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)
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.
> 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.
[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.]
Nigel Gamble nigel@nrg.org
Mountain View, CA, USA. http://www.nrg.org/
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