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SubjectRe: 2.6.31-rc6: Oops in fsnotify
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Date
On Fri, 2009-08-21 at 07:25 +0100, Grant Wilson wrote:
> On Thu, 20 Aug 2009 22:22:34 -0400
> Eric Paris <eparis@parisplace.org> wrote:
>
> > I'll take a close look in the morning. I don't offhand see how this
> > is possible without calling vfs_unlink on a negative dentry (does that
> > even make sense?)
> >
> > What was the filesystem you are dealing with? Odd, very, odd (as with
> > all the problems people find in my notify code)
> >
> The filesystem is ext4 (converted from ext3).

I got the assembly from Grant, it's actually very easy to map it back to
the code but I don't see any problems!

RIP: 0010:[<ffffffff8111bf93>] [<ffffffff8111bf93>] fsnotify+0x93/0x150
RSP: 0018:ffff880120c89d88 EFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: ffff88012b8aadcc RSI: 0000000000000000 RDI: ffff88012b8aadc8
RBP: ffff880120c89df8 R08: 0000000000000000 R09: 0000000000000001
R10: 0000000000000001 R11: 0000000000000001 R12: 0000000000000200
R13: 0000000008000200 R14: ffff8801286bf760 R15: 0000000000000000
FS: 00007f10fd749750(0000) GS:ffff88002f000000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
DR2: 0000000000000000 CR3: 0000000120c59000 CR4: 00000000000026f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
ffffffff8111bf77: e8 94 ce f6 ff callq ffffffff81088e10 <srcu_read_lock>
ffffffff8111bf7c: 48 8b 1d 3d b5 6d 00 mov 0x6db53d(%rip),%rbx # ffffffff817f74c0 <fsnotify_groups>
ffffffff8111bf83: 89 45 ac mov %eax,-0x54(%rbp)
ffffffff8111bf86: 48 c7 45 c8 00 00 00 movq $0x0,-0x38(%rbp)
ffffffff8111bf8d: 00
ffffffff8111bf8e: eb 03 jmp ffffffff8111bf93 <fsnotify+0x93>
ffffffff8111bf90: 48 8b 1b mov (%rbx),%rbx
ffffffff8111bf93: 48 8b 03 mov (%rbx),%rax <----------------------IP IS HERE
ffffffff8111bf96: 48 81 fb c0 74 7f 81 cmp $0xffffffff817f74c0,%rbx
ffffffff8111bf9d: 0f 18 08 prefetcht0 (%rax)
ffffffff8111bfa0: 74 36 je ffffffff8111bfd8 <fsnotify+0xd8>
ffffffff8111bfa2: 44 85 63 10 test %r12d,0x10(%rbx) <--- [if (test_mask &group->mask)]
ffffffff8111bfa6: 74 e8 je ffffffff8111bf90 <fsnotify+0x90>
ffffffff8111bfa8: 48 8b 43 20 mov 0x20(%rbx),%rax
ffffffff8111bfac: 44 89 ea mov %r13d,%edx
ffffffff8111bfaf: 4c 89 f6 mov %r14,%rsi
ffffffff8111bfb2: 48 89 df mov %rbx,%rdi
ffffffff8111bfb5: ff 10 callq *(%rax) <-------------- [group->ops->should_send_event()]
ffffffff8111bfb7: 84 c0 test %al,%al
ffffffff8111bfb9: 74 d5 je ffffffff8111bf90 <fsnotify+0x90>
ffffffff8111bfbb: 48 83 7d c8 00 cmpq $0x0,-0x38(%rbp)
ffffffff8111bfc0: 74 46 je ffffffff8111c008 <fsnotify+0x108> <- [fsnotify_create_event()]
ffffffff8111bfc2: 48 8b 43 20 mov 0x20(%rbx),%rax
ffffffff8111bfc6: 48 8b 75 c8 mov -0x38(%rbp),%rsi
ffffffff8111bfca: 48 89 df mov %rbx,%rdi
ffffffff8111bfcd: ff 50 08 callq *0x8(%rax) <----------- [group->ops->handle_event()]
ffffffff8111bfd0: eb be jmp ffffffff8111bf90 <fsnotify+0x90>
ffffffff8111bfd2: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
ffffffff8111bfd8: 8b 75 ac mov -0x54(%rbp),%esi
ffffffff8111bfdb: 48 c7 c7 60 b3 3f 82 mov $0xffffffff823fb360,%rdi
ffffffff8111bfe2: e8 c9 cd f6 ff callq ffffffff81088db0 <srcu_read_unlock>
Here is the code segment from fsnotify():

idx = srcu_read_lock(&fsnotify_grp_srcu);
list_for_each_entry_rcu(group, &fsnotify_groups, group_list) {
if (test_mask & group->mask) {
if (!group->ops->should_send_event(group, to_tell, mask))
continue;
if (!event) {
event = fsnotify_create_event(to_tell, mask, data,
data_is, file_name, cookie,
GFP_KERNEL);
/* shit, we OOM'd and now we can't tell, maybe
* someday someone else will want to do something
* here */
if (!event)
break;
}
group->ops->handle_event(group, event);
}
}
srcu_read_unlock(&fsnotify_grp_srcu, idx);
So the IP is clearly inside the 'list_for_each_entry_rcu.' This means
that somehow a ->next pointer = NULL. The anchor for this list
(fsnotify_groups) is declared as LIST_HEAD(fsnotify_groups); so it can't
point to NULL. So I have to look at things that can get added/removed.
fsnotify_obtain_group() does the only addition (using list_add_rcu) and
holding the fsnotify_grp_mutex. __fsnotify_evict_group() does a
list_del_rcu() (which doesn't change the forward pointer) and also hold
the correct mutex. These are the only 2 manipulation sites for objects
on the list and they are clearly protected by the fsnotify_grp_mutex.

So if all of the list manipulation has proper synchronization the
concern then becomes if an object was removed from the list (properly)
and freed (improperly) and had it's ->next pntr set to NULL before the
rcu timeouts. But in fsnotify_put_group() I have:

__fsnotify_evict_group(group);
mutex_unlock(&fsnotify_grp_mutex);
synchronize_srcu(&fsnotify_grp_srcu);
fsnotify_destroy_group(group);

So the group is taken off of the list in __fsnotify_evict_group() and
then we imediately wait for the fsnotify_grp_srcu which is protecting
the read section where we hit a bug. So it doesn't look possible that
the object could have been freed while it was still in use by this list.

Maybe someone a lot smarter than me or someone who understands {s,}rcu a
lot better than I do can show me something that I've done wrong, but I'm
having problems seeing anything. The only thing I can imagine is that
something was scribbling in random memory it didn't own. I'm sorry
Grant, but I don't see what is wrong here! If you keep getting bugs,
especially if they happen in random places, I'd suggest either dying RAM
or blame some other random subsystem :)
-Eric



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