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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This archive was generated by hypermail 2b29 : Sat Aug 31 2002 - 22:00:31 EST