Re: Huge race in lockd for async lock requests?

[Rob Gardner <rob.gardner@hp.com>]

Tom Talpey wrote:
> At 10:59 PM 5/28/2009, Rob Gardner wrote:
>   
>> J. Bruce Fields wrote:
>>     
>>> Looking at the code....  This is all under the BKL, and as far as I can
>>> tell there aren't any blocking operations anywhere there, so I don't
>>> think this should happen if the filesystem is careful.  Have you seen it
>>> happen?
>>>       
>> Aha, I just figured it out and you were right. The filesystem in this 
>> case was not careful. It broke the rules and actually made the fl_grant 
>> call *before* even returning to nlmsvc_lock's call to vfs_lock_file, and 
>> it did it in the lockd thread! So the BKL was of no use, and I saw 
>> nlmsvc_grant_deferred print "grant for unknown block". So I think 
>> everything is ok, no huge race in lockd for async lock requests. Thank 
>> you for clearing this up.
>>     
>
> Gack! I'm surprised it worked at all. The fact that the BKL allows itself to
> be taken recursively really masked your filesystem bug. If the BKL had
> blocked, or asserted, the bug would never have happened.
>   

Yeah, recall that I'm using a very old kernel (circa 2.6.18) which I 
think must still allow the BKL to be acquired recursively.

> This is as good a time as any to point out that the BKL's use in the lockd
> code is insidious and needs some serious attention. 
No disagreement here! I think I almost understand enough about lockd to 
remove the BKL, but the operative word there is "almost".


Rob Gardner