[PATCH 1/1] UBUNTU: SAUCE: replace gfs2_bitfit with upstream version to prevent oops

[Andy Whitcroft]

OriginalAuthor: Sergio Tosti <zeno979 at gmail.com>
Bug: 276641

Backport of the recent mainline gfs2_bitfit() to fix occasional OOPS.
This is already fixed in mainline and does not apply to Intrepid or
later.

Signed-off-by: Andy Whitcroft <apw at canonical.com>
---
 fs/gfs2/rgrp.c |  112 +++++++++++++++++++++++++++++++++++--------------------
 1 files changed, 71 insertions(+), 41 deletions(-)

diff --git a/fs/gfs2/rgrp.c b/fs/gfs2/rgrp.c
index 3552110..caf564a 100644
--- a/fs/gfs2/rgrp.c
+++ b/fs/gfs2/rgrp.c
@@ -14,6 +14,7 @@
 #include <linux/fs.h>
 #include <linux/gfs2_ondisk.h>
 #include <linux/lm_interface.h>
+#include <linux/prefetch.h>
 
 #include "gfs2.h"
 #include "incore.h"
@@ -33,6 +34,16 @@
 #define BFITNOENT ((u32)~0)
 #define NO_BLOCK ((u64)~0)
 
+#if BITS_PER_LONG == 32
+#define LBITMASK   (0x55555555UL)
+#define LBITSKIP55 (0x55555555UL)
+#define LBITSKIP00 (0x00000000UL)
+#else
+#define LBITMASK   (0x5555555555555555UL)
+#define LBITSKIP55 (0x5555555555555555UL)
+#define LBITSKIP00 (0x0000000000000000UL)
+#endif
+
 /*
  * These routines are used by the resource group routines (rgrp.c)
  * to keep track of block allocation.  Each block is represented by two
@@ -126,48 +137,67 @@ static unsigned char gfs2_testbit(struct gfs2_rgrpd *rgd, unsigned char *buffer,
  * Return: the block number (bitmap buffer scope) that was found
  */
 
-static u32 gfs2_bitfit(unsigned char *buffer, unsigned int buflen, u32 goal,
-		       unsigned char old_state)
+static u32 gfs2_bitfit(const u8 *buffer, unsigned int buflen, u32 goal,
+                       u8 old_state)
 {
-	unsigned char *byte;
-	u32 blk = goal;
-	unsigned int bit, bitlong;
-	unsigned long *plong, plong55;
-
-	byte = buffer + (goal / GFS2_NBBY);
-	plong = (unsigned long *)(buffer + (goal / GFS2_NBBY));
-	bit = (goal % GFS2_NBBY) * GFS2_BIT_SIZE;
-	bitlong = bit;
-#if BITS_PER_LONG == 32
-	plong55 = 0x55555555;
-#else
-	plong55 = 0x5555555555555555;
-#endif
-	while (byte < buffer + buflen) {
-
-		if (bitlong == 0 && old_state == 0 && *plong == plong55) {
-			plong++;
-			byte += sizeof(unsigned long);
-			blk += sizeof(unsigned long) * GFS2_NBBY;
-			continue;
-		}
-		if (((*byte >> bit) & GFS2_BIT_MASK) == old_state)
-			return blk;
-		bit += GFS2_BIT_SIZE;
-		if (bit >= 8) {
-			bit = 0;
-			byte++;
-		}
-		bitlong += GFS2_BIT_SIZE;
-		if (bitlong >= sizeof(unsigned long) * 8) {
-			bitlong = 0;
-			plong++;
-		}
-
-		blk++;
-	}
-
-	return BFITNOENT;
+        const u8 *byte, *start, *end;
+        int bit, startbit;
+        u32 g1, g2, misaligned;
+        unsigned long *plong;
+        unsigned long lskipval;
+
+        lskipval = (old_state & GFS2_BLKST_USED) ? LBITSKIP00 : LBITSKIP55;
+        g1 = (goal / GFS2_NBBY);
+        start = buffer + g1;
+        byte = start;
+        end = buffer + buflen;
+        g2 = ALIGN(g1, sizeof(unsigned long));
+        plong = (unsigned long *)(buffer + g2);
+        startbit = bit = (goal % GFS2_NBBY) * GFS2_BIT_SIZE;
+        misaligned = g2 - g1;
+        if (!misaligned)
+                goto ulong_aligned;
+/* parse the bitmap a byte at a time */
+misaligned:
+        while (byte < end) {
+                if (((*byte >> bit) & GFS2_BIT_MASK) == old_state) {
+                        return goal +
+                                (((byte - start) * GFS2_NBBY) +
+                                 ((bit - startbit) >> 1));
+                }
+                bit += GFS2_BIT_SIZE;
+                if (bit >= GFS2_NBBY * GFS2_BIT_SIZE) {
+                        bit = 0;
+                        byte++;
+                        misaligned--;
+                        if (!misaligned) {
+                                plong = (unsigned long *)byte;
+                                goto ulong_aligned;
+                        }
+                }
+        }
+        return BFITNOENT;
+
+/* parse the bitmap a unsigned long at a time */
+ulong_aligned:
+        /* Stop at "end - 1" or else prefetch can go past the end and segfault.
+           We could "if" it but we'd lose some of the performance gained.
+           This way will only slow down searching the very last 4/8 bytes
+           depending on architecture.  I've experimented with several ways
+           of writing this section such as using an else before the goto
+           but this one seems to be the fastest. */
+        while ((unsigned char *)plong < end - sizeof(unsigned long)) {
+                prefetch(plong + 1);
+                if (((*plong) & LBITMASK) != lskipval)
+                        break;
+                plong++;
+        }
+        if ((unsigned char *)plong < end) {
+                byte = (const u8 *)plong;
+                misaligned += sizeof(unsigned long) - 1;
+                goto misaligned;
+        }
+        return BFITNOENT;
 }
 
 /**
-- 
1.5.6.3