kernel - Fix race in bio_track_wait()

* Remove an improper optimization prior to an atomic cmpset function call.

diff --git a/sys/kern/vfs_bio.c b/sys/kern/vfs_bio.c
index 3f480dc..5f4148c 100644 (file)
--- a/sys/kern/vfs_bio.c
+++ b/sys/kern/vfs_bio.c
@@ -591,15 +591,17 @@ bio_track_wait(struct bio_track *track, int slp_flags, int slp_timo)
        /*
         * Full-on.  Note that the wait flag may only be atomically set if
         * the active count is non-zero.
+        *
+        * NOTE: We cannot optimize active == desired since a wakeup could
+        *       clear active prior to our tsleep_interlock().
         */
        error = 0;
        while ((active = track->bk_active) != 0) {
                desired = active | 0x80000000;
                tsleep_interlock(track, slp_flags);
-               if (active == desired ||
-                   atomic_cmpset_int(&track->bk_active, active, desired)) {
+               if (atomic_cmpset_int(&track->bk_active, active, desired)) {
                        error = tsleep(track, slp_flags | PINTERLOCKED,
-                                      "iowait", slp_timo);
+                                      "trwait", slp_timo);
                        if (error)
                                break;
                }