Date: Mon, 14 Dec 2009 18:02:54 +0100 From: Attilio Rao <attilio@freebsd.org> To: John Baldwin <jhb@freebsd.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r200447 - in head: share/man/man9 sys/kern sys/sys Message-ID: <3bbf2fe10912140902m407fa766q3a5e5bb6993723f9@mail.gmail.com> In-Reply-To: <200912141013.32839.jhb@freebsd.org> References: <200912122131.nBCLV71f064304@svn.freebsd.org> <200912141013.32839.jhb@freebsd.org>
next in thread | previous in thread | raw e-mail | index | archive | help
2009/12/14 John Baldwin <jhb@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). Attilio -- Peace can only be achieved by understanding - A. Einstein
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?3bbf2fe10912140902m407fa766q3a5e5bb6993723f9>