On Wed, May 20, 2015 at 9:00 AM, Alexander Kabaev <kab...@gmail.com> wrote:
> On Fri, 15 May 2015 13:50:38 +0000 (UTC) > John Baldwin <j...@freebsd.org> wrote: > > > Author: jhb > > Date: Fri May 15 13:50:37 2015 > > New Revision: 282971 > > URL: https://svnweb.freebsd.org/changeset/base/282971 > > > > Log: > > Previously, cv_waiters was only updated by cv_signal or cv_wait. If > > a thread awakened due to a time out, then cv_waiters was not > > decremented. If INT_MAX threads timed out on a cv without an > > intervening cv_broadcast, then cv_waiters could overflow. To fix > > this, have each sleeping thread decrement cv_waiters when it resumes. > > > > Note that previously cv_waiters was protected by the sleepq chain > > lock. However, that lock is not held when threads resume from sleep. > > In addition, the interlock is also not always reacquired after > > resuming (cv_wait_unlock), nor is it always held by callers of > > cv_signal() or cv_broadcast(). Instead, use atomic ops to update > > cv_waiters. Since the sleepq chain lock is still held on every > > increment, it should still be safe to compare cv_waiters against zero > > while holding the lock in the wakeup routines as the only way the > > race should be lost would result in extra calls to sleepq_signal() or > > sleepq_broadcast(). > > Differential Revision: https://reviews.freebsd.org/D2427 > > Reviewed by: benno > > Reported by: benno (wrap of cv_waiters in the field) > > MFC after: 2 weeks > > > > Modified: > > head/sys/kern/kern_condvar.c > > head/sys/sys/condvar.h > > > > This breaks ZFS range locking code, which expects to be able to wakeup > everyone on the condition variable and then free the structure that > contains it. Having woken up threads modify cv_waiters results in a > race that leads to already freed memory to be accessed. > > It is debatable just how correct ZFS code in its expectations, but I > think this commit should probably be reverted until either ZFS is > changed not to expect cv modifiable by waking threads or until > alternative solution is found to the cv_waiters overflow issue fixed by > this commit. > > It isn't clear to me how the zfs_range_unlock() code could know when all the waiters have woken up and updated the CV, and thus it's safe to destroy/free the CV. Would the woken threads ask, "was I the last thread to be woken by this CV" and if so free the struct containing the CV? Obviously such a check would need to ensure that the other threads have completed their updates to the CV. --matt _______________________________________________ svn-src-head@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
