Messages in this thread | | | Date | Mon, 3 Jul 2023 11:43:18 -0400 | From | Steven Rostedt <> | Subject | Buggy rwsem locking code in fs/smb/client/file.c |
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I just reviewed a patch that copied a solution from fs/smb/client/file.c (original was fs/cifs/file.c), which is really just hiding a bug. And because this code could have possibly caused this buggy solution to be repeated, I believe it should be fixed, before others use it as precedent in other areas of the kernel.
Commit d46b0da7a33dd ("cifs: Fix cifsInodeInfo lock_sem deadlock when reconnect occurs") has in its change log:
There's a deadlock that is possible and can easily be seen with a test where multiple readers open/read/close of the same file and a disruption occurs causing reconnect. The deadlock is due a reader thread inside cifs_strict_readv calling down_read and obtaining lock_sem, and then after reconnect inside cifs_reopen_file calling down_read a second time. If in between the two down_read calls, a down_write comes from another process, deadlock occurs.
CPU0 CPU1 ---- ---- cifs_strict_readv() down_read(&cifsi->lock_sem); _cifsFileInfo_put OR cifs_new_fileinfo down_write(&cifsi->lock_sem); cifs_reopen_file() down_read(&cifsi->lock_sem); Fix the above by changing all down_write(lock_sem) calls to down_write_trylock(lock_sem)/msleep() loop, which in turn makes the second down_read call benign since it will never block behind the writer while holding lock_sem.
And hides the bug by wrapping the down_write() with:
+void +cifs_down_write(struct rw_semaphore *sem) +{ + while (!down_write_trylock(sem)) + msleep(10); +} +
The comment above down_read_nested() has:
/* * nested locking. NOTE: rwsems are not allowed to recurse * (which occurs if the same task tries to acquire the same * lock instance multiple times), but multiple locks of the * same lock class might be taken, if the order of the locks * is always the same. This ordering rule can be expressed * to lockdep via the _nested() APIs, but enumerating the * subclasses that are used. (If the nesting relationship is * static then another method for expressing nested locking is * the explicit definition of lock class keys and the use of * lockdep_set_class() at lock initialization time. * See Documentation/locking/lockdep-design.rst for more details.) */
As the NOTE above states, down_read() is not a recursive lock, which appears to be what cifs is using it for. I wonder if it could be converted to using RCU instead.
I'm just bringing this to everyone's attention because that code really needs to be fixed.
-- Steve
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