On Tue, Sep 10, 2013 at 03:20:32PM +1000, NeilBrown wrote:
> On Tue, 10 Sep 2013 12:24:38 +0800 Shaohua Li <shli@xxxxxxxxxx> wrote:
>
> > On Tue, Sep 10, 2013 at 02:06:29PM +1000, NeilBrown wrote:
> > > On Tue, 10 Sep 2013 10:35:55 +0800 Shaohua Li <shli@xxxxxxxxxx> wrote:
> > >
> > > > On Tue, Sep 10, 2013 at 11:13:18AM +1000, NeilBrown wrote:
> > > > > On Mon, 9 Sep 2013 12:33:18 +0800 Shaohua Li <shli@xxxxxxxxxx> wrote:
> > > > > > } else {
> > > > > > + spin_lock(&conf->device_lock);
> > > > > > +
> > > > > > if (atomic_read(&sh->count)) {
> > > > > > BUG_ON(!list_empty(&sh->lru)
> > > > > > && !test_bit(STRIPE_EXPANDING, &sh->state)
> > > > > > @@ -611,13 +725,14 @@ get_active_stripe(struct r5conf *conf, s
> > > > > > sh->group = NULL;
> > > > > > }
> > > > > > }
> > > > > > + spin_unlock(&conf->device_lock);
> > > > >
> > > > > The device_lock is only really needed in the 'else' branch of the if
> > > > > statement. So can we have it only there. i.e. don't take the lock if
> > > > > sh->count is non-zero.
> > > >
> > > > This is correct, I assume this isn't worthy optimizing before. Will fix soon.
> > >
> > > It isn't really about optimising performance. It is about making the code
> > > easier to understand. If we keep the region covered by the lock as small as
> > > reasonably possible, it makes it more obvious to the reader which values are
> > > being protected.
> > >
> > >
> > > > > > - spin_lock_irqsave(&conf->device_lock, flags);
> > > > > > + lock_all_device_hash_locks_irqsave(conf, &flags);
> > > > > > clear_bit(In_sync, &rdev->flags);
> > > > > > mddev->degraded = calc_degraded(conf);
> > > > > > - spin_unlock_irqrestore(&conf->device_lock, flags);
> > > > > > + unlock_all_device_hash_locks_irqrestore(conf, &flags);
> > > > > > set_bit(MD_RECOVERY_INTR, &mddev->recovery);
> > > > >
> > > > > Why do you think you need to take all the hash locks here and elsewhere when
> > > > > ->degraded is set?
> > > > > The lock is only need to ensure that the 'In_sync' flags are consistent with
> > > > > the 'degraded' count.
> > > > > ->degraded isn't used in get_active_stripe so I cannot see how it is relevant
> > > > > to the hash locks.
> > > > >
> > > > > We need to lock everything in raid5_quiesce(). I don't think we need to
> > > > > anywhere else.
> > > >
> > > > init_stripe() accesses some filelds, don't need to protect?
> > >
> > > What fields? Not ->degraded.
> > >
> > > I think the fields that it accesses are effectively protected by the new
> > > seqlock.
> > > If you don't think so, please be explicit.
> >
> > Like raid_disks, previous_raid_disks, chunk_sectors, prev_chunk_sectors,
> > algorithm and so on. They are used in raid5_compute_sector(), stripe_set_idx()
> > and init_stripe(). The former two are called by init_stripe().
>
> Yes. Those are only changed in raid5_start_reshape() and are protected by
> conf->gen_lock.
Ok, I thought I misread degraded as max_degraded, so added unnecessary code.
The last question, in raid5_start_reshape(), I thought we should use seqlock to
protect the '!mddev->sync_thread' case, no?
> If they change while init_stripe is running, the read_seqcount_retry() call in
> make_request() will notice the inconsistency, release the stripe, and try
> again.
>
> I guess we probably need an extra check on gen_lock inside init_stripe().
> i.e. a
> do {
> seq = read_seqcount_begin(&conf->gen_lock);
>
> just after the "remove_hash(sh)", and a
>
> } while (read_seqcount_retry(&conf->gen_lock, seq));
>
> just before the "insert_hash(sh)". That will ensure the stripe inserted into
> the hash is consistent. The read_seqcount_retry() in make_request is still
> needed to ensure that the correct stripe_head is used.
Good point. If it's in hash list, the seqcount check could be skiped.
Thanks,
Shaohua
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