[PATCH v3] jffs2: Fix lock acquisition order bug in jffs2_write_begin

[Thomas Betker]

jffs2_write_begin() first acquires the page lock, then f->sem. This 
causes an AB-BA deadlock with jffs2_garbage_collect_live(), which first 
acquires f->sem, then the page lock:

jffs2_garbage_collect_live
    mutex_lock(&f->sem)                         (A)
    jffs2_garbage_collect_dnode
        jffs2_gc_fetch_page
            read_cache_page_async
                do_read_cache_page
                    lock_page(page)             (B)

jffs2_write_begin
    grab_cache_page_write_begin
        find_lock_page
            lock_page(page)                     (B)
    mutex_lock(&f->sem)                         (A)

We fix this by restructuring jffs2_write_begin() to take f->sem before 
the page lock. However, we make sure that f->sem is not held when 
calling jffs2_reserve_space(), as this is not permitted by the locking 
rules.

The deadlock above was observed multiple times on an SoC with a dual 
ARMv7 (Cortex-A9), running the long-term 3.4.11 kernel; it occurred 
when using scp to copy files from a host system to the ARM target 
system. The fix was heavily tested on the same target system.

Cc: Joakim Tjernlund <Joakim.Tjernlund at transmode.se>
Cc: stable at vger.kernel.org [v3.4+],
Signed-off-by: Thomas Betker <thomas.betker at rohde-schwarz.com>

diff -Naur linux-3.4.11-orig/fs/jffs2/file.c linux-3.4.11/fs/jffs2/file.c
--- linux-3.4.11-orig/fs/jffs2/file.c	2012-09-14 22:18:55.000000000 +0000
+++ linux-3.4.11/fs/jffs2/file.c	2012-10-05 09:01:10.000000000 +0000
@@ -138,33 +138,39 @@
 	struct page *pg;
 	struct inode *inode = mapping->host;
 	struct jffs2_inode_info *f = JFFS2_INODE_INFO(inode);
+	struct jffs2_sb_info *c = JFFS2_SB_INFO(inode->i_sb);
+	struct jffs2_raw_inode ri;
+	uint32_t alloc_len = 0;
 	pgoff_t index = pos >> PAGE_CACHE_SHIFT;
 	uint32_t pageofs = index << PAGE_CACHE_SHIFT;
 	int ret = 0;
 
+	jffs2_dbg(1, "%s()\n", __func__);
+
+	if (pageofs > inode->i_size) {
+		ret = jffs2_reserve_space(c, sizeof(ri), &alloc_len,
+					  ALLOC_NORMAL, JFFS2_SUMMARY_INODE_SIZE);
+		if (ret)
+			return ret;
+	}
+
+	mutex_lock(&f->sem);
 	pg = grab_cache_page_write_begin(mapping, index, flags);
-	if (!pg)
+	if (!pg) {
+		if (alloc_len)
+			jffs2_complete_reservation(c);
+		mutex_unlock(&f->sem);
 		return -ENOMEM;
+	}
 	*pagep = pg;
 
-	jffs2_dbg(1, "%s()\n", __func__);
-
-	if (pageofs > inode->i_size) {
+	if (alloc_len) {
 		/* Make new hole frag from old EOF to new page */
-		struct jffs2_sb_info *c = JFFS2_SB_INFO(inode->i_sb);
-		struct jffs2_raw_inode ri;
 		struct jffs2_full_dnode *fn;
-		uint32_t alloc_len;
 
 		jffs2_dbg(1, "Writing new hole frag 0x%x-0x%x between current EOF and new page\n",
 			  (unsigned int)inode->i_size, pageofs);
 
-		ret = jffs2_reserve_space(c, sizeof(ri), &alloc_len,
-					  ALLOC_NORMAL, JFFS2_SUMMARY_INODE_SIZE);
-		if (ret)
-			goto out_page;
-
-		mutex_lock(&f->sem);
 		memset(&ri, 0, sizeof(ri));
 
 		ri.magic = cpu_to_je16(JFFS2_MAGIC_BITMASK);
@@ -191,7 +197,6 @@
 		if (IS_ERR(fn)) {
 			ret = PTR_ERR(fn);
 			jffs2_complete_reservation(c);
-			mutex_unlock(&f->sem);
 			goto out_page;
 		}
 		ret = jffs2_add_full_dnode_to_inode(c, f, fn);
@@ -206,12 +211,10 @@
 			jffs2_mark_node_obsolete(c, fn->raw);
 			jffs2_free_full_dnode(fn);
 			jffs2_complete_reservation(c);
-			mutex_unlock(&f->sem);
 			goto out_page;
 		}
 		jffs2_complete_reservation(c);
 		inode->i_size = pageofs;
-		mutex_unlock(&f->sem);
 	}
 
 	/*
@@ -220,18 +223,18 @@
 	 * case of a short-copy.
 	 */
 	if (!PageUptodate(pg)) {
-		mutex_lock(&f->sem);
 		ret = jffs2_do_readpage_nolock(inode, pg);
-		mutex_unlock(&f->sem);
 		if (ret)
 			goto out_page;
 	}
+	mutex_unlock(&f->sem);
 	jffs2_dbg(1, "end write_begin(). pg->flags %lx\n", pg->flags);
 	return ret;
 
 out_page:
 	unlock_page(pg);
 	page_cache_release(pg);
+	mutex_unlock(&f->sem);
 	return ret;
 }