Re: Question about f_count in struct nlm_file

[Wendy Cheng <wcheng@redhat.com>]

Neil Brown wrote:
> On Thursday March 22, wcheng@redhat.com wrote:
>   
>> Wendy Cheng wrote:
>>
>>     
>>> client does posix lock -->
>>>     server calls nlm4svc_proc_lock() ->
>>>         * server lookup file (f_count++)
>>>         * server lock the file
>>>         * server calls nlm_release_host
>>>         * server calls nlm_release_file (f_count--)
>>>         * server return to client with status 0
>>>
>>> This will cause any call into nlm_traverse_files() to crash in the 
>>> following path, if the file happens to be of "no interest" of the search 
>>> (for example, the "match" function returns FALSE in all cases). Is this 
>>> intentional or oversight ? Would 2.6.21-rc4 be a good base to do NLM 
>>> development work ?
>>>       
>
> Hmmm.... definitely seems to be a bug in there!!
>
> The key to the issue seems to be to make sure we take account of all
> the current locks.
> One way might be to replace
> 			if (list_empty(&file->f_blocks) && !file->f_locks
> 			 && !file->f_shares && !file->f_count) {
> in nlm_traverse_files with a call to nlm_file_inuse(file).
>   

Look reasonable ... an easy and quick fix. This one has my vote.
> Another way would be to try a bit harder to keep f_count uptodate by
> incrementing it in nlmsvc_lock:
>
> diff .prev/fs/lockd/svclock.c ./fs/lockd/svclock.c
> --- .prev/fs/lockd/svclock.c	2007-03-23 15:30:37.000000000 +1100
> +++ ./fs/lockd/svclock.c	2007-03-23 15:33:41.000000000 +1100
> @@ -370,6 +370,7 @@ again:
>  
>  	switch(error) {
>  		case 0:
> +			file->f_locks ++;
>  			ret = nlm_granted;
>  			goto out;
>  		case -EAGAIN:
>
> The former is probably bit safer... but I wish I know what we have
> that f_locks count.  It is not at all clear what it is needed for.
> Maybe some legacy issue that doesn't exist any more...
>   

This f_locks is definitely another awkward counter - better not messing 
with it. My vote is "no". How about removing it from nlm_file for good ?
>
>   
>> I should have made it clear... after nlm_inspect_file(), the logic 
>> unconditionally checks for possible removing of this file. Since the 
>> file is not blocked, nothing to do with shares, and f_count is zero, it 
>> will get removed from hash and fclose() invoked (even it still owns a 
>> plock). This will make VFS very unhappy and BUG() in fs/locks.c:1988 in 
>> the middle of __fput -> locks_remove_flock.
>>
>> On the other hand, the more I think (about this issue), maybe just 
>> looping back after nlm_inspect_file finds no match would be good enough. 
>> Anyway, that's what I'm going to do. Any objection ? Please let me know.
>>     
>
> Could you explain this possible fix a bit more.  I'm not sure what you
> are proposing..
>
>   
No, scratch what I said here (since it is not related) ... will submit 
the first set of failover patches shortly after this mail.

Great thanks (as always) for looking into this.

-- Wendy


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