On 01/13/2015 04:44 PM, Jeff Layton wrote: > On Tue, 13 Jan 2015 00:11:37 -0500 > Sasha Levin <sasha.levin@xxxxxxxxxx> wrote: > >> Hey Jeff, >> >> While fuzzing with trinity inside a KVM tools guest running the latest -next >> kernel, I've stumbled on the following spew: >> >> [ 887.078606] WARNING: CPU: 16 PID: 4296 at fs/locks.c:236 locks_free_lock_context+0x10d/0x240() >> [ 887.079703] Modules linked in: >> [ 887.080288] CPU: 16 PID: 4296 Comm: trinity-c273 Not tainted 3.19.0-rc4-next-20150112-sasha-00053-g23c147e02e-dirty #1710 >> [ 887.082229] 0000000000000000 0000000000000000 0000000000000000 ffff8804c9f4f8e8 >> [ 887.083773] ffffffff9154e0a6 0000000000000000 ffff8804cad98000 ffff8804c9f4f938 >> [ 887.085280] ffffffff8140a4d0 0000000000000001 ffffffff81bf0d2d ffff8804c9f4f988 >> [ 887.086792] Call Trace: >> [ 887.087320] dump_stack (lib/dump_stack.c:52) >> [ 887.088247] warn_slowpath_common (kernel/panic.c:447) >> [ 887.089342] ? locks_free_lock_context (fs/locks.c:236 (discriminator 3)) >> [ 887.090514] warn_slowpath_null (kernel/panic.c:481) >> [ 887.091629] locks_free_lock_context (fs/locks.c:236 (discriminator 3)) >> [ 887.092782] __destroy_inode (fs/inode.c:243) >> [ 887.093817] destroy_inode (fs/inode.c:268) >> [ 887.094833] evict (fs/inode.c:574) >> [ 887.095808] iput (fs/inode.c:1503) >> [ 887.096687] __dentry_kill (fs/dcache.c:323 fs/dcache.c:508) >> [ 887.097683] ? _raw_spin_trylock (kernel/locking/spinlock.c:136) >> [ 887.098733] ? dput (fs/dcache.c:545 fs/dcache.c:648) >> [ 887.099672] dput (fs/dcache.c:649) >> [ 887.100552] __fput (fs/file_table.c:227) > > So, looking at this a bit more... > > It's clear that we're at the dput in __fput at this point. Much earlier > in __fput, we call locks_remove_file to remove all of the locks that > are associated with the file description. > > Evidently though, something didn't go right there. The two most likely > scenarios to my mind are: > > A) a lock raced onto the list somehow after that point. That seems > unlikely since presumably the fcheck should have failed at that point. > > ...or... > > B) the CPU that called locks_remove_file mistakenly thought that > inode->i_flctx was NULL when it really wasn't (stale cache, perhaps?). > That would make it skip trying to remove any flock locks. > > B seems more likely to me, and if it's the case then that would seem to > imply that we need some memory barriers (or maybe some ACCESS_ONCE > calls) in these codepaths. I'll have to sit down and work through it to > see what makes the most sense. > > If your debugging seems to jive with this, then one thing that might be > interesting would be to comment out these two lines in > locks_remove_flock: > > if (!file_inode(filp)->i_flctx) > return; > > ...and see if it's still reproducible. That's obviously not a real fix > for this problem, but it might help prove whether the above suspicion > is correct. Removing those two lines makes the issue go away. I'm guessing that figuring out which filesystem we were abusing isn't interesting anymore... Thanks, Sasha -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
