linux-sg2042/fs/jffs2
Jason Lunz fc0e01974c [JFFS2] fix write deadlock regression
I've bisected the deadlock when many small appends are done on jffs2 down to
this commit:

commit 6fe6900e1e
Author: Nick Piggin <npiggin@suse.de>
Date:   Sun May 6 14:49:04 2007 -0700

    mm: make read_cache_page synchronous

    Ensure pages are uptodate after returning from read_cache_page, which allows
    us to cut out most of the filesystem-internal PageUptodate calls.

    I didn't have a great look down the call chains, but this appears to fixes 7
    possible use-before uptodate in hfs, 2 in hfsplus, 1 in jfs, a few in
    ecryptfs, 1 in jffs2, and a possible cleared data overwritten with readpage in
    block2mtd.  All depending on whether the filler is async and/or can return
    with a !uptodate page.

It introduced a wait to read_cache_page, as well as a
read_cache_page_async function equivalent to the old read_cache_page
without any callers.

Switching jffs2_gc_fetch_page to read_cache_page_async for the old
behavior makes the deadlocks go away, but maybe reintroduces the
use-before-uptodate problem? I don't understand the mm/fs interaction
well enough to say.

[It's fine. dwmw2.]

Signed-off-by: Jason Lunz <lunz@falooley.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>
2007-09-02 18:18:38 +01:00
..
LICENCE [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00
Makefile [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00
README.Locking [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00
TODO [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00
acl.c Introduce is_owner_or_cap() to wrap CAP_FOWNER use with fsuid check 2007-07-17 12:00:03 -07:00
acl.h [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00
background.c [JFFS2] Fix suspend failure with JFFS2 GC thread. 2007-08-02 21:43:03 +01:00
build.c [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00
compr.c [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00
compr.h [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00
compr_rtime.c [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00
compr_rubin.c [JFFS2] Fix compr_rubin.c build after include file elimination. 2007-04-26 07:27:04 +01:00
compr_zlib.c [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00
debug.c [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00
debug.h [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00
dir.c [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00
erase.c [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00
file.c sendfile: remove .sendfile from filesystems that use generic_file_sendfile() 2007-07-10 08:04:13 +02:00
fs.c [JFFS2] fix write deadlock regression 2007-09-02 18:18:38 +01:00
gc.c [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00
ioctl.c [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00
jffs2_fs_i.h [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00
jffs2_fs_sb.h [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00
malloc.c mm: Remove slab destructors from kmem_cache_create(). 2007-07-20 10:11:58 +09:00
nodelist.c [JFFS2] Handle inodes with only a single metadata node with non-zero isize 2007-04-25 17:04:23 +01:00
nodelist.h [JFFS2] Deletion dirents should be REF_NORMAL, not REF_PRISTINE. 2007-08-02 21:39:50 +01:00
nodemgmt.c [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00
os-linux.h Merge branch 'master' of git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6 2007-04-27 19:16:19 +01:00
read.c [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00
readinode.c [JFFS2] Print correct node offset when complaining about broken data CRC 2007-08-02 21:43:46 +01:00
scan.c [JFFS2] Deletion dirents should be REF_NORMAL, not REF_PRISTINE. 2007-08-02 21:39:50 +01:00
security.c [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00
summary.c [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00
summary.h [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00
super.c mm: Remove slab destructors from kmem_cache_create(). 2007-07-20 10:11:58 +09:00
symlink.c [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00
wbuf.c [JFFS2] Don't advance c->wbuf_ofs to next eraseblock after wbuf flush 2007-05-05 09:52:49 +01:00
write.c JFFS2 locking regression fix. 2007-08-20 22:44:27 -07:00
writev.c [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00
xattr.c [JFFS2] Fix potential memory leak of dead xattrs on unmount. 2007-05-20 11:30:38 -04:00
xattr.h [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00
xattr_trusted.c [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00
xattr_user.c [JFFS2] Tidy up licensing/copyright boilerplate. 2007-04-25 14:16:47 +01:00

README.Locking

	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.


	alloc_sem
	---------

The alloc_sem is a per-filesystem semaphore, used primarily to ensure
contiguous allocation of space on the medium. It is automatically
obtained during space allocations (jffs2_reserve_space()) and freed
upon write completion (jffs2_complete_reservation()). Note that
the garbage collector will obtain this right at the beginning of
jffs2_garbage_collect_pass() and release it at the end, thereby
preventing any other write activity on the file system during a
garbage collect pass.

When writing new nodes, the alloc_sem must be held until the new nodes
have been properly linked into the data structures for the inode to
which they belong. This is for the benefit of NAND flash - adding new
nodes to an inode may obsolete old ones, and by holding the alloc_sem
until this happens we ensure that any data in the write-buffer at the
time this happens are part of the new node, not just something that
was written afterwards. Hence, we can ensure the newly-obsoleted nodes
don't actually get erased until the write-buffer has been flushed to
the medium.

With the introduction of NAND flash support and the write-buffer, 
the alloc_sem is also used to protect the wbuf-related members of the
jffs2_sb_info structure. Atomically reading the wbuf_len member to see
if the wbuf is currently holding any data is permitted, though.

Ordering constraints: See f->sem.


	File Semaphore f->sem
	---------------------

This is the JFFS2-internal equivalent of the inode semaphore i->i_sem.
It protects the contents of the jffs2_inode_info private inode data,
including the linked list of node fragments (but see the notes below on
erase_completion_lock), etc.

The reason that the i_sem itself isn't used for this purpose is to
avoid deadlocks with garbage collection -- the VFS will lock the i_sem
before calling a function which may need to allocate space. The
allocation may trigger garbage-collection, which may need to move a
node belonging to the inode which was locked in the first place by the
VFS. If the garbage collection code were to attempt to lock the i_sem
of the inode from which it's garbage-collecting a physical node, this
lead to deadlock, unless we played games with unlocking the i_sem
before calling the space allocation functions.

Instead of playing such games, we just have an extra internal
semaphore, which is obtained by the garbage collection code and also
by the normal file system code _after_ allocation of space.

Ordering constraints: 

	1. Never attempt to allocate space or lock alloc_sem with 
	   any f->sem held.
	2. Never attempt to lock two file semaphores in one thread.
	   No ordering rules have been made for doing so.


	erase_completion_lock spinlock
	------------------------------

This is used to serialise access to the eraseblock lists, to the
per-eraseblock lists of physical jffs2_raw_node_ref structures, and
(NB) the per-inode list of physical nodes. The latter is a special
case - see below.

As the MTD API no longer permits erase-completion callback functions
to be called from bottom-half (timer) context (on the basis that nobody
ever actually implemented such a thing), it's now sufficient to use
a simple spin_lock() rather than spin_lock_bh().

Note that the per-inode list of physical nodes (f->nodes) is a special
case. Any changes to _valid_ nodes (i.e. ->flash_offset & 1 == 0) in
the list are protected by the file semaphore f->sem. But the erase
code may remove _obsolete_ nodes from the list while holding only the
erase_completion_lock. So you can walk the list only while holding the
erase_completion_lock, and can drop the lock temporarily mid-walk as
long as the pointer you're holding is to a _valid_ node, not an
obsolete one.

The erase_completion_lock is also used to protect the c->gc_task
pointer when the garbage collection thread exits. The code to kill the
GC thread locks it, sends the signal, then unlocks it - while the GC
thread itself locks it, zeroes c->gc_task, then unlocks on the exit path.


	inocache_lock spinlock
	----------------------

This spinlock protects the hashed list (c->inocache_list) of the
in-core jffs2_inode_cache objects (each inode in JFFS2 has the
correspondent jffs2_inode_cache object). So, the inocache_lock
has to be locked while walking the c->inocache_list hash buckets.

This spinlock also covers allocation of new inode numbers, which is
currently just '++->highest_ino++', but might one day get more complicated
if we need to deal with wrapping after 4 milliard inode numbers are used.

Note, the f->sem guarantees that the correspondent jffs2_inode_cache
will not be removed. So, it is allowed to access it without locking
the inocache_lock spinlock. 

Ordering constraints: 

	If both erase_completion_lock and inocache_lock are needed, the
	c->erase_completion has to be acquired first.


	erase_free_sem
	--------------

This semaphore is only used by the erase code which frees obsolete
node references and the jffs2_garbage_collect_deletion_dirent()
function. The latter function on NAND flash must read _obsolete_ nodes
to determine whether the 'deletion dirent' under consideration can be
discarded or whether it is still required to show that an inode has
been unlinked. Because reading from the flash may sleep, the
erase_completion_lock cannot be held, so an alternative, more
heavyweight lock was required to prevent the erase code from freeing
the jffs2_raw_node_ref structures in question while the garbage
collection code is looking at them.

Suggestions for alternative solutions to this problem would be welcomed.


	wbuf_sem
	--------

This read/write semaphore protects against concurrent access to the
write-behind buffer ('wbuf') used for flash chips where we must write
in blocks. It protects both the contents of the wbuf and the metadata
which indicates which flash region (if any) is currently covered by 
the buffer.

Ordering constraints:
	Lock wbuf_sem last, after the alloc_sem or and f->sem.


	c->xattr_sem
	------------

This read/write semaphore protects against concurrent access to the
xattr related objects which include stuff in superblock and ic->xref.
In read-only path, write-semaphore is too much exclusion. It's enough
by read-semaphore. But you must hold write-semaphore when updating,
creating or deleting any xattr related object.

Once xattr_sem released, there would be no assurance for the existence
of those objects. Thus, a series of processes is often required to retry,
when updating such a object is necessary under holding read semaphore.
For example, do_jffs2_getxattr() holds read-semaphore to scan xref and
xdatum at first. But it retries this process with holding write-semaphore
after release read-semaphore, if it's necessary to load name/value pair
from medium.

Ordering constraints:
	Lock xattr_sem last, after the alloc_sem.