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Diffstat (limited to 'kernel/fs/jffs2/README.Locking')
| -rw-r--r-- | kernel/fs/jffs2/README.Locking | 172 |
1 files changed, 172 insertions, 0 deletions
diff --git a/kernel/fs/jffs2/README.Locking b/kernel/fs/jffs2/README.Locking new file mode 100644 index 000000000..3ea365541 --- /dev/null +++ b/kernel/fs/jffs2/README.Locking @@ -0,0 +1,172 @@ + + 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 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. + + + 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 mutex 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 mutex 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. |