jffs2: GC deadlock reading a page that is used in jffs2_write_begin()

GC task can deadlock in read_cache_page() because it may attempt
to release a page that is actually allocated by another task in
jffs2_write_begin().
The reason is that in jffs2_write_begin() there is a small window
a cache page is allocated for use but not set Uptodate yet.

This ends up with a deadlock between two tasks:
1) A task (e.g. file copy)
- jffs2_write_begin() locks a cache page
- jffs2_write_end() tries to lock "alloc_sem" from
jffs2_reserve_space() <-- STUCK
2) GC task (jffs2_gcd_mtd3)
- jffs2_garbage_collect_pass() locks "alloc_sem"
- try to lock the same cache page in read_cache_page() <-- STUCK

So to avoid this deadlock, hold "alloc_sem" in jffs2_write_begin()
while reading data in a cache page.

Signed-off-by: Kyeong Yoo <[e-mail]>
Signed-off-by: Richard Weinberger <[e-mail]>

fs/jffs2/file.c

···
 	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);
 	pgoff_t index = pos >> PAGE_SHIFT;
 	uint32_t pageofs = index << PAGE_SHIFT;
 	int ret = 0;
-
-	pg = grab_cache_page_write_begin(mapping, index, flags);
-	if (!pg)
-		return -ENOMEM;
-	*pagep = pg;
 
 	jffs2_dbg(1, "%s()\n", __func__);
 
 	if (pageofs > inode->i_size) {
 		/* 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;
···
 		ret = jffs2_reserve_space(c, sizeof(ri), &alloc_len,
 					  ALLOC_NORMAL, JFFS2_SUMMARY_INODE_SIZE);
 		if (ret)
-			goto out_page;
+			goto out_err;
 
 		mutex_lock(&f->sem);
 		memset(&ri, 0, sizeof(ri));
···
 			ret = PTR_ERR(fn);
 			jffs2_complete_reservation(c);
 			mutex_unlock(&f->sem);
-			goto out_page;
+			goto out_err;
 		}
 		ret = jffs2_add_full_dnode_to_inode(c, f, fn);
 		if (f->metadata) {
···
 			jffs2_free_full_dnode(fn);
 			jffs2_complete_reservation(c);
 			mutex_unlock(&f->sem);
-			goto out_page;
+			goto out_err;
 		}
 		jffs2_complete_reservation(c);
 		inode->i_size = pageofs;
 		mutex_unlock(&f->sem);
 	}
+
+	/*
+	 * While getting a page and reading data in, lock c->alloc_sem until
+	 * the page is Uptodate. Otherwise GC task may attempt to read the same
+	 * page in read_cache_page(), which causes a deadlock.
+	 */
+	mutex_lock(&c->alloc_sem);
+	pg = grab_cache_page_write_begin(mapping, index, flags);
+	if (!pg) {
+		ret = -ENOMEM;
+		goto release_sem;
+	}
+	*pagep = pg;
 
 	/*
 	 * Read in the page if it wasn't already present. Cannot optimize away
···
 		mutex_lock(&f->sem);
 		ret = jffs2_do_readpage_nolock(inode, pg);
 		mutex_unlock(&f->sem);
-		if (ret)
-			goto out_page;
+		if (ret) {
+			unlock_page(pg);
+			put_page(pg);
+			goto release_sem;
+		}
 	}
 	jffs2_dbg(1, "end write_begin(). pg->flags %lx\n", pg->flags);
-	return ret;
 
-out_page:
-	unlock_page(pg);
-	put_page(pg);
+release_sem:
+	mutex_unlock(&c->alloc_sem);
+out_err:
 	return ret;
 }