jffs2: Fix page lock / f->sem deadlock

	JFFS2 LOCKING DOCUMENTATION

	---------------------------

At least theoretically, JFFS2 does not require the Big Kernel Lock

(BKL), which was always helpfully obtained for it by Linux 2.4 VFS

code. It has its own locking, as described below.

This document attempts to describe the existing locking rules for

JFFS2. It is not expected to remain perfectly up to date, but ought to

be fairly close.

	   any f->sem held.

	2. Never attempt to lock two file mutexes in one thread.

	   No ordering rules have been made for doing so.

	3. Never lock a page cache page with f->sem held.

	erase_completion_lock spinlock

		BUG_ON(start > orig_start);

	}

	/* First, use readpage() to read the appropriate page into the page cache */

	/* Q: What happens if we actually try to GC the _same_ page for which commit_write()

	 *    triggered garbage collection in the first place?

	 * A: I _think_ it's OK. read_cache_page shouldn't deadlock, we'll write out the

	 *    page OK. We'll actually write it out again in commit_write, which is a little

	 *    suboptimal, but at least we're correct.

	 */

	/* The rules state that we must obtain the page lock *before* f->sem, so

	 * drop f->sem temporarily. Since we also hold c->alloc_sem, nothing's

	 * actually going to *change* so we're safe; we only allow reading.

	 *

	 * It is important to note that jffs2_write_begin() will ensure that its

	 * page is marked Uptodate before allocating space. That means that if we

	 * end up here trying to GC the *same* page that jffs2_write_begin() is

	 * trying to write out, read_cache_page() will not deadlock. */

	mutex_unlock(&f->sem);

	pg_ptr = jffs2_gc_fetch_page(c, f, start, &pg);

	mutex_lock(&f->sem);

	if (IS_ERR(pg_ptr)) {

		pr_warn("read_cache_page() returned error: %ld\n",