3 files changed, 16 insertions, 1 deletions

diff --git a/share/man/man9/sx.9 b/share/man/man9/sx.9
index 473a47c27cf4..f96ee4ffccf7 100644
--- a/share/man/man9/sx.9
+++ b/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,
diff --git a/sys/compat/linuxkpi/common/include/linux/rwsem.h b/sys/compat/linuxkpi/common/include/linux/rwsem.h
index b7a800b12e18..43688ab93796 100644
--- a/sys/compat/linuxkpi/common/include/linux/rwsem.h
+++ b/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 */
diff --git a/sys/sys/sx.h b/sys/sys/sx.h
index d28cae9d01e5..dab4fffb6f15 100644
--- a/sys/sys/sx.h
+++ b/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)				\