1 2 JFFS2 LOCKING DOCUMENTATION 3 --------------------------- 4 5At least theoretically, JFFS2 does not require the Big Kernel Lock 6(BKL), which was always helpfully obtained for it by Linux 2.4 VFS 7code. It has its own locking, as described below. 8 9This document attempts to describe the existing locking rules for 10JFFS2. It is not expected to remain perfectly up to date, but ought to 11be fairly close. 12 13 14 alloc_sem 15 --------- 16 17The alloc_sem is a per-filesystem semaphore, used primarily to ensure 18contiguous allocation of space on the medium. It is automatically 19obtained during space allocations (jffs2_reserve_space()) and freed 20upon write completion (jffs2_complete_reservation()). Note that 21the garbage collector will obtain this right at the beginning of 22jffs2_garbage_collect_pass() and release it at the end, thereby 23preventing any other write activity on the file system during a 24garbage collect pass. 25 26When writing new nodes, the alloc_sem must be held until the new nodes 27have been properly linked into the data structures for the inode to 28which they belong. This is for the benefit of NAND flash - adding new 29nodes to an inode may obsolete old ones, and by holding the alloc_sem 30until this happens we ensure that any data in the write-buffer at the 31time this happens are part of the new node, not just something that 32was written afterwards. Hence, we can ensure the newly-obsoleted nodes 33don't actually get erased until the write-buffer has been flushed to 34the medium. 35 36With the introduction of NAND flash support and the write-buffer, 37the alloc_sem is also used to protect the wbuf-related members of the 38jffs2_sb_info structure. Atomically reading the wbuf_len member to see 39if the wbuf is currently holding any data is permitted, though. 40 41Ordering constraints: See f->sem. 42 43 44 File Semaphore f->sem 45 --------------------- 46 47This is the JFFS2-internal equivalent of the inode semaphore i->i_sem. 48It protects the contents of the jffs2_inode_info private inode data, 49including the linked list of node fragments (but see the notes below on 50erase_completion_lock), etc. 51 52The reason that the i_sem itself isn't used for this purpose is to 53avoid deadlocks with garbage collection -- the VFS will lock the i_sem 54before calling a function which may need to allocate space. The 55allocation may trigger garbage-collection, which may need to move a 56node belonging to the inode which was locked in the first place by the 57VFS. If the garbage collection code were to attempt to lock the i_sem 58of the inode from which it's garbage-collecting a physical node, this 59lead to deadlock, unless we played games with unlocking the i_sem 60before calling the space allocation functions. 61 62Instead of playing such games, we just have an extra internal 63semaphore, which is obtained by the garbage collection code and also 64by the normal file system code _after_ allocation of space. 65 66Ordering constraints: 67 68 1. Never attempt to allocate space or lock alloc_sem with 69 any f->sem held. 70 2. Never attempt to lock two file semaphores in one thread. 71 No ordering rules have been made for doing so. 72 73 74 erase_completion_lock spinlock 75 ------------------------------ 76 77This is used to serialise access to the eraseblock lists, to the 78per-eraseblock lists of physical jffs2_raw_node_ref structures, and 79(NB) the per-inode list of physical nodes. The latter is a special 80case - see below. 81 82As the MTD API no longer permits erase-completion callback functions 83to be called from bottom-half (timer) context (on the basis that nobody 84ever actually implemented such a thing), it's now sufficient to use 85a simple spin_lock() rather than spin_lock_bh(). 86 87Note that the per-inode list of physical nodes (f->nodes) is a special 88case. Any changes to _valid_ nodes (i.e. ->flash_offset & 1 == 0) in 89the list are protected by the file semaphore f->sem. But the erase 90code may remove _obsolete_ nodes from the list while holding only the 91erase_completion_lock. So you can walk the list only while holding the 92erase_completion_lock, and can drop the lock temporarily mid-walk as 93long as the pointer you're holding is to a _valid_ node, not an 94obsolete one. 95 96The erase_completion_lock is also used to protect the c->gc_task 97pointer when the garbage collection thread exits. The code to kill the 98GC thread locks it, sends the signal, then unlocks it - while the GC 99thread itself locks it, zeroes c->gc_task, then unlocks on the exit path. 100 101 102 inocache_lock spinlock 103 ---------------------- 104 105This spinlock protects the hashed list (c->inocache_list) of the 106in-core jffs2_inode_cache objects (each inode in JFFS2 has the 107correspondent jffs2_inode_cache object). So, the inocache_lock 108has to be locked while walking the c->inocache_list hash buckets. 109 110This spinlock also covers allocation of new inode numbers, which is 111currently just '++->highest_ino++', but might one day get more complicated 112if we need to deal with wrapping after 4 milliard inode numbers are used. 113 114Note, the f->sem guarantees that the correspondent jffs2_inode_cache 115will not be removed. So, it is allowed to access it without locking 116the inocache_lock spinlock. 117 118Ordering constraints: 119 120 If both erase_completion_lock and inocache_lock are needed, the 121 c->erase_completion has to be acquired first. 122 123 124 erase_free_sem 125 -------------- 126 127This semaphore is only used by the erase code which frees obsolete 128node references and the jffs2_garbage_collect_deletion_dirent() 129function. The latter function on NAND flash must read _obsolete_ nodes 130to determine whether the 'deletion dirent' under consideration can be 131discarded or whether it is still required to show that an inode has 132been unlinked. Because reading from the flash may sleep, the 133erase_completion_lock cannot be held, so an alternative, more 134heavyweight lock was required to prevent the erase code from freeing 135the jffs2_raw_node_ref structures in question while the garbage 136collection code is looking at them. 137 138Suggestions for alternative solutions to this problem would be welcomed. 139 140 141 wbuf_sem 142 -------- 143 144This read/write semaphore protects against concurrent access to the 145write-behind buffer ('wbuf') used for flash chips where we must write 146in blocks. It protects both the contents of the wbuf and the metadata 147which indicates which flash region (if any) is currently covered by 148the buffer. 149 150Ordering constraints: 151 Lock wbuf_sem last, after the alloc_sem or and f->sem. 152 153 154 c->xattr_sem 155 ------------ 156 157This read/write semaphore protects against concurrent access to the 158xattr related objects which include stuff in superblock and ic->xref. 159In read-only path, write-semaphore is too much exclusion. It's enough 160by read-semaphore. But you must hold write-semaphore when updating, 161creating or deleting any xattr related object. 162 163Once xattr_sem released, there would be no assurance for the existence 164of those objects. Thus, a series of processes is often required to retry, 165when updating such a object is necessary under holding read semaphore. 166For example, do_jffs2_getxattr() holds read-semaphore to scan xref and 167xdatum at first. But it retries this process with holding write-semaphore 168after release read-semaphore, if it's necessary to load name/value pair 169from medium. 170 171Ordering constraints: 172 Lock xattr_sem last, after the alloc_sem. 173