Re: [RFC] Are you good with Lockdep?

[Byungchul Park]

On Thu, Nov 12, 2020 at 02:52:51PM +0000, Matthew Wilcox wrote:
> On Thu, Nov 12, 2020 at 09:26:12AM -0500, Steven Rostedt wrote:
> > > FYI, roughly Lockdep is doing:
> > > 
> > >    1. Dependency check
> > >    2. Lock usage correctness check (including RCU)
> > >    3. IRQ related usage correctness check with IRQFLAGS
> > > 
> > > 2 and 3 should be there forever which is subtle and have gotten matured.
> > > But 1 is not. I've been talking about 1. But again, it's not about
> > > replacing it right away but having both for a while. I'm gonna try my
> > > best to make it better.
> > 
> > And I believe lockdep does handle 1. Perhaps show some tangible use case
> > that you want to cover that you do not believe that lockdep can handle. If
> > lockdep cannot handle it, it will show us where lockdep is lacking. If it
> > can handle it, it will educate you on other ways that lockdep can be
> > helpful in your development ;-)
> 
> Something I believe lockdep is missing is a way to annotate "This lock
> will be released by a softirq".  If we had lockdep for lock_page(), this
> would be a great case to show off.  The filesystem locks the page, then
> submits it to a device driver.  On completion, the filesystem's bio
> completion handler will be called in softirq context and unlock the page.
> 
> So if the filesystem has another lock which is acquired by the completion
> handler. we could get an ABBA deadlock that lockdep would be unable to see.
> 
> There are other similar things; if you look at the remaining semaphore
> users in the kernel, you'll see the general pattern is that they're
> acquired in process context and then released in interrupt context.
> If we had a way to transfer ownership of the semaphore to a generic
> "interrupt context", they could become mutexes and lockdep could check
> that nothing else will cause a deadlock.

Yes. Those are exactly what Cross-release feature solves. Those problems
can be achieved with Cross-release. But even with Cross-release, we
still cannot solve the problem of (1) readlock handling (2) and false
positives preventing further reporting.

Thanks,
Byungchul