volser: ensure GCTrans transaction walk remains valid

[openafs.git] / src / volser / voltrans.c

diff --git a/src/volser/voltrans.c b/src/volser/voltrans.c
index 158fec6..7a7af3e 100644 (file)
--- a/src/volser/voltrans.c
+++ b/src/volser/voltrans.c
@@ -180,7 +180,6 @@ GCTrans(void)
 
     VTRANS_LOCK;
     for (tt = allTrans; tt; tt = nt) {
-       nt = tt->next;          /* remember in case we zap it */
        if (tt->time + OLDTRANSWARN < now) {
            Log("trans %u on volume %" AFS_VOLID_FMT " %s than %d seconds\n", tt->tid,
                afs_printable_VolumeId_lu(tt->volid),
@@ -206,12 +205,27 @@ GCTrans(void)
 
                VTRANS_UNLOCK;
                VPurgeVolume(&error, tt->volume);
+               /*
+                * While the lock was dropped, tt->next may have changed.
+                * Therefore, defer reading tt->next until _after_ we regain the lock.
+                */
                VTRANS_LOCK;
            }
-
-           DeleteTrans(tt, 0); /* drops refCount or deletes it */
+           nt = tt->next;
+           /*
+            * DeleteTrans() will decrement tt->refCount; if it falls to 0, it will
+            * also delete tt itself.  Therefore, we must read tt->next _before_
+            * calling DeleteTrans().
+            */
+           DeleteTrans(tt, 0);
            GCDeletes++;
+           continue;
        }
+       /*
+        * This path never dropped VTRANS_LOCK or modified the allTrans list.
+        * Therefore, no special care is required to determine the next trans in the chain.
+        */
+       nt = tt->next;
     }
     VTRANS_UNLOCK;
     return 0;