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Date:   Wed, 18 Nov 2020 03:30:05 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Boqun Feng <boqun.feng@gmail.com>
Cc:     Byungchul Park <byungchul.park@lge.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, torvalds@linux-foundation.org,
        peterz@infradead.org, mingo@redhat.com, will@kernel.org,
        linux-kernel@vger.kernel.org, joel@joelfernandes.org,
        alexander.levin@microsoft.com, daniel.vetter@ffwll.ch,
        chris@chris-wilson.co.uk, duyuyang@gmail.com,
        johannes.berg@intel.com, tj@kernel.org, tytso@mit.edu,
        david@fromorbit.com, amir73il@gmail.com, bfields@fieldses.org,
        gregkh@linuxfoundation.org, kernel-team@lge.com
Subject: Re: [RFC] Are you good with Lockdep?
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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.