ufs: avoid grabbing ->truncate_mutex if possible

tail unpacking is done in a wrong place; the deadlocks galore
is best dealt with by doing that in ->write_iter() (and switching
to iomap, while we are at it), but that's rather painful to
backport.  The trouble comes from grabbing pages that cover
the beginning of tail from inside of ufs_new_fragments(); ongoing
pageout of any of those is going to deadlock on ->truncate_mutex
with process that got around to extending the tail holding that
and waiting for page to get unlocked, while ->writepage() on
that page is waiting on ->truncate_mutex.

The thing is, we don't need ->truncate_mutex when the fragment
we are trying to map is within the tail - the damn thing is
allocated (tail can't contain holes).

Let's do a plain lookup and if the fragment is present, we can
just pretend that we'd won the race in almost all cases.  The
only exception is a fragment between the end of tail and the
end of block containing tail.

Protect ->i_lastfrag with ->meta_lock - read_seqlock_excl() is
sufficient.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/ufs/balloc.c

@@ -423,12 +423,12 @@ u64 ufs_new_fragments(struct inode *inode, void *p, u64 fragment,
 		if (result) {
 			ufs_clear_frags(inode, result + oldcount,
 					newcount - oldcount, locked_page != NULL);
+			*err = 0;
 			write_seqlock(&UFS_I(inode)->meta_lock);
 			ufs_cpu_to_data_ptr(sb, p, result);
-			write_sequnlock(&UFS_I(inode)->meta_lock);
-			*err = 0;
 			UFS_I(inode)->i_lastfrag =
 				max(UFS_I(inode)->i_lastfrag, fragment + count);
+			write_sequnlock(&UFS_I(inode)->meta_lock);
 		}
 		mutex_unlock(&UFS_SB(sb)->s_lock);
 		UFSD("EXIT, result %llu\n", (unsigned long long)result);
@@ -441,8 +441,10 @@ u64 ufs_new_fragments(struct inode *inode, void *p, u64 fragment,
 	result = ufs_add_fragments(inode, tmp, oldcount, newcount);
 	if (result) {
 		*err = 0;
+		read_seqlock_excl(&UFS_I(inode)->meta_lock);
 		UFS_I(inode)->i_lastfrag = max(UFS_I(inode)->i_lastfrag,
 						fragment + count);
+		read_sequnlock_excl(&UFS_I(inode)->meta_lock);
 		ufs_clear_frags(inode, result + oldcount, newcount - oldcount,
 				locked_page != NULL);
 		mutex_unlock(&UFS_SB(sb)->s_lock);
@@ -479,12 +481,12 @@ u64 ufs_new_fragments(struct inode *inode, void *p, u64 fragment,
 		ufs_change_blocknr(inode, fragment - oldcount, oldcount,
 				   uspi->s_sbbase + tmp,
 				   uspi->s_sbbase + result, locked_page);
+		*err = 0;
 		write_seqlock(&UFS_I(inode)->meta_lock);
 		ufs_cpu_to_data_ptr(sb, p, result);
-		write_sequnlock(&UFS_I(inode)->meta_lock);
-		*err = 0;
 		UFS_I(inode)->i_lastfrag = max(UFS_I(inode)->i_lastfrag,
 						fragment + count);
+		write_sequnlock(&UFS_I(inode)->meta_lock);
 		mutex_unlock(&UFS_SB(sb)->s_lock);
 		if (newcount < request)
 			ufs_free_fragments (inode, result + newcount, request - newcount);

fs/ufs/inode.c

@@ -401,13 +401,20 @@ static int ufs_getfrag_block(struct inode *inode, sector_t fragment, struct buff
 	u64 phys64 = 0;
 	unsigned frag = fragment & uspi->s_fpbmask;
 
-	if (!create) {
-		phys64 = ufs_frag_map(inode, offsets, depth);
-		if (phys64)
-			map_bh(bh_result, sb, phys64 + frag);
-		return 0;
-	}
+	phys64 = ufs_frag_map(inode, offsets, depth);
+	if (!create)
+		goto done;
 
+	if (phys64) {
+		if (fragment >= UFS_NDIR_FRAGMENT)
+			goto done;
+		read_seqlock_excl(&UFS_I(inode)->meta_lock);
+		if (fragment < UFS_I(inode)->i_lastfrag) {
+			read_sequnlock_excl(&UFS_I(inode)->meta_lock);
+			goto done;
+		}
+		read_sequnlock_excl(&UFS_I(inode)->meta_lock);
+	}
         /* This code entered only while writing ....? */
 
 	mutex_lock(&UFS_I(inode)->truncate_mutex);
@@ -451,6 +458,11 @@ static int ufs_getfrag_block(struct inode *inode, sector_t fragment, struct buff
 	}
 	mutex_unlock(&UFS_I(inode)->truncate_mutex);
 	return err;
+
+done:
+	if (phys64)
+		map_bh(bh_result, sb, phys64 + frag);
+	return 0;
 }
 
 static int ufs_writepage(struct page *page, struct writeback_control *wbc)
@@ -1161,7 +1173,9 @@ static void ufs_truncate_blocks(struct inode *inode)
 			free_full_branch(inode, block, i - UFS_IND_BLOCK + 1);
 		}
 	}
+	read_seqlock_excl(&ufsi->meta_lock);
 	ufsi->i_lastfrag = DIRECT_FRAGMENT;
+	read_sequnlock_excl(&ufsi->meta_lock);
 	mark_inode_dirty(inode);
 	mutex_unlock(&ufsi->truncate_mutex);
 }