Re: [Drbd-dev] FLUSH/FUA documentation & code discrepancy

From: Kent Overstreet <koverstreet@google.com>
To: Tejun Heo <tj@kernel.org>
Cc: Lars Ellenberg <lars.ellenberg@linbit.com>,
	Philipp Reisner <philipp.reisner@linbit.com>,
	Jens Axboe <axboe@kernel.dk>,
	linux-kernel@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
	drbd-dev@lists.linbit.com, Vivek Goyal <vgoyal@redhat.com>,
	NeilBrown <neilb@suse.de>
Subject: Re: [Drbd-dev] FLUSH/FUA documentation & code discrepancy
Date: Mon, 10 Sep 2012 16:31:59 -0700	[thread overview]
Message-ID: <20120910233159.GE19739@google.com> (raw)
In-Reply-To: <20120910230654.GF7677@google.com>

cc'ing Neil

On Mon, Sep 10, 2012 at 04:06:54PM -0700, Tejun Heo wrote:
> Hello, again.
> 
> cc'ing Kent and Vivek.  The original thread is at
> 
>   http://thread.gmane.org/gmane.linux.network.drbd.devel/2130
> 
> On Mon, Sep 10, 2012 at 03:54:42PM -0700, Tejun Heo wrote:
> > > We can possibly work around that by introducing an additional submitter thread,
> > > or at least our own list where we queue assembled bios until the lower
> > > level device queue drains.
> > > 
> > > But we'd rather have the elevator see the FLUSH/FUA,
> > > and treat them as at least a soft barrier/reorder boundary.
> > > 
> > > I may be wrong here, but all the necessary bits for this seem to be in
> > > place already, if the information would even reach the elevator in one
> > > way or other, and not be completely stripped away early.
> > > 
> > > What would you rather see, the elevator recognizing reorder boundaries?
> > > Or additional higher level queueing and extra thread/work queue/whatever?
> > > 
> > > Both are fine with me, I'm just asking for an opinion.
> > 
> > First of all, using FLUSH/FUA for such purpose is an error-prone
> > abuse.  You're trying to exploit an implementation detail which may
> > change at any time.  I think what you want is to be able to specify
> > REQ_SOFTBARRIER on bio submission, which shouldn't be too hard but I'm
> > still lost why this is necessary.  Can you please explain it a bit
> > more?
> 
> The problem with exposing REQ_SOFTBARRIER at bio submission is that it
> would require block layer not to reorder bios while passing through
> stacked adrivers until it reaches a rq-based driver.  I *suspect* this
> has been true until now but Kent's pending patch to fix possible
> deadlock issue breaks that.

Yeah, you might be right about that. I think Neil Brown would know
better than I if this ordering was ever explicitly broken.

But I don't think anything else is relying on that kind of ordering any
more.

>   http://thread.gmane.org/gmane.linux.kernel.bcache.devel/1017/focus=1356250
> 
> As for what the resolution should be, urgh... I don't know. :(