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.

Reported by:	avg, kib, pho
Reviewed by:	kib
Tested by:	pho

1 files changed, 5 insertions, 1 deletions

diff --git a/sys/kern/kern_sx.c b/sys/kern/kern_sx.c
index 2d777a2..37e5d03 100644
--- a/sys/kern/kern_sx.c
+++ b/sys/kern/kern_sx.c
@@ -705,8 +705,12 @@ _sx_xunlock_hard(struct sx *sx, uintptr_t tid, const char *file, int line)
 	 * ideal.  It gives precedence to shared waiters if they are
 	 * present.  For this condition, we have to preserve the
 	 * state of the exclusive waiters flag.
+	 * If interruptible sleeps left the shared queue empty avoid a
+	 * starvation for the threads sleeping on the exclusive queue by giving
+	 * them precedence and cleaning up the shared waiters bit anyway.
 	 */
-	if (sx->sx_lock & SX_LOCK_SHARED_WAITERS) {
+	if ((sx->sx_lock & SX_LOCK_SHARED_WAITERS) != 0 &&
+	    sleepq_sleepcnt(&sx->lock_object, SQ_SHARED_QUEUE) != 0) {
 		queue = SQ_SHARED_QUEUE;
 		x |= (sx->sx_lock & SX_LOCK_EXCLUSIVE_WAITERS);
 	} else