Re: Report 2 in ext4 and journal based on v5.17-rc1

[Byungchul Park]

On Wed, Feb 23, 2022 at 03:48:59PM +0100, Jan Kara wrote:
> On Wed 23-02-22 09:35:34, Byungchul Park wrote:
> > On Mon, Feb 21, 2022 at 08:02:04PM +0100, Jan Kara wrote:
> > > On Thu 17-02-22 20:10:04, Byungchul Park wrote:
> > > > [    9.008161] ===================================================
> > > > [    9.008163] DEPT: Circular dependency has been detected.
> > > > [    9.008164] 5.17.0-rc1-00015-gb94f67143867-dirty #2 Tainted: G        W
> > > > [    9.008166] ---------------------------------------------------
> > > > [    9.008167] summary
> > > > [    9.008167] ---------------------------------------------------
> > > > [    9.008168] *** DEADLOCK ***
> > > > [    9.008168]
> > > > [    9.008168] context A
> > > > [    9.008169]     [S] (unknown)(&(&journal->j_wait_transaction_locked)->dmap:0)
> > > > [    9.008171]     [W] wait(&(&journal->j_wait_commit)->dmap:0)
> > > > [    9.008172]     [E] event(&(&journal->j_wait_transaction_locked)->dmap:0)
> > > > [    9.008173]
> > > > [    9.008173] context B
> > > > [    9.008174]     [S] down_write(mapping.invalidate_lock:0)
> > > > [    9.008175]     [W] wait(&(&journal->j_wait_transaction_locked)->dmap:0)
> > > > [    9.008176]     [E] up_write(mapping.invalidate_lock:0)
> > > > [    9.008177]
> > > > [    9.008178] context C
> > > > [    9.008179]     [S] (unknown)(&(&journal->j_wait_commit)->dmap:0)
> > > > [    9.008180]     [W] down_write(mapping.invalidate_lock:0)
> > > > [    9.008181]     [E] event(&(&journal->j_wait_commit)->dmap:0)
> > > > [    9.008181]
> > > > [    9.008182] [S]: start of the event context
> > > > [    9.008183] [W]: the wait blocked
> > > > [    9.008183] [E]: the event not reachable
> > > 
> > > So what situation is your tool complaining about here? Can you perhaps show
> > > it here in more common visualization like:
> > 
> > Sure.
> > 
> > > TASK1				TASK2
> > > 				does foo, grabs Z
> > > does X, grabs lock Y
> > > blocks on Z
> > > 				blocks on Y
> > > 
> > > or something like that? Because I was not able to decipher this from the
> > > report even after trying for some time...
> > 
> > KJOURNALD2(kthread)	TASK1(ksys_write)	TASK2(ksys_write)
> > 
> > wait A
> > --- stuck
> > 			wait B
> > 			--- stuck
> > 						wait C
> > 						--- stuck
> > 
> > wake up B		wake up C		wake up A
> > 
> > where:
> > A is a wait_queue, j_wait_commit
> > B is a wait_queue, j_wait_transaction_locked
> > C is a rwsem, mapping.invalidate_lock
> 
> I see. But a situation like this is not necessarily a guarantee of a
> deadlock, is it? I mean there can be task D that will eventually call say
> 'wake up B' and unblock everything and this is how things were designed to
> work? Multiple sources of wakeups are quite common I'd say... What does

Yes. At the very beginning when I desgined Dept, I was thinking whether
to support multiple wakeup sources or not for a quite long time.
Supporting it would be a better option to aovid non-critical reports.
However, I thought anyway we'd better fix it - not urgent tho - if
there's any single circle dependency. That's why I decided not to
support it for now and wanted to gather the kernel guys' opinions. Thing
is which policy we should go with.

> Dept do to prevent false reports in cases like this?
> 
> > The above is the simplest form. And it's worth noting that Dept focuses
> > on wait and event itself rather than grabing and releasing things like
> > lock. The following is the more descriptive form of it.
> > 
> > KJOURNALD2(kthread)	TASK1(ksys_write)	TASK2(ksys_write)
> > 
> > wait @j_wait_commit
> > 			ext4_truncate_failed_write()
> > 			   down_write(mapping.invalidate_lock)
> > 
> > 			   ext4_truncate()
> > 			      ...
> > 			      wait @j_wait_transaction_locked
> > 
> > 						ext_truncate_failed_write()
> > 						   down_write(mapping.invalidate_lock)
> > 
> > 						ext4_should_retry_alloc()
> > 						   ...
> > 						   __jbd2_log_start_commit()
> > 						      wake_up(j_wait_commit)
> > jbd2_journal_commit_transaction()
> >    wake_up(j_wait_transaction_locked)
> > 			   up_write(mapping.invalidate_lock)
> > 
> > I hope this would help you understand the report.
> 
> I see, thanks for explanation! So the above scenario is impossible because

My pleasure.

> for anyone to block on @j_wait_transaction_locked the transaction must be
> committing, which is done only by kjournald2 kthread and so that thread
> cannot be waiting at @j_wait_commit. Essentially blocking on
> @j_wait_transaction_locked means @j_wait_commit wakeup was already done.

kjournal2 repeatedly does the wait and the wake_up so the above scenario
looks possible to me even based on what you explained. Maybe I should
understand how the journal things work more for furhter discussion. Your
explanation is so helpful. Thank you really.

Thanks,
Byungchul

> I guess this shows there can be non-trivial dependencies between wait
> queues which are difficult to track in an automated way and without such
> tracking we are going to see false positives...
> 
> 								Honza
> 
> -- 
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR