dma-debug: change hash_bucket_find from first-fit to best-fit

Some device drivers map the same physical address multiple times to a
dma address. Without an IOMMU this results in the same dma address being
put into the dma-debug hash multiple times. With a first-fit match in
hash_bucket_find() this function may return the wrong dma_debug_entry.

This can result in false positive warnings. This patch fixes it by
changing the first-fit behavior of hash_bucket_find() into a best-fit
algorithm.

Reported-by: Torsten Kaiser <just.for.lkml@googlemail.com>
Reported-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Cc: lethal@linux-sh.org
Cc: just.for.lkml@googlemail.com
Cc: hancockrwd@gmail.com
Cc: jens.axboe@oracle.com
Cc: bharrosh@panasas.com
Cc: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: <stable@kernel.org>
LKML-Reference: <20090605104132.GE24836@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 39 insertions, 4 deletions

diff --git a/lib/dma-debug.c b/lib/dma-debug.c
index cdd205d6bf7c..8fcc09c91e1b 100644
--- a/lib/dma-debug.c
+++ b/lib/dma-debug.c
@@ -186,15 +186,50 @@ static void put_hash_bucket(struct hash_bucket *bucket,
 static struct dma_debug_entry *hash_bucket_find(struct hash_bucket *bucket,
                                                 struct dma_debug_entry *ref)
 {
-        struct dma_debug_entry *entry;
+        struct dma_debug_entry *entry, *ret = NULL;
+        int matches = 0, match_lvl, last_lvl = 0;
 
         list_for_each_entry(entry, &bucket->list, list) {
-                if ((entry->dev_addr == ref->dev_addr) &&
-                    (entry->dev == ref->dev))
+                if ((entry->dev_addr != ref->dev_addr) ||
+                    (entry->dev != ref->dev))
+                        continue;
+
+                /*
+                 * Some drivers map the same physical address multiple
+                 * times. Without a hardware IOMMU this results in the
+                 * same device addresses being put into the dma-debug
+                 * hash multiple times too. This can result in false
+                 * positives being reported. Therfore we implement a
+                 * best-fit algorithm here which returns the entry from
+                 * the hash which fits best to the reference value
+                 * instead of the first-fit.
+                 */
+                matches += 1;
+                match_lvl = 0;
+                entry->size      == ref->size      ? ++match_lvl : match_lvl;
+                entry->type      == ref->type      ? ++match_lvl : match_lvl;
+                entry->direction == ref->direction ? ++match_lvl : match_lvl;
+
+                if (match_lvl == 3) {
+                        /* perfect-fit - return the result */
                         return entry;
+                } else if (match_lvl > last_lvl) {
+                        /*
+                         * We found an entry that fits better then the
+                         * previous one
+                         */
+                        last_lvl = match_lvl;
+                        ret      = entry;
+                }
         }
 
-        return NULL;
+        /*
+         * If we have multiple matches but no perfect-fit, just return
+         * NULL.
+         */
+        ret = (matches == 1) ? ret : NULL;
+
+        return ret;
 }
 
 /*