Re: [RFC] Are you good with Lockdep?

[Matthew Wilcox]

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.