Date:   Mon, 17 Jan 2022 15:38:07 +0100
From:   Jan Kara <jack@...e.cz>
To:     Ritesh Harjani <riteshh@...ux.ibm.com>
Cc:     Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jan Kara <jack@...e.com>,
        Andreas Dilger <adilger.kernel@...ger.ca>, tytso@....edu,
        Eric Whitney <enwlinux@...il.com>
Subject: Re: [PATCH 6/6] jbd2: No need to use t_handle_lock in
 jbd2_journal_wait_updates

On Mon 17-01-22 18:25:27, Ritesh Harjani wrote:
> On 22/01/13 06:08PM, Ritesh Harjani wrote:
> > On 22/01/13 12:27PM, Jan Kara wrote:
> > > On Thu 13-01-22 08:56:29, Ritesh Harjani wrote:
> > > > Since jbd2_journal_wait_updates() uses waitq based on t_updates atomic_t
> > > > variable. So from code review it looks like we don't need to use
> > > > t_handle_lock spinlock for checking t_updates value.
> > > > Hence this patch gets rid of the spinlock protection in
> > > > jbd2_journal_wait_updates()
> > > >
> > > > Signed-off-by: Ritesh Harjani <riteshh@...ux.ibm.com>
> > >
> > > This patch looks good. Feel free to add:
> > >
> > > Reviewed-by: Jan Kara <jack@...e.cz>
> > >
> > > Actually looking at it, t_handle_lock seems to be very much unused. I agree
> 
> Thanks Jan for your help in this.
> I have dropped this patch from v2 in order to discuss few more things and I felt
> killing t_handle_lock completely can be sent in a seperate patch series.

Yes, probably a good choice.

> > I too had this thought in mind. Thanks for taking a deeper look into it :)
> >
> > >
> > > we don't need it when waiting for outstanding handles but the only
> > > remaining uses are:
> > >
> > > 1) jbd2_journal_extend() where it is not needed either - we use
> > > atomic_add_return() to manipulate t_outstanding_credits and hold
> > > j_state_lock for reading which provides us enough exclusion.
> 
> I looked into jbd2_journal_extend and yes, we don't need t_handle_lock
> for updating transaction->t_outstanding_credits, since it already happens with
> atomic API calls.
> 
> Now I do see we update handle->h_**_credits in that function.
> But I think this is per process (based on task_struct, current->journal_info)
> and doesn't need a lock protection right?

Yes, handle is per process so no lock is needed there.

> > > 2) update_t_max_wait() - this is the only valid use of t_handle_lock but we
> > > can just switch it to cmpxchg loop with a bit of care. Something like:
> > >
> > > 	unsigned long old;
> > >
> > > 	ts = jbd2_time_diff(ts, transaction->t_start);
> > > 	old = transaction->t_max_wait;
> > > 	while (old < ts)
> > > 		old = cmpxchg(&transaction->t_max_wait, old, ts);
> 
> I think there might be a simpler and more straight forward way for updating
> t_max_wait.
> 
> I did look into the t_max_wait logic and where all we are updating it.
> 
> t_max_wait is the max wait time in starting (&attaching) a _new_ running
> transaction by a handle. Is this understaning correct?

Correct. It is the maximum time we had to wait for a new transaction to be
created.

> From code I don't see t_max_wait getting updated for the time taken in order
> to start the handle by a existing running transaction.
> 
> Here is how -
> update_t_max_wait() will only update t_max_wait if the
> transaction->t_start is after ts
> (ts is nothing but when start_this_handle() was called).
> 
> 1. This means that for transaction->t_start to be greater than ts, it has to be
>    the new transaction that gets started right (in start_this_handle() func)?
>
> 2. Second place where transaction->t_start is updated is just after the start of
>    commit phase 7. But this only means that this transaction has become the
>    commit transaction. That means someone has to alloc a new running transaction
>    which again is case-1.
> 
> Now I think this spinlock was added since multiple processes can start a handle
> in parallel and attach a running transaction.
> 
> Also this was then moved within CONFIG_JBD2_DEBUG since to avoid spinlock
> contention on a SMP system in starting multiple handles by different processes.
> 
> Now looking at all of above, I think we can move update_t_max_wait()
> inside jbd2_get_transaction() in start_this_handle(). Because that is where
> a new transaction will be started and transaction->t_start will be greater then
> ts. This also is protected within j_state_lock write_lock, so we don't need
> spinlock.

All above is correct upto this point. The catch is there can be (and often
are) more processes in start_this_handle() waiting in
wait_transaction_switching() and then racing to create the new transaction.
The process calling jbd2_get_transaction() is not necessarily the one which
entered start_this_handle() first and thus t_max_wait would not be really
the maximum time someone had to wait.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

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