On Sat, 3 Jun 2006, Willy Tarreau wrote:
Hi Davide,
On Fri, Jun 02, 2006 at 04:28:25PM -0700, Davide Libenzi wrote:
A few days ago Arjan signaled a lockdep red flag on epoll locks, and
precisely between the epoll's device structure lock (->lock) and the wait
queue head lock (->lock). Like I explained in another email, and directly
to Arjan, this can't happen in reality because of the explicit check at
eventpoll.c:592, that does not allow to drop an epoll fd inside the same
epoll fd. Since lockdep is working on per-structure locks, it will never
be able to know of policies enforced in other parts of the code. It was
decided time ago of having the ability to drop epoll fds inside other
epoll fds, that triggers a very trick wakeup operations (due to possibly
reentrant callback-driven wakeups) handled by the ep_poll_safewake()
function.
While looking again at the code though, I noticed that all the operations
done on the epoll's main structure wait queue head (->wq) are already
protected by the epoll lock (->lock), so that locked-style functions can
be used to manipulate the ->wq member. This makes both a lock-acquire
save, and lockdep happy.
Running totalmess on my dual opteron for a while did not reveal any
problem so far:
http://www.xmailserver.org/totalmess.c
Shouldn't we notice a tiny performance boost by avoiding those useless
locks, or do you consider they are not located in the fast path anyway ?
Well, we take a lock less but I can't say if it'll be measureable. The
test program above is not a performance thing though, just some code to
verify multiple threads doing waits on the same epoll fd.
- Davide
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