[PATCH] Fix race condition in AFFS

["Vladimir 'φ-coder/phcoder' Serbinenko" <phcoder@gmail.com>]

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AFFS preallocates few blocks per inode as an optimisation. Unfortunately
there is a race condition and the same blocks ends up being allocated
for both data and metadata, metadata gets overwritten and so the file is
partially unreadable. Here is a fix.
Please indicate if this isn't the right place to submit this or my
previous fs-related patches.

-- 
Regards
Vladimir 'φ-coder/phcoder' Serbinenko


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=== modified file 'affs.h'
--- affs.h	2012-05-07 16:42:19 +0000
+++ affs.h	2012-05-08 21:24:08 +0000
@@ -66,8 +66,11 @@ struct affs_inode_info {
 	struct buffer_head *i_ext_bh;		/* bh of last extended block */
 	loff_t	 mmu_private;
 	u32	 i_protect;			/* unused attribute bits */
+
 	u32	 i_lastalloc;			/* last allocated block */
 	int	 i_pa_cnt;			/* number of preallocated blocks */
+	struct mutex i_alloc;		        /* Protects last 2 fields. */
+
 	struct inode vfs_inode;
 };
 

=== modified file 'bitmap.c'
--- bitmap.c	2012-05-06 21:40:49 +0000
+++ bitmap.c	2012-05-08 21:24:08 +0000
@@ -151,12 +151,18 @@ affs_alloc_block(struct inode *inode, u3
 
 	pr_debug("AFFS: balloc(inode=%lu,goal=%u): ", inode->i_ino, goal);
 
+	mutex_lock(&AFFS_I (inode)->i_alloc);
+
 	if (AFFS_I(inode)->i_pa_cnt) {
-		pr_debug("%d\n", AFFS_I(inode)->i_lastalloc+1);
+		u32 ret;
 		AFFS_I(inode)->i_pa_cnt--;
-		return ++AFFS_I(inode)->i_lastalloc;
+		ret = ++AFFS_I(inode)->i_lastalloc;
+		mutex_unlock(&AFFS_I (inode)->i_alloc);
+		return ret;
 	}
 
+	mutex_unlock(&AFFS_I (inode)->i_alloc);
+
 	if (!goal || goal > sbi->s_partition_size) {
 		if (goal)
 			affs_warning(sb, "affs_balloc", "invalid goal %d", goal);
@@ -230,16 +236,22 @@ find_bit:
 	bit = ffs(tmp & mask) - 1;
 	blk += bit + sbi->s_reserved;
 	mask2 = mask = 1 << (bit & 31);
-	AFFS_I(inode)->i_lastalloc = blk;
 
-	/* prealloc as much as possible within this word */
-	while ((mask2 <<= 1)) {
-		if (!(tmp & mask2))
-			break;
-		AFFS_I(inode)->i_pa_cnt++;
-		mask |= mask2;
+	mutex_lock(&AFFS_I (inode)->i_alloc);
+	if (!AFFS_I(inode)->i_pa_cnt) {
+		AFFS_I(inode)->i_lastalloc = blk;
+
+		/* prealloc as much as possible within this word */
+		while ((mask2 <<= 1)) {
+			if (!(tmp & mask2))
+				break;
+			AFFS_I(inode)->i_pa_cnt++;
+			mask |= mask2;
+		}
+		bm->bm_free -= AFFS_I(inode)->i_pa_cnt + 1;
 	}
-	bm->bm_free -= AFFS_I(inode)->i_pa_cnt + 1;
+
+	mutex_unlock(&AFFS_I (inode)->i_alloc);
 
 	*data = cpu_to_be32(tmp & ~mask);
 

=== modified file 'file.c'
--- file.c	2012-05-06 21:40:49 +0000
+++ file.c	2012-05-08 21:24:08 +0000
@@ -795,12 +795,21 @@ void
 affs_free_prealloc(struct inode *inode)
 {
 	struct super_block *sb = inode->i_sb;
+	u32 first, cnt;
 
 	pr_debug("AFFS: free_prealloc(ino=%lu)\n", inode->i_ino);
 
-	while (AFFS_I(inode)->i_pa_cnt) {
-		AFFS_I(inode)->i_pa_cnt--;
-		affs_free_block(sb, ++AFFS_I(inode)->i_lastalloc);
+	mutex_lock(&AFFS_I (inode)->i_alloc);
+	first = AFFS_I(inode)->i_lastalloc;
+	cnt = AFFS_I(inode)->i_pa_cnt;
+	AFFS_I(inode)->i_lastalloc += cnt;
+	AFFS_I(inode)->i_pa_cnt = 0;
+
+	mutex_unlock(&AFFS_I (inode)->i_alloc);
+
+	while (cnt) {
+		cnt--;
+		affs_free_block(sb, ++first);
 	}
 }
 

=== modified file 'inode.c'
--- inode.c	2012-05-07 16:23:07 +0000
+++ inode.c	2012-05-08 21:24:08 +0000
@@ -70,6 +70,7 @@ struct inode *affs_iget(struct super_blo
 	AFFS_I(inode)->mmu_private = 0;
 	AFFS_I(inode)->i_lastalloc = 0;
 	AFFS_I(inode)->i_pa_cnt = 0;
+	mutex_init(&AFFS_I (inode)->i_alloc);
 
 	if (sbi->s_flags & SF_SETMODE)
 		inode->i_mode = sbi->s_mode;
@@ -326,6 +327,7 @@ affs_new_inode(struct inode *dir)
 	AFFS_I(inode)->i_pa_cnt = 0;
 	AFFS_I(inode)->i_extcnt = 1;
 	AFFS_I(inode)->i_ext_last = ~1;
+	mutex_init(&AFFS_I (inode)->i_alloc);
 
 	insert_inode_hash(inode);
 


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