MFC r200447,201703,201709-201710:

In current code, threads performing an interruptible sleep
will leave the waiters flag on forcing the owner to do a wakeup even
when the waiter queue is empty.
That operation may lead to a deadlock in the case of doing a fake wakeup
on the "preferred" 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.

Add a sleepqueue interface which does report the actual number of waiters
on a specified queue of a waitchannel and track if at least one sleepfail
waiter is present or not. In presence of this or empty "preferred" queue,
wakeup both waiters queues.

Discussed with:	kib
Tested by:	Pete French <petefrench at ticketswitch dot com>,
		Justin Head <justin at encarnate dot com>

1 files changed, 12 insertions, 1 deletions

diff --git a/share/man/man9/sleepqueue.9 b/share/man/man9/sleepqueue.9
index 144bdc0..d1d17cd 100644
--- a/share/man/man9/sleepqueue.9
+++ b/share/man/man9/sleepqueue.9
@@ -23,7 +23,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd December 12, 2009
+.Dd January 18, 2010
 .Dt SLEEPQUEUE 9
 .Os
 .Sh NAME
@@ -41,6 +41,7 @@
 .Nm sleepq_remove ,
 .Nm sleepq_signal ,
 .Nm sleepq_set_timeout ,
+.Nm sleepq_sleepcnt ,
 .Nm sleepq_timedwait ,
 .Nm sleepq_timedwait_sig ,
 .Nm sleepq_wait ,
@@ -77,6 +78,8 @@
 .Fn sleepq_signal "void *wchan" "int flags" "int pri" "int queue"
 .Ft void
 .Fn sleepq_set_timeout "void *wchan" "int timo"
+.Ft u_int
+.Fn sleepq_sleepcnt "void *wchan" "int queue"
 .Ft int
 .Fn sleepq_timedwait "void *wchan"
 .Ft int
@@ -355,6 +358,14 @@ One possible use is waking up a specific thread from a widely shared sleep
 channel.
 .Pp
 The
+.Fn sleepq_sleepcnt
+function offer a simple way to retrieve the number of threads sleeping for
+the specified
+.Fa queue ,
+given a
+.Fa wchan .
+.Pp
+The
 .Fn sleepq_abort ,
 .Fn sleepq_broadcast ,
 and