svn commit: r200447 - in head: share/man/man9 sys/kern sys/sys
John Baldwin
jhb at freebsd.org
Mon Dec 14 08:46:55 PST 2009
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?
--
John Baldwin
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