linux-kernel - Re: [PATCH 1/2] btrfs: drop trace_btrfs_all_work_done() from normal_work

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Date:   Wed, 21 Dec 2016 09:28:37 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Qu Wenruo <quwenruo@...fujitsu.com>
Cc:     dsterba@...e.cz, Chris Mason <clm@...com>,
        Josef Bacik <jbacik@...com>, David Sterba <dsterba@...e.com>,
        linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] btrfs: drop trace_btrfs_all_work_done() from
 normal_work_helper()

On 2016-12-21 08:33:03 [+0800], Qu Wenruo wrote:
> The trace point only uses the pointer, and this helps us to pair with
> btrfs_work_queued/sched.

| /* For situiations that the work is freed */
| DECLARE_EVENT_CLASS(btrfs__work__done,
| 
|         TP_PROTO(struct btrfs_work *work),
| 
|         TP_ARGS(work),
| 
|         TP_STRUCT__entry_btrfs(
|                 __field(        void *, work                    )
|         ),
| 
|         TP_fast_assign_btrfs(btrfs_work_owner(work),
|                 __entry->work           = work;
|         ),
| 
|         TP_printk_btrfs("work->%p", __entry->work)
| );

and btrfs_work_owner exapnds to:

| struct btrfs_fs_info *
| btrfs_work_owner(struct btrfs_work *work)
| {
|         return work->wq->fs_info;
| }
 
voilà

> But I still don't understand why backtrace is triggered.
> Since we're just recording a pointer, not touching it.
> 
> Would you please explain the problem with more details on how it trigger the
> problem?

enabled all events played with the fs which was just an upgrade and git
tree sync + checkout so nothing special.

> > 
> > So I think we should either remove the tracepoint completely or change
> > the arguments to take something else than a potentially freed 'work'.
> 
> I'm mostly OK to remove the tracepoint, but such all_workd_done() trace
> should still help to determine if it's a workqueue stalled.
> 
> Thanks,
> Qu

Sebastian