jffs2: Fix lock acquisition order bug in jffs2_write_begin

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: stable@vger.kernel.org
Signed-off-by: Thomas Betker <thomas.betker@rohde-schwarz.com>
Acked-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>

1 files changed, 21 insertions, 18 deletions

diff --git a/fs/jffs2/file.c b/fs/jffs2/file.c
index 60ef3fb707ff..1506673c087e 100644
--- a/fs/jffs2/file.c
+++ b/fs/jffs2/file.c
@@ -138,33 +138,39 @@ static int jffs2_write_begin(struct file *filp, struct address_space *mapping,
         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 @@ static int jffs2_write_begin(struct file *filp, struct address_space *mapping,
                 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 @@ static int jffs2_write_begin(struct file *filp, struct address_space *mapping,
                         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 @@ static int jffs2_write_begin(struct file *filp, struct address_space *mapping,
          * 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;
 }