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

[Tejun Heo]

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 at I-love.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