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authorArtem Bityutskiy <artem.bityutskiy@linux.intel.com>2014-06-29 17:00:45 +0300
committerArtem Bityutskiy <artem.bityutskiy@linux.intel.com>2014-09-08 15:55:02 +0300
commit052c28073ff26f771d44ef33952a41d18dadd255 (patch)
tree9b15a30e3d8db2e3f367b3587ad2bbd278ef79c0 /fs/file.c
parent2ce7598c9a453e0acd0e07be7be3f5eb39608ebd (diff)
downloadlinux-052c28073ff26f771d44ef33952a41d18dadd255.tar.bz2
UBIFS: fix a race condition
Hu (hujianyang@huawei.com) discovered a race condition which may lead to a situation when UBIFS is unable to mount the file-system after an unclean reboot. The problem is theoretical, though. In UBIFS, we have the log, which basically a set of LEBs in a certain area. The log has the tail and the head. Every time user writes data to the file-system, the UBIFS journal grows, and the log grows as well, because we append new reference nodes to the head of the log. So the head moves forward all the time, while the log tail stays at the same position. At any time, the UBIFS master node points to the tail of the log. When we mount the file-system, we scan the log, and we always start from its tail, because this is where the master node points to. The only occasion when the tail of the log changes is the commit operation. The commit operation has 2 phases - "commit start" and "commit end". The former is relatively short, and does not involve much I/O. During this phase we mostly just build various in-memory lists of the things which have to be written to the flash media during "commit end" phase. During the commit start phase, what we do is we "clean" the log. Indeed, the commit operation will index all the data in the journal, so the entire journal "disappears", and therefore the data in the log become unneeded. So we just move the head of the log to the next LEB, and write the CS node there. This LEB will be the tail of the new log when the commit operation finishes. When the "commit start" phase finishes, users may write more data to the file-system, in parallel with the ongoing "commit end" operation. At this point the log tail was not changed yet, it is the same as it had been before we started the commit. The log head keeps moving forward, though. The commit operation now needs to write the new master node, and the new master node should point to the new log tail. After this the LEBs between the old log tail and the new log tail can be unmapped and re-used again. And here is the possible problem. We do 2 operations: (a) We first update the log tail position in memory (see 'ubifs_log_end_commit()'). (b) And then we write the master node (see the big lock of code in 'do_commit()'). But nothing prevents the log head from moving forward between (a) and (b), and the log head may "wrap" now to the old log tail. And when the "wrap" happens, the contends of the log tail gets erased. Now a power cut happens and we are in trouble. We end up with the old master node pointing to the old tail, which was erased. And replay fails because it expects the master node to point to the correct log tail at all times. This patch merges the abovementioned (a) and (b) operations by moving the master node change code to the 'ubifs_log_end_commit()' function, so that it runs with the log mutex locked, which will prevent the log from being changed benween operations (a) and (b). Cc: stable@vger.kernel.org # 07e19df UBIFS: remove mst_mutex Cc: stable@vger.kernel.org Reported-by: hujianyang <hujianyang@huawei.com> Tested-by: hujianyang <hujianyang@huawei.com> Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Diffstat (limited to 'fs/file.c')
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