From: Al Viro Date: Fri, 19 Jul 2013 23:13:55 +0000 (+0400) Subject: livelock avoidance in sget() X-Git-Tag: v3.11-rc2~4^2~1 X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=acfec9a5a892f98461f52ed5770de99a3e571ae2;p=pandora-kernel.git livelock avoidance in sget() Eric Sandeen has found a nasty livelock in sget() - take a mount(2) about to fail. The superblock is on ->fs_supers, ->s_umount is held exclusive, ->s_active is 1. Along comes two more processes, trying to mount the same thing; sget() in each is picking that superblock, bumping ->s_count and trying to grab ->s_umount. ->s_active is 3 now. Original mount(2) finally gets to deactivate_locked_super() on failure; ->s_active is 2, superblock is still ->fs_supers because shutdown will *not* happen until ->s_active hits 0. ->s_umount is dropped and now we have two processes chasing each other: s_active = 2, A acquired ->s_umount, B blocked A sees that the damn thing is stillborn, does deactivate_locked_super() s_active = 1, A drops ->s_umount, B gets it A restarts the search and finds the same superblock. And bumps it ->s_active. s_active = 2, B holds ->s_umount, A blocked on trying to get it ... and we are in the earlier situation with A and B switched places. The root cause, of course, is that ->s_active should not grow until we'd got MS_BORN. Then failing ->mount() will have deactivate_locked_super() shut the damn thing down. Fortunately, it's easy to do - the key point is that grab_super() is called only for superblocks currently on ->fs_supers, so it can bump ->s_count and grab ->s_umount first, then check MS_BORN and bump ->s_active; we must never increment ->s_count for superblocks past ->kill_sb(), but grab_super() is never called for those. The bug is pretty old; we would've caught it by now, if not for accidental exclusion between sget() for block filesystems; the things like cgroup or e.g. mtd-based filesystems don't have anything of that sort, so they get bitten. The right way to deal with that is obviously to fix sget()... Signed-off-by: Al Viro --- Reading git-diff-tree failed