tsleep(9): add global "nowake" channel for threads avoiding wakeup(9)

It would be convenient if there were a channel a thread could sleep on
to indicate they do not want any wakeup(9) broadcasts.  The easiest way
to do this is to add an "int nowake" to kern_synch.c and extern it in
sys/systm.h.  You use it like this:

	#include <sys/systm.h>

	tsleep_nsec(&nowait, ...);

There is now no need to handroll a local dead channel, e.g.

	int chan;

	tsleep_nsec(&chan, ...);

which expands the stack.  Local dead channels will be replaced with
&nowake in later patches.

One possible problem with this "one global channel" approach is sleep
queue congestion.  If you have lots of threads sleeping on &nowake you
might slow down a wakeup(9) on a different channel that hashes into
the same queue.  Unsure how much of problem this actually is, if at all.

NetBSD and FreeBSD have a "pause" interface in the kernel that chooses
a suitable channel automatically.  To keep things simple and avoid
adding a new interface we will start with this global channel.

Discussed with mpi@, claudio@, kettenis@, and deraadt@.

Basically designed by kettenis@, who vetoed my other proposals.

Bugs caught by deraadt@, tb@, and patrick@.

2 files changed, 12 insertions, 2 deletions

diff --git a/sys/kern/kern_synch.c b/sys/kern/kern_synch.c
index c41f35e54ba..e266de8ef5d 100644
--- a/sys/kern/kern_synch.c
+++ b/sys/kern/kern_synch.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: kern_synch.c,v 1.172 2020/12/07 16:55:29 mpi Exp $	*/
+/*	$OpenBSD: kern_synch.c,v 1.173 2020/12/24 01:16:14 cheloha Exp $	*/
 /*	$NetBSD: kern_synch.c,v 1.37 1996/04/22 01:38:37 christos Exp $	*/
 
 /*
@@ -87,6 +87,11 @@ sleep_queue_init(void)
 		TAILQ_INIT(&slpque[i]);
 }
 
+/*
+ * Global sleep channel for threads that do not want to
+ * receive wakeup(9) broadcasts.
+ */
+int nowake;
 
 /*
  * During autoconfiguration or after a panic, a sleep will simply
@@ -119,6 +124,7 @@ tsleep(const volatile void *ident, int priority, const char *wmesg, int timo)
 #endif
 
 	KASSERT((priority & ~(PRIMASK | PCATCH)) == 0);
+	KASSERT(ident != &nowake || ISSET(priority, PCATCH) || timo != 0);
 
 #ifdef MULTIPROCESSOR
 	KASSERT(timo || _kernel_lock_held());
@@ -213,6 +219,7 @@ msleep(const volatile void *ident, struct mutex *mtx, int priority,
 #endif
 
 	KASSERT((priority & ~(PRIMASK | PCATCH | PNORELOCK)) == 0);
+	KASSERT(ident != &nowake || ISSET(priority, PCATCH) || timo != 0);
 	KASSERT(mtx != NULL);
 
 	if (priority & PCATCH)
@@ -301,6 +308,7 @@ rwsleep(const volatile void *ident, struct rwlock *rwl, int priority,
 	int error, status;
 
 	KASSERT((priority & ~(PRIMASK | PCATCH | PNORELOCK)) == 0);
+	KASSERT(ident != &nowake || ISSET(priority, PCATCH) || timo != 0);
 	rw_assert_anylock(rwl);
 	status = rw_status(rwl);
 
diff --git a/sys/sys/systm.h b/sys/sys/systm.h
index c98203833b3..49556975a7d 100644
--- a/sys/sys/systm.h
+++ b/sys/sys/systm.h
@@ -1,4 +1,4 @@
-/*	$OpenBSD: systm.h,v 1.148 2020/08/26 03:29:07 visa Exp $	*/
+/*	$OpenBSD: systm.h,v 1.149 2020/12/24 01:16:14 cheloha Exp $	*/
 /*	$NetBSD: systm.h,v 1.50 1996/06/09 04:55:09 briggs Exp $	*/
 
 /*-
@@ -107,6 +107,8 @@ extern struct vnode *rootvp;	/* vnode equivalent to above */
 extern dev_t swapdev;		/* swapping device */
 extern struct vnode *swapdev_vp;/* vnode equivalent to above */
 
+extern int nowake;		/* dead wakeup(9) channel */
+
 struct proc;
 struct process;
 #define curproc curcpu()->ci_curproc