diff options
author | Peter Zijlstra | 2009-02-18 14:48:30 -0800 |
---|---|---|
committer | Linus Torvalds | 2009-02-18 15:37:55 -0800 |
commit | ada723dcd681e2dffd7d73345cc8fda0eb0df9bd (patch) | |
tree | 5f1b114d225721ae7a65fca08a6d053f89f36c43 /fs | |
parent | 27c0c8e511fa9e2389503926840fac606d90a049 (diff) |
fs/super.c: add lockdep annotation to s_umount
Li Zefan said:
Thread 1:
for ((; ;))
{
mount -t cpuset xxx /mnt > /dev/null 2>&1
cat /mnt/cpus > /dev/null 2>&1
umount /mnt > /dev/null 2>&1
}
Thread 2:
for ((; ;))
{
mount -t cpuset xxx /mnt > /dev/null 2>&1
umount /mnt > /dev/null 2>&1
}
(Note: It is irrelevant which cgroup subsys is used.)
After a while a lockdep warning showed up:
=============================================
[ INFO: possible recursive locking detected ]
2.6.28 #479
---------------------------------------------
mount/13554 is trying to acquire lock:
(&type->s_umount_key#19){--..}, at: [<c049d888>] sget+0x5e/0x321
but task is already holding lock:
(&type->s_umount_key#19){--..}, at: [<c049da0c>] sget+0x1e2/0x321
other info that might help us debug this:
1 lock held by mount/13554:
#0: (&type->s_umount_key#19){--..}, at: [<c049da0c>] sget+0x1e2/0x321
stack backtrace:
Pid: 13554, comm: mount Not tainted 2.6.28-mc #479
Call Trace:
[<c044ad2e>] validate_chain+0x4c6/0xbbd
[<c044ba9b>] __lock_acquire+0x676/0x700
[<c044bb82>] lock_acquire+0x5d/0x7a
[<c049d888>] ? sget+0x5e/0x321
[<c061b9b8>] down_write+0x34/0x50
[<c049d888>] ? sget+0x5e/0x321
[<c049d888>] sget+0x5e/0x321
[<c045a2e7>] ? cgroup_set_super+0x0/0x3e
[<c045959f>] ? cgroup_test_super+0x0/0x2f
[<c045bcea>] cgroup_get_sb+0x98/0x2e7
[<c045cfb6>] cpuset_get_sb+0x4a/0x5f
[<c049dfa4>] vfs_kern_mount+0x40/0x7b
[<c049e02d>] do_kern_mount+0x37/0xbf
[<c04af4a0>] do_mount+0x5c3/0x61a
[<c04addd2>] ? copy_mount_options+0x2c/0x111
[<c04af560>] sys_mount+0x69/0xa0
[<c0403251>] sysenter_do_call+0x12/0x31
The cause is after alloc_super() and then retry, an old entry in list
fs_supers is found, so grab_super(old) is called, but both functions hold
s_umount lock:
struct super_block *sget(...)
{
...
retry:
spin_lock(&sb_lock);
if (test) {
list_for_each_entry(old, &type->fs_supers, s_instances) {
if (!test(old, data))
continue;
if (!grab_super(old)) <--- 2nd: down_write(&old->s_umount);
goto retry;
if (s)
destroy_super(s);
return old;
}
}
if (!s) {
spin_unlock(&sb_lock);
s = alloc_super(type); <--- 1th: down_write(&s->s_umount)
if (!s)
return ERR_PTR(-ENOMEM);
goto retry;
}
...
}
It seems like a false positive, and seems like VFS but not cgroup needs to
be fixed.
Peter said:
We can simply put the new s_umount instance in a but lockdep doesn't
particularly cares about subclass order.
If there's any issue with the callers of sget() assuming the s_umount lock
being of sublcass 0, then there is another annotation we can use to fix
that, but lets not bother with that if this is sufficient.
Addresses http://bugzilla.kernel.org/show_bug.cgi?id=12673
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Tested-by: Li Zefan <lizf@cn.fujitsu.com>
Reported-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Paul Menage <menage@google.com>
Cc: Arjan van de Ven <arjan@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'fs')
-rw-r--r-- | fs/super.c | 17 |
1 files changed, 16 insertions, 1 deletions
diff --git a/fs/super.c b/fs/super.c index 61dce001dd57..8349ed6b1412 100644 --- a/fs/super.c +++ b/fs/super.c @@ -82,7 +82,22 @@ static struct super_block *alloc_super(struct file_system_type *type) * lock ordering than usbfs: */ lockdep_set_class(&s->s_lock, &type->s_lock_key); - down_write(&s->s_umount); + /* + * sget() can have s_umount recursion. + * + * When it cannot find a suitable sb, it allocates a new + * one (this one), and tries again to find a suitable old + * one. + * + * In case that succeeds, it will acquire the s_umount + * lock of the old one. Since these are clearly distrinct + * locks, and this object isn't exposed yet, there's no + * risk of deadlocks. + * + * Annotate this by putting this lock in a different + * subclass. + */ + down_write_nested(&s->s_umount, SINGLE_DEPTH_NESTING); s->s_count = S_BIAS; atomic_set(&s->s_active, 1); mutex_init(&s->s_vfs_rename_mutex); |