xen-blkback: workaround compiler bug in gcc 4.1

The code generat with gcc (GCC) 4.1.2 20080704 (Red Hat 4.1.2-54)
creates an unbound loop for the second foreach_grant_safe loop in
purge_persistent_gnt.

The workaround is to avoid having this second loop and instead
perform all the work inside the first loop by adding a new variable,
clean_used, that will be set when all the desired persistent grants
have been removed and we need to iterate over the remaining ones to
remove the WAS_ACTIVE flag.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reported-by: Tom O'Neill <toneill@vmem.com>
Reported-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

1 files changed, 14 insertions, 10 deletions

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index ea158fe0c9a4..4662217c61be 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -341,7 +341,7 @@ static void purge_persistent_gnt(struct xen_blkif *blkif)
         struct persistent_gnt *persistent_gnt;
         struct rb_node *n;
         unsigned int num_clean, total;
-        bool scan_used = false;
+        bool scan_used = false, clean_used = false;
         struct rb_root *root;
 
         if (blkif->persistent_gnt_c < xen_blkif_max_pgrants ||
@@ -358,9 +358,8 @@ static void purge_persistent_gnt(struct xen_blkif *blkif)
         num_clean = (xen_blkif_max_pgrants / 100) * LRU_PERCENT_CLEAN;
         num_clean = blkif->persistent_gnt_c - xen_blkif_max_pgrants + num_clean;
         num_clean = min(blkif->persistent_gnt_c, num_clean);
-        if (num_clean >
-            (blkif->persistent_gnt_c -
-            atomic_read(&blkif->persistent_gnt_in_use)))
+        if ((num_clean == 0) ||
+            (num_clean > (blkif->persistent_gnt_c - atomic_read(&blkif->persistent_gnt_in_use))))
                 return;
 
         /*
@@ -383,6 +382,11 @@ purge_list:
                 BUG_ON(persistent_gnt->handle ==
                         BLKBACK_INVALID_HANDLE);
 
+                if (clean_used) {
+                        clear_bit(PERSISTENT_GNT_WAS_ACTIVE, persistent_gnt->flags);
+                        continue;
+                }
+
                 if (test_bit(PERSISTENT_GNT_ACTIVE, persistent_gnt->flags))
                         continue;
                 if (!scan_used &&
@@ -400,18 +404,18 @@ purge_list:
          * grants that were used since last purge in order to cope
          * with the requested num
          */
-        if (!scan_used) {
+        if (!scan_used && !clean_used) {
                 pr_debug(DRV_PFX "Still missing %u purged frames\n", num_clean);
                 scan_used = true;
                 goto purge_list;
         }
 finished:
-        /* Remove the "used" flag from all the persistent grants */
-        foreach_grant_safe(persistent_gnt, n, root, node) {
-                BUG_ON(persistent_gnt->handle ==
-                        BLKBACK_INVALID_HANDLE);
-                clear_bit(PERSISTENT_GNT_WAS_ACTIVE, persistent_gnt->flags);
+        if (!clean_used) {
+                pr_debug(DRV_PFX "Finished scanning for grants to clean, removing used flag\n");
+                clean_used = true;
+                goto purge_list;
         }
+
         blkif->persistent_gnt_c -= (total - num_clean);
         blkif->vbd.overflow_max_grants = 0;