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
