[Unionfs] Locking issue with unionfs

[Olivier Blin]

Erez Zadok <ezk at cs.sunysb.edu> writes:

>> Looking at above, we can see that unionfs is locked at unionfs_lookup, 
>> probably at "unionfs_read_lock(sb);". Then if you analyze the stack trace 
>> above looking at the other unionfs functions, you discover that in  
>> unionfs_remount_fs there is a "unionfs_write_lock(sb)", and because this the 
>> deadlock happens. I made a patch that at least seems to have worked here on a 
>> quick test as a workaround, I don't know yet if it has some side effect or if 
>> could be the proper fix for this locking issue, here it's the patch:
>> 
>> --- linux-2.6.22.10-0.2mdv/fs/unionfs/super.c	2007-10-18 
>> 17:11:39.000000000 -0200
>> +++ linux-2.6.22.10-0.2mdv.mod/fs/unionfs/super.c	2007-10-18 
>> 16:56:39.000000000 -0200
>> @@ -587,10 +587,12 @@ static int unionfs_remount_fs(struct sup
>>  		}
>>  
>>  		if (!strcmp("add", optname)) {
>> +			unionfs_write_unlock(sb);
>>  			err = do_remount_add_option(optarg, new_branches,
>>  						    tmp_data,
>>  						    tmp_lower_paths,
>>  						    &new_high_branch_id);
>> +			unionfs_write_lock(sb);
>>  			if (err)
>>  				goto out_release;
>>  			new_branches++;
>> 
>> I just unlock temporarily sb, probably not the right fix, as I didn't 
>> understand well the locking scheme yet.
>
> Herton, your fix is probably ok in the short term.  I'm looking into a way
> to solve this deadlock more cleanly.  It's not easy b/c a read-write
> semaphore in linux has no easy way to allow the same lock-owner who holds
> the write lock, to also get a readlock.

Of course, the same is valid for mode/del operations:

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Olivier Blin (blino) - Mandriva