Re: [git pull] vfs fixes
From: Andreas Dilger
Date: Tue Apr 11 2017 - 17:02:29 EST
On Apr 11, 2017, at 12:48 AM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> On Mon, Apr 10, 2017 at 11:10:19PM -0700, Linus Torvalds wrote:
>
>> It looks odd because the lock part is
>>
>> if (flags & LOOKUP_RCU)
>> rcu_read_lock();
>>
>> ie it's locked conditionally, and the code in between does not seem to
>> return every time LOOKUP_RCU is clear.
>>
>> So mind giving this a look? Is it as obviously buggy as I think it is,
>> or is there something I'm missing?
>
> It's more obscure than I would like, and can grow into a bug one day, but...
> nd_jump_root() can only return non-zero if you have LOOKUP_RCU. So without
> LOOKUP_RCU in flags, this
> if (flags & LOOKUP_RCU)
> rcu_read_lock();
> set_root(nd);
> if (likely(!nd_jump_root(nd)))
> return s;
> nd->root.mnt = NULL;
> rcu_read_unlock();
> won't get to that rcu_read_unlock() at all - it'll get zero from nd_jump_root()
> and proceed to return s;
So possibly a comment like the following would be helpful:
rcu_read_unlock(); /* nd_jump_root() returns if !LOOKUP_RCU */
so that us mere mortals have a chance to understand this in the future?
Cheers, Andreas
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