gfs2: Fix withdraw race

Function gfs2_withdraw() tries to synchronize concurrent callers by
atomically setting the SDF_WITHDRAWN flag in the first caller, setting
the SDF_WITHDRAW_IN_PROG flag to indicate that a withdraw is in
progress, performing the actual withdraw, and clearing the
SDF_WITHDRAW_IN_PROG flag when done.  All other callers wait for the
SDF_WITHDRAW_IN_PROG flag to be cleared before returning.

This leaves a small window in which callers can find the SDF_WITHDRAWN
flag set before the SDF_WITHDRAW_IN_PROG flag has been set, causing them
to return prematurely, before the withdraw has been completed.

Fix that by setting the SDF_WITHDRAWN and SDF_WITHDRAW_IN_PROG flags
atomically.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

diff --git a/fs/gfs2/util.c b/fs/gfs2/util.c
index 65a3c7b..da29faf 100644 (file)
--- a/fs/gfs2/util.c
+++ b/fs/gfs2/util.c
@@ -323,19 +323,19 @@ int gfs2_withdraw(struct gfs2_sbd *sdp)
        struct lm_lockstruct *ls = &sdp->sd_lockstruct;
        const struct lm_lockops *lm = ls->ls_ops;
 
-       if (sdp->sd_args.ar_errors == GFS2_ERRORS_WITHDRAW &&
-           test_and_set_bit(SDF_WITHDRAWN, &sdp->sd_flags)) {
-               if (!test_bit(SDF_WITHDRAW_IN_PROG, &sdp->sd_flags))
-                       return -1;
-
-               wait_on_bit(&sdp->sd_flags, SDF_WITHDRAW_IN_PROG,
-                           TASK_UNINTERRUPTIBLE);
-               return -1;
-       }
-
-       set_bit(SDF_WITHDRAW_IN_PROG, &sdp->sd_flags);
-
        if (sdp->sd_args.ar_errors == GFS2_ERRORS_WITHDRAW) {
+               unsigned long old = READ_ONCE(sdp->sd_flags), new;
+
+               do {
+                       if (old & BIT(SDF_WITHDRAWN)) {
+                               wait_on_bit(&sdp->sd_flags,
+                                           SDF_WITHDRAW_IN_PROG,
+                                           TASK_UNINTERRUPTIBLE);
+                               return -1;
+                       }
+                       new = old | BIT(SDF_WITHDRAWN) | BIT(SDF_WITHDRAW_IN_PROG);
+               } while (unlikely(!try_cmpxchg(&sdp->sd_flags, &old, new)));
+
                fs_err(sdp, "about to withdraw this file system\n");
                BUG_ON(sdp->sd_args.ar_debug);