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@xxxxxxxxxxxxxxxxxxx> > > 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
