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>

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index ea158fe0c9a420c87e8fe5c44c984a41b733aa72..4662217c61be0015c800f0c9305c26bb675b9745 100644 (file)
--- 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;