Messages in this thread | | | Date | Tue, 4 Dec 2001 14:06:51 -0800 (PST) | From | Nigel Gamble <> | Subject | Re: [PATCH] improve spinlock debugging |
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On 4 Dec 2001, Robert Love wrote: > On Tue, 2001-12-04 at 15:30, george anzinger wrote: > > > spin_lockirq > > > > spin_unlock > > > > restore_irq > > 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)
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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