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From: Tejun Heo <tj@kernel.org>
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: 0day robot <lkp@intel.com>, LKML <linux-kernel@vger.kernel.org>,
	lkp@lists.01.org, kernel test robot <oliver.sang@intel.com>
Subject: Re: [PATCH] workqueue: Use private WQ for schedule_on_each_cpu() API
Date: Wed, 23 Feb 2022 12:29:01 -1000	[thread overview]
Message-ID: <Yha1LeX4OK3cLCV5@slm.duckdns.org> (raw)
In-Reply-To: <9a883d72-ea7d-1936-93e6-5c2a290509d4@I-love.SAKURA.ne.jp>

On Thu, Feb 24, 2022 at 07:26:30AM +0900, Tetsuo Handa wrote:
> > The patch seems pretty wrong. What's problematic is system workqueue flushes
> > (which flushes the entire workqueue), not work item flushes.
> 
> Why? My understanding is that
> 
>   flushing a workqueue waits for completion of all work items in that workqueue
> 
>   flushing a work item waits for for completion of that work item using
>   a workqueue specified as of queue_work()
> 
> and
> 
>   if a work item in some workqueue is blocked by other work in that workqueue
>   (e.g. max_active limit, work items on that workqueue and locks they need),
>   it has a risk of deadlock
> 
> . Then, how can flushing a work item using system-wide workqueues be free of deadlock risk?
> Isn't it just "unlikely to deadlock" rather than "impossible to deadlock"?

If we're jamming system_wq with a combination of work items which need more
than max_active to make forward progress, we're stuck regardless of flushes.
What's needed at that point is increasing max_active (or something along
that line).

Thanks.

-- 
tejun

WARNING: multiple messages have this Message-ID (diff)
From: Tejun Heo <tj@kernel.org>
To: lkp@lists.01.org
Subject: Re: [PATCH] workqueue: Use private WQ for schedule_on_each_cpu() API
Date: Wed, 23 Feb 2022 12:29:01 -1000	[thread overview]
Message-ID: <Yha1LeX4OK3cLCV5@slm.duckdns.org> (raw)
In-Reply-To: <9a883d72-ea7d-1936-93e6-5c2a290509d4@I-love.SAKURA.ne.jp>

[-- Attachment #1: Type: text/plain, Size: 1111 bytes --]

On Thu, Feb 24, 2022 at 07:26:30AM +0900, Tetsuo Handa wrote:
> > The patch seems pretty wrong. What's problematic is system workqueue flushes
> > (which flushes the entire workqueue), not work item flushes.
> 
> Why? My understanding is that
> 
>   flushing a workqueue waits for completion of all work items in that workqueue
> 
>   flushing a work item waits for for completion of that work item using
>   a workqueue specified as of queue_work()
> 
> and
> 
>   if a work item in some workqueue is blocked by other work in that workqueue
>   (e.g. max_active limit, work items on that workqueue and locks they need),
>   it has a risk of deadlock
> 
> . Then, how can flushing a work item using system-wide workqueues be free of deadlock risk?
> Isn't it just "unlikely to deadlock" rather than "impossible to deadlock"?

If we're jamming system_wq with a combination of work items which need more
than max_active to make forward progress, we're stuck regardless of flushes.
What's needed at that point is increasing max_active (or something along
that line).

Thanks.

-- 
tejun

  reply	other threads:[~2022-02-23 22:29 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-02-21  8:33 [workqueue] 29bd199e4e: WARNING:possible_circular_locking_dependency_detected kernel test robot
2022-02-21  8:33 ` kernel test robot
2022-02-21 10:38 ` [PATCH] workqueue: Use private WQ for schedule_on_each_cpu() API Tetsuo Handa
2022-02-21 10:38   ` Tetsuo Handa
2022-02-22 17:29   ` Tejun Heo
2022-02-22 17:29     ` Tejun Heo
2022-02-23  0:57     ` Tetsuo Handa
2022-02-23  0:57       ` Tetsuo Handa
2022-02-23 21:33       ` Tejun Heo
2022-02-23 21:33         ` Tejun Heo
2022-02-23 22:26         ` Tetsuo Handa
2022-02-23 22:26           ` Tetsuo Handa
2022-02-23 22:29           ` Tejun Heo [this message]
2022-02-23 22:29             ` Tejun Heo
2022-02-28 14:03             ` [PATCH v3] workqueue: Warn flushing of kernel-global workqueues Tetsuo Handa
2022-02-28 14:03               ` Tetsuo Handa
2022-02-28 18:42               ` Tejun Heo
2022-02-28 18:42                 ` Tejun Heo
2022-03-17 22:25                 ` Tetsuo Handa
2022-03-17 22:25                   ` Tetsuo Handa

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