Re: [PATCH] bfq: silence lockdep for bfqd/ioc lock inversion

[Khazhy Kumykov]

On Wed, May 5, 2021 at 6:44 AM Paolo Valente <paolo.valente@xxxxxxxxxx> wrote:
>
> Hi Jan, Khazhy,
> sorry for my super delay.  Thanks Khazy for spotting this, and Jan for
> proposing alternative solutions.
>
> > Il giorno 15 apr 2021, alle ore 12:47, Jan Kara <jack@xxxxxxx> ha scritto:
> >
> > On Wed 14-04-21 11:33:14, Khazhy Kumykov wrote:
> >> On Wed, Apr 14, 2021 at 2:54 AM Jan Kara <jack@xxxxxxx> wrote:
> >>>
> >>> On Thu 18-03-21 23:00:15, Khazhismel Kumykov wrote:
> >>>> lockdep warns of circular locking due to inversion between
> >>>> bfq_insert_requests and bfq_exit_icq. If we end freeing a request when
> >>>> merging, we *may* grab an ioc->lock if that request is the last refcount
> >>>> to that ioc. bfq_bio_merge also potentially could have this ordering.
> >>>> bfq_exit_icq, conversely, grabs bfqd but is always called with ioc->lock
> >>>> held.
> >>>>
> >>>> bfq_exit_icq may either be called from put_io_context_active with ioc
> >>>> refcount raised, ioc_release_fn after the last refcount was already
> >>>> dropped, or ioc_clear_queue, which is only called while queue is
> >>>> quiesced or exiting, so the inverted orderings should never conflict.
> >>>>
> >>>> Fixes: aee69d78dec0 ("block, bfq: introduce the BFQ-v0 I/O scheduler as
> >>>> an extra scheduler")
> >>>>
> >>>> Signed-off-by: Khazhismel Kumykov <khazhy@xxxxxxxxxx>
> >>>
> >>> I've just hit the same lockdep complaint. When looking at this another
> >>> option to solve this complaint seemed to be to modify bfq_bio_merge() like:
> >>>
> >>>        ret = blk_mq_sched_try_merge(q, bio, nr_segs, &free);
> >>>
> >>> +       spin_unlock_irq(&bfqd->lock);
> >>>        if (free)
> >>>                blk_mq_free_request(free);
> >>> -       spin_unlock_irq(&bfqd->lock);
> >>>
> >>>        return ret;
> >>>
> >>> to release request outside of bfqd->lock. Because AFAICT there's no good
> >>> reason why we are actually freeing the request under bfqd->lock. And it
> >>> would seem a bit safer than annotating-away the lockdep complaint (as much
> >>> as I don't see a problem with your analysis). Paolo?
> >>
> >> If we can re-order the locking so we don't need the annotation, that
> >> seems better ("inversion is OK so long as either we're frozen or we
> >> have ioc refcount, and we only grab ioc->lock normally if we drop the
> >> last refcount" is a tad "clever"). Though we still need to deal with
> >> blk_mq_sched_try_insert_merge which can potentially free a request.
> >
> > I see, right.
> >
>
> Trying to put pieces together:
> 1) Moving ahead the invocation of spin_unlock_irq(&bfqd->lock) in
> bfq_bio_merge(), as suggested by Jan, seems ok to me as well.  I also
> proposed this change several years ago, but received no feedback.  So
> I followed the conservative approach of not touching what apparently
> works :)
> 2) If I'm not missing anything, then also what Jan suggests below is
> ok.  That is, in blk_mq_sched_try_insert_merge, ioc->lock gets grabbed
> only in case blk_mq_free_request is invoked.  So it is ok to simply move
> the invocation of blk_mq_free_request outside
> blk_mq_sched_try_insert_merge, using the same approach as with
> blk_mq_sched_try_merge in bfq_bio_merge.

Yes, I believe these two changes together should solve the lockdep
warning. The second change requires a bit more plumbing
(blk_mq_sched_try_insert_merge -> elv_attempt_insert_merge ->
blk_attempt_req_merge). In particular elv_attempt_insert_merge could
result in freeing multiple requests, so plumbing that up requires some
more thought

>
> Thanks,
> Paolo
>
> >> (See the first stacktrace). Something simple that I wasn't sure of is:
> >> could we delay bfq_exit_icq work, then avoid the inversion? Simpler to
> >> analyze then.
> >
> > That's problematic because ICQ (referencing BFQQs etc.) is going to be
> > freed after RCU grace period expires. So we cannot really postpone the
> > teardown of bfq_io_cq. What we could do is to modify
> > blk_mq_sched_try_insert_merge() so that it returns request to free
> > similarly to blk_mq_sched_try_merge().  Then we can free the request after
> > dropping bfqd->lock.
> >
> >                                                               Honza
> >
> >>>> ---
> >>>> block/bfq-iosched.c | 9 ++++++++-
> >>>> 1 file changed, 8 insertions(+), 1 deletion(-)
> >>>>
> >>>> Noticed this lockdep running xfstests (generic/464) on top of a bfq
> >>>> block device. I was also able to tease it out w/ binary trying to issue
> >>>> requests that would end up merging while rapidly swapping the active
> >>>> scheduler. As far as I could see, the deadlock would not actually occur,
> >>>> so this patch opts to change lock class for the inverted case.
> >>>>
> >>>> bfqd -> ioc :
> >>>> [ 2995.524557] __lock_acquire+0x18f5/0x2660
> >>>> [ 2995.524562] lock_acquire+0xb4/0x3a0
> >>>> [ 2995.524565] _raw_spin_lock_irqsave+0x3f/0x60
> >>>> [ 2995.524569] put_io_context+0x33/0x90.  -> ioc->lock grabbed
> >>>> [ 2995.524573] blk_mq_free_request+0x51/0x140
> >>>> [ 2995.524577] blk_put_request+0xe/0x10
> >>>> [ 2995.524580] blk_attempt_req_merge+0x1d/0x30
> >>>> [ 2995.524585] elv_attempt_insert_merge+0x56/0xa0
> >>>> [ 2995.524590] blk_mq_sched_try_insert_merge+0x4b/0x60
> >>>> [ 2995.524595] bfq_insert_requests+0x9e/0x18c0.    -> bfqd->lock grabbed
> >>>> [ 2995.524598] blk_mq_sched_insert_requests+0xd6/0x2b0
> >>>> [ 2995.524602] blk_mq_flush_plug_list+0x154/0x280
> >>>> [ 2995.524606] blk_finish_plug+0x40/0x60
> >>>> [ 2995.524609] ext4_writepages+0x696/0x1320
> >>>> [ 2995.524614] do_writepages+0x1c/0x80
> >>>> [ 2995.524621] __filemap_fdatawrite_range+0xd7/0x120
> >>>> [ 2995.524625] sync_file_range+0xac/0xf0
> >>>> [ 2995.524642] __x64_sys_sync_file_range+0x44/0x70
> >>>> [ 2995.524646] do_syscall_64+0x31/0x40
> >>>> [ 2995.524649] entry_SYSCALL_64_after_hwframe+0x44/0xae
> >>>>
> >>>> ioc -> bfqd
> >>>> [ 2995.524490] _raw_spin_lock_irqsave+0x3f/0x60
> >>>> [ 2995.524498] bfq_exit_icq+0xa3/0xe0 -> bfqd->lock grabbed
> >>>> [ 2995.524512] put_io_context_active+0x78/0xb0 -> ioc->lock grabbed
> >>>> [ 2995.524516] exit_io_context+0x48/0x50
> >>>> [ 2995.524519] do_exit+0x7e9/0xdd0
> >>>> [ 2995.524526] do_group_exit+0x54/0xc0
> >>>> [ 2995.524530] __x64_sys_exit_group+0x18/0x20
> >>>> [ 2995.524534] do_syscall_64+0x31/0x40
> >>>> [ 2995.524537] entry_SYSCALL_64_after_hwframe+0x44/0xae
> >>>>
> >>>> Another trace where we grab ioc -> bfqd through bfq_exit_icq is when
> >>>> changing elevator
> >>>>               -> #1 (&(&bfqd->lock)->rlock){-.-.}:
> >>>> [  646.890820]        lock_acquire+0x9b/0x140
> >>>> [  646.894868]        _raw_spin_lock_irqsave+0x3b/0x50
> >>>> [  646.899707]        bfq_exit_icq_bfqq+0x47/0x1f0
> >>>> [  646.904196]        bfq_exit_icq+0x21/0x30
> >>>> [  646.908160]        ioc_destroy_icq+0xf3/0x130
> >>>> [  646.912466]        ioc_clear_queue+0xb8/0x140
> >>>> [  646.916771]        elevator_switch_mq+0xa4/0x3c0
> >>>> [  646.921333]        elevator_switch+0x5f/0x340
> >>>>
> >>>> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> >>>> index 95586137194e..cb50ac0ffe80 100644
> >>>> --- a/block/bfq-iosched.c
> >>>> +++ b/block/bfq-iosched.c
> >>>> @@ -5027,7 +5027,14 @@ static void bfq_exit_icq_bfqq(struct bfq_io_cq *bic, bool is_sync)
> >>>>      if (bfqq && bfqd) {
> >>>>              unsigned long flags;
> >>>>
> >>>> -             spin_lock_irqsave(&bfqd->lock, flags);
> >>>> +             /* bfq_exit_icq is usually called with ioc->lock held, which is
> >>>> +              * inverse order from elsewhere, which may grab ioc->lock
> >>>> +              * under bfqd->lock if we merge requests and drop the last ioc
> >>>> +              * refcount. Since exit_icq is either called with a refcount,
> >>>> +              * or with queue quiesced, use a differnet lock class to
> >>>> +              * silence lockdep
> >>>> +              */
> >>>> +             spin_lock_irqsave_nested(&bfqd->lock, flags, 1);
> >>>>              bfqq->bic = NULL;
> >>>>              bfq_exit_bfqq(bfqd, bfqq);
> >>>>              bic_set_bfqq(bic, NULL, is_sync);
> >>>> --
> >>>> 2.31.0.rc2.261.g7f71774620-goog
> >>>>
> >>> --
> >>> Jan Kara <jack@xxxxxxxx>
> >>> SUSE Labs, CR
> >
> >
> > --
> > Jan Kara <jack@xxxxxxxx>
> > SUSE Labs, CR
>

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