linux-kernel - Re: [PATCH 38/40] cifs: use workqueue instead of slow-work

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Date:	Wed, 20 Jan 2010 09:15:14 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Jeff Layton <jlayton@...hat.com>
CC:	torvalds@...ux-foundation.org, mingo@...e.hu, peterz@...radead.org,
	awalls@...ix.net, linux-kernel@...r.kernel.org, jeff@...zik.org,
	akpm@...ux-foundation.org, jens.axboe@...cle.com,
	rusty@...tcorp.com.au, cl@...ux-foundation.org,
	dhowells@...hat.com, arjan@...ux.intel.com, avi@...hat.com,
	johannes@...solutions.net, andi@...stfloor.org,
	Steve French <sfrench@...ba.org>
Subject: Re: [PATCH 38/40] cifs: use workqueue instead of slow-work

Hello,

On 01/19/2010 09:20 PM, Jeff Layton wrote:
>> @@ -584,13 +583,13 @@ is_valid_oplock_break(struct smb_hdr *buf, struct TCP_Server_Info *srv)
>>  				pCifsInode->clientCanCacheAll = false;
>>  				if (pSMB->OplockLevel == 0)
>>  					pCifsInode->clientCanCacheRead = false;
>> -				rc = slow_work_enqueue(&netfile->oplock_break);
>> -				if (rc) {
>> -					cERROR(1, ("failed to enqueue oplock "
>> -						   "break: %d\n", rc));
>> -				} else {
>> -					netfile->oplock_break_cancelled = false;
>> -				}
>> +
>> +				cifs_oplock_break_get(netfile);
>> +				if (!queue_work(system_single_wq,
>> +						&netfile->oplock_break))
>> +					cifs_oplock_break_put(netfile);
>> +				netfile->oplock_break_cancelled = false;
>> +
>>  				read_unlock(&GlobalSMBSeslock);
>>  				read_unlock(&cifs_tcp_ses_lock);
>>  				return true;
> 
> This block of code looks problematic. This code is run by the
> cifs_demultiplex_thread (cifsd). We can't do an oplock_break_put in
> this context, since it might trigger a blocking SMB and cause a
> deadlock.

Okay, thanks for pointing it out.

> A while back, I backported this code to earlier kernels and used a
> standard workqueue there. What I did there was to only do the "get" if
> the queue_work succeeded, and then had the queued work take and
> immediately drop the GlobalSMBSeslock first thing. Yes, it's ugly, but
> it prevented the possible deadlock and didn't require adding anything
> like completion vars to the struct.

Hmmm... Why is locking GlobalSMBSeslock necessary?
cifs_oplock_break_get() can never fail and it seems that
is_valid_oplock_break() should be holding valid reference by the time
it enqueues the work, so wouldn't the following be sufficient?

	if (queue_work(system_single_wq, &netfile->oplock_break))
		cifs_oplock_break_get(netfile);

> Also, this change seems to have changed the logic a bit. The
> oplock_break_cancelled flag is being set to false unconditionally, and
> the printk was dropped. Not a big deal on the last part, but we can't
> really do much with errors in this codepath so it might be helpful to
> have some indication that there are problems here.

The thing is that slow_work_enqueue() can only fail if getting a
reference fails.  In cifs' case, it always succeeds so there's no
failure case to handle there.

> Other than the above problems (which are easily fixable), this patch
> seems fine.

Thanks.

-- 
tejun
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