sx: Add sx_has_waiters() macro

This macro will return non-zero if there are threads waiting for this
lock; otherwise, it will return zero.

The function assumes (but does not assert) that the caller already holds
the lock and that it is interested in other threads waiting for it to
release the lock.

The motivation to add this is the implementation of
`rwsem_is_contended()` in linuxkpi.

This Linux function indicates the same thing to the caller: if other
threads are waiting for this semaphore.

The amdgpu DRM driver started to use `rwsem_is_contended()` in Linux
6.12.

Reviewed by:	bz, olce
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D56443

Author: dumbbell

share/man/man9/sx.9

@@ -24,7 +24,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
 .\" DAMAGE.
 .\"
-.Dd November 11, 2017
+.Dd Apri 30, 2026
 .Dt SX 9
 .Os
 .Sh NAME
@@ -46,6 +46,7 @@
 .Nm sx_sleep ,
 .Nm sx_xholder ,
 .Nm sx_xlocked ,
+.Nm sx_has_waiters ,
 .Nm sx_assert ,
 .Nm SX_SYSINIT ,
 .Nm SX_SYSINIT_FLAGS
@@ -88,6 +89,8 @@
 .Fn sx_xholder "struct sx *sx"
 .Ft int
 .Fn sx_xlocked "const struct sx *sx"
+.Ft int
+.Fn sx_has_waiters "const struct sx *sx"
 .Pp
 .Cd "options INVARIANTS"
 .Cd "options INVARIANT_SUPPORT"
@@ -268,6 +271,13 @@ is returned instead.
 will return non-zero if the current thread holds the exclusive lock;
 otherwise, it will return zero.
 .Pp
+.Fn sx_has_waiters
+will return non-zero if there are threads waiting for this lock;
+otherwise, it will return zero.
+The function assumes (but does not assert) that the caller already holds the
+lock and that it is interested in other threads waiting for it to release the
+lock.
+.Pp
 For ease of programming,
 .Fn sx_unlock
 is provided as a macro frontend to the respective functions,

sys/compat/linuxkpi/common/include/linux/rwsem.h

@@ -51,6 +51,7 @@ struct rw_semaphore {
 #define	down_read_nested(_rw, _sc)	down_read(_rw)
 #define	init_rwsem(_rw)			linux_init_rwsem(_rw, rwsem_name("lnxrwsem"))
 #define	down_write_nest_lock(sem, _rw)	down_write(_rw)
+#define	rwsem_is_contended(_rw)		sx_has_waiters(&(_rw)->sx);
 
 #ifdef WITNESS_ALL
 /* NOTE: the maximum WITNESS name is 64 chars */

sys/sys/sx.h

@@ -259,6 +259,10 @@ __sx_xunlock(struct sx *sx, struct thread *td, const char *file, int line)
 	(void)0; /* ensure void type for expression */			\
 })
 
+/* Return true if there are threads waiting to acquire this sx lock. */
+#define	sx_has_waiters(sx)						\
+	((SX_READ_VALUE(sx) & SX_LOCK_WAITERS) != 0)
+
 #define	sx_unlock(sx)	sx_unlock_((sx), LOCK_FILE, LOCK_LINE)
 
 #define	sx_sleep(chan, sx, pri, wmesg, timo)				\