Fix x86_64 _spin_lock_irqsave()

[Martin Bligh <mbligh@mbligh.org>]

Andrew Morton wrote:
> On Thu, 24 Aug 2006 13:10:09 +1000
> Nick Piggin <nickpiggin@yahoo.com.au> wrote:
>
>
> > Edward Falk wrote:
> >
> > > 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.
> > >
> > > This fix is courtesy of Michael Davidson
> >
> > So, what's the bug? You shouldn't rely on these semantics anyway
> > because you should never expect to wait for a spinlock for so long
> > (and it may be the case that irqs can't be enabled anyway).
> >
> > BTW. you should be cc'ing Andi Kleen (x86+/-64 maintainer) on
> > this type of stuff.
> >
> > No comments on the merits of adding this feature. I suppose parity
> > with i386 is a good thing, though.
>
>
> We put this into x86 ages ago and Andi ducked the x86_64 patch at the time.
>
> I don't recall any reports about the x86 patch (Zwane?) improving or
> worsening anything.  I guess there are some theoretical interrupt latency
> benefits.

Spinlocks are indeed meant to be held for a short time, but irq
disabling is meant to be shorter.

I think the real question is: what is the justification for disabling
interrupts when spinning for a lock? We should never disable interrupts
unless we have to.

M.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/