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SubjectRe: [patch] scheduler fixes, 2.5.32-BK

On Fri, 30 Aug 2002, Ingo Molnar wrote:
>
> it touches the waitqueue spinlock - and the __down() path [ie. the process
> that gets woken up, which has the semaphore on the stack] takes the
> spinlock after waking up. Ie. there's guaranteed synchronization, the
> semaphore will not be 'unused' before the __down() path takes the spinlock
> - ie. after the __up() path releases the spinlock. What am i missing?

So why couldn't this happen? This is what used to happen before, I don't
see that consolidating the spinlock had any impact at all.

CPU #0 CPU #1

down() up()

lock decl (negative)
__down() lock incl
spinlock() __up()
atomic_add_negative()
success - break
spinunlock();
} wake_up()
return - semaphore is now invalid spin_lock()

BOOM!


The fact is, that as long as down() and up() avoid taking the spinlock
_before_ they touch "count", they aren't synchronized.

And we definitely do _not_ want to take the spinlock before we touch
count, since that would make the fast path a lot slower.

What?

Linus

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