svn commit: r200447 - in head: share/man/man9 sys/kern sys/sys
John Baldwin
jhb at freebsd.org
Mon Dec 14 11:49:57 PST 2009
On Monday 14 December 2009 12:02:54 pm Attilio Rao wrote:
> 2009/12/14 John Baldwin <jhb at freebsd.org>:
> > On Saturday 12 December 2009 4:31:07 pm Attilio Rao wrote:
> >> Author: attilio
> >> Date: Sat Dec 12 21:31:07 2009
> >> New Revision: 200447
> >> URL: http://svn.freebsd.org/changeset/base/200447
> >>
> >> Log:
> >> In current code, threads performing an interruptible sleep (on both
> >> sxlock, via the sx_{s, x}lock_sig() interface, or plain lockmgr), will
> >> leave the waiters flag on forcing the owner to do a wakeup even when if
> >> the waiter queue is empty.
> >> That operation may lead to a deadlock in the case of doing a fake wakeup
> >> on the "preferred" (based on the wakeup algorithm) queue while the other
> >> queue has real waiters on it, because nobody is going to wakeup the 2nd
> >> queue waiters and they will sleep indefinitively.
> >>
> >> A similar bug, is present, for lockmgr in the case the waiters are
> >> sleeping with LK_SLEEPFAIL on. In this case, even if the waiters queue
> >> is not empty, the waiters won't progress after being awake but they will
> >> just fail, still not taking care of the 2nd queue waiters (as instead the
> >> lock owned doing the wakeup would expect).
> >>
> >> In order to fix this bug in a cheap way (without adding too much locking
> >> and complicating too much the semantic) add a sleepqueue interface which
> >> does report the actual number of waiters on a specified queue of a
> >> waitchannel (sleepq_sleepcnt()) and use it in order to determine if the
> >> exclusive waiters (or shared waiters) are actually present on the lockmgr
> >> (or sx) before to give them precedence in the wakeup algorithm.
> >> This fix alone, however doesn't solve the LK_SLEEPFAIL bug. In order to
> >> cope with it, add the tracking of how many exclusive LK_SLEEPFAIL waiters
> >> a lockmgr has and if all the waiters on the exclusive waiters queue are
> >> LK_SLEEPFAIL just wake both queues.
> >>
> >> The sleepq_sleepcnt() introduction and ABI breakage require
> >> __FreeBSD_version bumping.
> >
> > Hmm, do you need an actual count of waiters or would a 'sleepq_empty()'
> > (similar to turnstile_empty()) method be sufficient?
>
> I need the count in order to fix properly LK_SLEEPFAIL case (the idea
> is: track exclusive waiters with LK_SLEEPFAIL on; if the number is
> equal to the actual sleepers on the queue then wake up both queues,
> otherwise nobody is going to take care of the shared waiters queue).
Ok.
--
John Baldwin
More information about the svn-src-head
mailing list