linux-kernel - Re: [RFC] Dept(Dependency Tracker) Report Example

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Message-ID: <20201123121439.GD9464@X58A-UD3R>
Date:   Mon, 23 Nov 2020 21:14:39 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     torvalds@...ux-foundation.org, peterz@...radead.org,
        mingo@...hat.com, will@...nel.org
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de,
        rostedt@...dmis.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,
        willy@...radead.org, david@...morbit.com, amir73il@...il.com,
        bfields@...ldses.org, gregkh@...uxfoundation.org,
        kernel-team@....com
Subject: Re: [RFC] Dept(Dependency Tracker) Report Example

On Mon, Nov 23, 2020 at 08:13:32PM +0900, Byungchul Park wrote:
> [    0.995081] ===================================================
> [    0.995619] Dept: Circular dependency has been detected.
> [    0.995816] 5.9.0+ #8 Tainted: G        W        
> [    0.996493] ---------------------------------------------------
> [    0.996493] summary
> [    0.996493] ---------------------------------------------------
> [    0.996493] *** AA DEADLOCK ***
> [    0.996493] 
> [    0.996493] context A
> [    0.996493]     [S] __mutex_lock(&dev->mutex:0)
> [    0.996493]     [W] __mutex_lock(&dev->mutex:0)
> [    0.996493]     [E] __mutex_unlock(&dev->mutex:0)
> [    0.996493] 
> [    0.996493] [S]: start of the event context
> [    0.996493] [W]: the wait blocked
> [    0.996493] [E]: the event not reachable

Let me explain what [S], [W] and [E] mean using example:

1. In the case of typical locks:

   if condition a
      lock(&a);		<- [S] start of the event context for the event, unlock(&a)
			   [W] wait as well
      ...
      unlock(&a);	<- [E] event to someone who has been waiting lock &a to
			       be released
   else
      lock(&b);		<- [S] start of the event context for the event, unlock(&b)
			   [W] wait as well
      ...
      unlock(&b);	<- [E] event to someone who has been waiting lock &b to
			       be released

2. In the case of general wait and event:

   THREAD 1
      trigger_the_event_context_to_go();
      ...
      wait_for_something(&c); <- [W] wait
         store_timestamp();

   THREAD 2
      notice_someone_triggered_me();
      ...

      (somewhere can see the timestamp of wait_for_something(&c) in THREAD 1)
      <- [S] start of the event context for the event, do_something(&c)

      ...
      do_something(&c); <- [E] event the wait is waiting for in THREAD 1

Thanks,
Byungchul