linux-kernel - Re: [RFC] Are you good with Lockdep?

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Message-ID: <20201118033005.GD29991@casper.infradead.org>
Date:   Wed, 18 Nov 2020 03:30:05 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Byungchul Park <byungchul.park@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, torvalds@...ux-foundation.org,
        peterz@...radead.org, mingo@...hat.com, will@...nel.org,
        linux-kernel@...r.kernel.org, joel@...lfernandes.org,
        alexander.levin@...rosoft.com, daniel.vetter@...ll.ch,
        chris@...is-wilson.co.uk, duyuyang@...il.com,
        johannes.berg@...el.com, tj@...nel.org, tytso@....edu,
        david@...morbit.com, amir73il@...il.com, bfields@...ldses.org,
        gregkh@...uxfoundation.org, kernel-team@....com
Subject: Re: [RFC] Are you good with Lockdep?

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.