Document sx_xholder().

share/man/man9/Makefile

@@ -1068,6 +1068,7 @@ MLINKS+=sx.9 sx_assert.9 \
 	sx.9 sx_try_xlock.9 \
 	sx.9 sx_sleep.9 \
 	sx.9 sx_unlock.9 \
+	sx.9 sx_xholder.9 \
 	sx.9 sx_xlock.9 \
 	sx.9 sx_xlocked.9 \
 	sx.9 sx_xunlock.9

share/man/man9/sx.9

@@ -44,6 +44,7 @@
 .Nm sx_try_upgrade ,
 .Nm sx_downgrade ,
 .Nm sx_sleep ,
+.Nm sx_xholder ,
 .Nm sx_xlocked ,
 .Nm sx_assert ,
 .Nm SX_SYSINIT
@@ -78,6 +79,8 @@
 .Fn sx_downgrade "struct sx *sx"
 .Ft int
 .Fn sx_sleep "void *chan" "struct sx *sx" "int priority" "const char *wmesg" "int timo"
+.Ft "struct thread *"
+.Fn sx_xholder "struct sx *sx"
 .Ft int
 .Fn sx_xlocked "struct sx *sx"
 .Pp
@@ -235,6 +238,15 @@ Assert that the current thread does not have a recursed lock on
 .Fa sx .
 .El
 .Pp
+.Fn sx_xholder
+will return a pointer to the thread which currently holds an exclusive lock on
+.Fa sx .
+If no thread holds an exclusive lock on
+.Fa sx ,
+then
+.Dv NULL
+is returned instead.
+.Pp
 .Fn sx_xlocked
 will return non-zero if the current thread holds the exclusive lock;
 otherwise, it will return zero.