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From: Michel Lespinasse on 20 May 2010 18:40
On Wed, May 19, 2010 at 5:25 AM, David Howells <dhowells(a)redhat.com> wrote:
> Michel Lespinasse <walken(a)google.com> wrote:
>> ... When there are waiter threads on a rwsem and the spinlock is held, other
>> threads can only increment the active count by trying to grab the rwsem in
> That's not true. �A thread attempting to get an rwsem by issuing a down_read()
> or down_write() will also unconditionally increment the active count before it
> considers calling out to the slow path.
> Maybe what you mean is that other threads wanting to do a wake up can only
> increase the active count for the processes being woken up whilst holding the
> rwsem's spinlock.
Damn, this was a pretty bad commit comment - I wrote up_xxxx instead
of down_xxxx here.
down_xxxx can increase the active count but it will then take the
down_failed path and wait to acquire the spinlock - so we don't need
to worry about it. Other code paths can't even increase the active
count, since they don't hold the spinlock.
(I got mixed up due to a legitimate mention of the up_xxxx path higher
up. Semaphore operation names confuse me with the active count going
up in down() and down in up() - I suppose it made more sense in the
era of mechanical train semaphores :)
>> + � � /* If we come here from up_xxxx(), another thread might have reached
>> + � � �* rwsem_down_failed_common() before we acquired the spinlock and
>> + � � �* woken up an active locker.
> Do you mean a waiter rather than an active locker? �If a process is still
> registering activity on the rwsem, then it can't be woken yet.
Yes, I meant a waiter (but since it got woken it's now an active locker).
> Apart from that, the patch looks fine. �That's all comment/description fixes.
Thanks. Updated the comments. And I thought I could speak english... :)
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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