On Tue, Jun 12, 2012 at 2:02 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> On Wed, Jun 6, 2012 at 11:52 PM, Shaohua Li <shli@xxxxxxxxxx> wrote:
>> On Thu, Jun 07, 2012 at 04:35:22PM +1000, NeilBrown wrote:
>>> On Thu, 7 Jun 2012 14:29:39 +0800 Shaohua Li <shli@xxxxxxxxxx> wrote:
>>>
>>> > On Thu, Jun 07, 2012 at 10:54:10AM +1000, NeilBrown wrote:
>>> > > On Mon, 04 Jun 2012 16:01:53 +0800 Shaohua Li <shli@xxxxxxxxxx> wrote:
>>> > >
>>> > > > Add a per-stripe lock to protect stripe specific data, like dev->read,
>>> > > > written, ... The purpose is to reduce lock contention of conf->device_lock.
>>> > >
>>> > > I'm not convinced that you need to add a lock.
>>> > > I am convinced that if you do add one you need to explain exactly what it is
>>> > > protecting.
>>> > >
>>> > > The STRIPE_ACTIVE bit serves as a lock and ensures that only one process can
>>> > > be in handle_stripe at a time.
>>> > > So I don't think dev->read actually needs any protection (though I haven't
>>> > > checked thoroughly).
>>> > >
>>> > > I think the only things that device_lock protects are things shared by
>>> > > multiple stripes, so adding a per-stripe spinlock isn't going to help remove
>>> > > device_lock.
>>> >
>>> > This sounds not true to me. both the async callbacks and request completion
>>> > access stripe data, like dev->read. Such things are not protected by
>>> > STRIPE_ACTIVE bit. Thought we can delete STRIPE_ACTIVE bit with stripe lock
>>> > introduced.
>>>
>>> Please give specifics. What race do you see with access to dev->read that is
>>> not protected by STRIPE_ACTIVE ?
>>
>> For example, ops_complete_biofill() will change dev->read which isn't protected
>> by STRIPE_ACTIVE. add_stripe_bio() checks ->toread ->towrite, which isn't
>> protected by the bit too. Am I missing anything?
>
> STRIPE_ACTIVE is the replacement for the old per-stripe lock. That
> lock never was meant/able to synchronize add_stripe_bio() vs ops_run_*
> (producer vs consumer). That's always been device_lock's job because
> an individual bio may be added to several stripes. If device_lock is
> gone we need a different scheme. That's what tripped me up last time
> I looked at this.
Actually now that I look at add_stripe_bio again, I think it could be
made to work if:
1/ bi_phys_segments is incremented prior to publishing the bio on
to{read|write} otherwise we potentially race with a consumer without a
reference
2/ making sure the overlap checking does not walk off into invalid
bios as it may do once we no longer have a global lock
Outside of that we need a lock for making sure bi->next fields are
updated properly and two threads only collide there on the same
stripe. But I need to think more about the other usages.
--
Dan
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