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

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Date:   Sat, 5 Mar 2022 23:15:38 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Theodore Ts'o <tytso@....edu>
Cc:     damien.lemoal@...nsource.wdc.com, linux-ide@...r.kernel.org,
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Subject: Re: Report 2 in ext4 and journal based on v5.17-rc1

On Fri, Mar 04, 2022 at 10:26:23PM -0500, Theodore Ts'o wrote:
> On Fri, Mar 04, 2022 at 09:42:37AM +0900, Byungchul Park wrote:
> > 
> > All contexts waiting for any of the events in the circular dependency
> > chain will be definitely stuck if there is a circular dependency as I
> > explained. So we need another wakeup source to break the circle. In
> > ext4 code, you might have the wakeup source for breaking the circle.
> > 
> > What I agreed with is:
> > 
> >    The case that 1) the circular dependency is unevitable 2) there are
> >    another wakeup source for breadking the circle and 3) the duration
> >    in sleep is short enough, should be acceptable.
> > 
> > Sounds good?
> 
> These dependencies are part of every single ext4 metadata update,
> and if there were any unnecessary sleeps, this would be a major
> performance gap, and this is a very well studied part of ext4.
> 
> There are some places where we sleep, sure.  In some case
> start_this_handle() needs to wait for a commit to complete, and the
> commit thread might need to sleep for I/O to complete.  But the moment
> the thing that we're waiting for is complete, we wake up all of the
> processes on the wait queue.  But in the case where we wait for I/O
> complete, that wakeupis coming from the device driver, when it
> receives the the I/O completion interrupt from the hard drive.  Is
> that considered an "external source"?  Maybe DEPT doesn't recognize
> that this is certain to happen just as day follows the night?  (Well,
> maybe the I/O completion interrupt might not happen if the disk drive
> bursts into flames --- but then, you've got bigger problems. :-)

Almost all you've been blaming at Dept are totally non-sense. Based on
what you're saying, I'm conviced that you don't understand how Dept
works even 1%. You don't even try to understand it before blame.

You don't have to understand and support it. But I can't response to you
if you keep saying silly things that way.

> In any case, if DEPT is going to report these "circular dependencies
> as bugs that MUST be fixed", it's going to be pure noise and I will
> ignore all DEPT reports, and will push back on having Lockdep replaced

Dept is going to be improved so that what you are concerning about won't
be reported.

> by DEPT --- because Lockdep give us actionable reports, and if DEPT

Right. Dept should give actionable reports, too.

> can't tell the difference between a valid programming pattern and a
> bug, then it's worse than useless.

Needless to say.