fs/jffs2/README.Locking

Source file repositories/reference/linux-study-clean/fs/jffs2/README.Locking

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System
Linux kernel
Corpus path
fs/jffs2/README.Locking
Extension
.Locking
Size
7068 bytes
Lines
170
Domain
Core OS
Bucket
VFS And Filesystem Core
Inferred role
Core OS: VFS And Filesystem Core
Status
atlas-only

Why This File Exists

Core operating-system implementation surface: boot, tasks, memory, VFS, syscall-facing interfaces, synchronization, credentials, and isolation.

Dependency Surface

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Annotated Snippet

JFFS2 LOCKING DOCUMENTATION
	---------------------------

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 mutex, 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 Mutex f->sem
	---------------------

This is the JFFS2-internal equivalent of the inode mutex 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
mutex, 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 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

Annotation

Implementation Notes