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


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