Re: [RFC] Are you good with Lockdep?

[Matthew Wilcox]

On Wed, Nov 18, 2020 at 09:45:40AM +0800, Boqun Feng wrote:
> Hi Matthew,
> 
> On Mon, Nov 16, 2020 at 03:37:29PM +0000, Matthew Wilcox wrote:
> [...]
> > 
> > It's not just about lockdep for semaphores.  Mutexes will spin if the
> > current owner is still running, so to convert an interrupt-released
> > semaphore to a mutex, we need a way to mark the mutex as being released
> 
> Could you provide an example for the conversion from interrupt-released
> semaphore to a mutex? I'd like to see if we can improve lockdep to help
> on that case.

How about adb_probe_mutex in drivers/macintosh/adb.c.  Most of
the acquires/releases are within the same task.  But adb_reset_bus()
calls down(&adb_probe_mutex), then schedules adb_reset_work() which runs
adb_probe_task() which calls up(&adb_probe_mutex).

Ideally adb_probe_mutex would become a mutex instead of the semaphore
it currently is.  adb_reset_bus() would pass ownership of the mutex to
kadbprobe since it's the one which must run in order to release the mutex.