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SubjectRe: [PATCH] improve spinlock debugging
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On Tue, 2001-12-04 at 17:06, Nigel Gamble wrote:

> > Given this order, couldn't we _always_ not touch the preempt count since
> > irq's are off?
>
> Not with the current spinlock usage in the kernel.
> spin_lock/spin_unlock are used both nested when interrupts are known to
> be disabled (as above) or, more commonly,
>
> spin_lock_irqsave(a, flags)
>
> spin_lock(b)
>
> spin_unlock(b)
>
> spin_unlock_irqrestore(a, flags)
>
> and when interrupts are enabled:
>
> spin_lock(c)
>
> spin_unlock(c)

Right, I meant just the spin_lock_irq case, which would be fine except
for the case where:

spin_lock_irq
spin_unlock
restore_irq

to solve this, we need a spin_unlock_irq_on macro that didn't touch the
preemption count.

> We don't need to preempt count the former but we do the latter, but
> there's no way to tell the difference without a runtime check for
> interrupt state.
>
> In IRIX we changed the name of the former, nested versions to:
>
> spin_lock_irqsave(a, flags)
>
> nested_spin_lock(b)
>
> nested_spin_unlock(b)
>
> spin_unlock_irqrestore(a, flags)
>
> The nested version contained an assertion that interrupts were disabled
> in DEBUG kernels. We wouldn't need to count the nested_spin_lock
> versions.

Ah, another optimization wrt preempt count. This goes further than just
making spin_lock_irqsave not touch the preempt count, in that it also
let's us say (this lock is _always_ nested inside another, we don't need
to touch the preempt count).

It would apply anywhere we grab multiple locks and drop the first one
last.

> > Further, since I doubt we ever see:
> >
> > spin_lock_irq
> > restore_irq
> > spin_unlock
>
> I hope not, since that would be a bug.
>
> > and the common use is:
> >
> > spin_lock_irq
> > spin_unlock_irq
> >
> > Isn't it safe to have spin_lock_irq *never* touch the preempt count?
>
> No, because of
>
> > > spin_lockirq
> > >
> > > spin_unlock
> > >
> > > restore_irq
>
> (which does occasionally occur in the kernel). The spin_unlock is going
> to decrement the count, so the spin_lock_irqsave must increment it. If
> we had, and used, a nested_spin_unlock, we could then have spin_lock_irq
> never touch the preempt count.

Right.

> [And if we could guarantee that all spinlocks we held for only a few 10s
> of microseconds at the most (a big "if"), we could make them all into
> spin_lock_irqs and then we wouldn't need the preempt count at all. This
> is how REAL/IX and IRIX implemented kernel preemption.]

I offered that idea and George convinced me otherwise :)

Robert Love

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