Prev: [PATCH 1/1] perf tui: Allow disabling the TUI on a per command basis in ~/.perfconfig
Next: [PATCH 6/5] adjust mm_take_all_locks to anon-vma-root locking
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
>> up_xxxx().
>
> 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.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo(a)vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
 | 
Pages: 1
Prev: [PATCH 1/1] perf tui: Allow disabling the TUI on a per command basis in ~/.perfconfig
Next: [PATCH 6/5] adjust mm_take_all_locks to anon-vma-root locking