linux-kernel - Re: [PATCH v2] workqueue: Warn flush attempt using system-wide workqueues

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Message-ID: <Yhaop3T53bHo7v2I@slm.duckdns.org>
Date:   Wed, 23 Feb 2022 11:35:35 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Marek Szyprowski <m.szyprowski@...sung.com>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        linux-rpi-kernel <linux-rpi-kernel@...ts.infradead.org>,
        Bart Van Assche <bvanassche@....org>, jgg@...pe.ca,
        linux-kernel@...r.kernel.org,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Haakon Bugge <haakon.bugge@...cle.com>,
        DRI mailing list <dri-devel@...ts.freedesktop.org>,
        Nicolas Saenz Julienne <nsaenz@...nel.org>
Subject: Re: [PATCH v2] workqueue: Warn flush attempt using system-wide
 workqueues

On Wed, Feb 23, 2022 at 10:20:47PM +0100, Marek Szyprowski wrote:
> Hi All,
> 
> On 17.02.2022 12:22, Tetsuo Handa wrote:
> > syzbot found a circular locking dependency which is caused by flushing
> > system_long_wq WQ [1]. Tejun Heo commented that it makes no sense at all
> > to call flush_workqueue() on the shared workqueues as the caller has no
> > idea what it's gonna end up waiting for.
> >
> > Although there is flush_scheduled_work() which flushes system_wq WQ with
> > "Think twice before calling this function! It's very easy to get into
> > trouble if you don't take great care." warning message, it will be too
> > difficult to guarantee that all users safely flush system-wide WQs.
> >
> > Therefore, let's change the direction to that developers had better use
> > their own WQs if flushing is inevitable. To give developers time to update
> > their modules, for now just emit a warning message when flush_workqueue()
> > or flush_work() is called on system-wide WQs. We will eventually convert
> > this warning message into WARN_ON() and kill flush_scheduled_work().
> >
> > Link: https://syzkaller.appspot.com/bug?extid=831661966588c802aae9 [1]
> > Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> 
> This patch landed in linux next-20220222 as commit 4a6a0ce060e4 
> ("workqueue: Warn flush attempt using system-wide workqueues"). As it 
> might be expected it exposed some calls to flush work. However it also 
> causes boot failure of the Raspberry Pi 3 and 4 boards (kernel compiled 
> from arm64/defconfig). In the log I see one call from the 
> deferred_probe_initcall(), but it isn't critical for the boot process. 
> The deadlock occurs when DRM registers emulated framebuffer on RPi4. 
> RPi3 boots a bit further, to the shell prompt, but then the console is 
> freezed. Reverting this patch on top of linux-next 'fixes' the boot.

Tetsuo, can you please revert the patch? The patch is incorrect in that it's
triggering also on work item flushes, not just workqueue flushes.

Thanks.

-- 
tejun