[Xen-devel] [PATCH 1/4] xen-blkback: workaround compiler bug in gcc 4.1

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

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 Monnà <roger.pau@xxxxxxxxxx>
Reported-by: Tom O'Neill <toneill@xxxxxxxx>
Reported-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
---
 drivers/block/xen-blkback/blkback.c |   24 ++++++++++++++----------
 1 files changed, 14 insertions(+), 10 deletions(-)

diff --git a/drivers/block/xen-blkback/blkback.c 
b/drivers/block/xen-blkback/blkback.c
index 4119bcd..d622d86 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;
 
-- 
1.7.7.5 (Apple Git-26)


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