On Wed 2021-11-10 10:37:26, Daniel Vetter wrote: > On Tue, Nov 09, 2021 at 10:38:47PM +0100, Peter Zijlstra wrote: > > On Tue, Nov 09, 2021 at 12:06:48PM -0800, Sultan Alsawaf wrote: > > > Hi, > > > > > > I encountered a printk deadlock on 5.13 which appears to still affect the latest > > > kernel. The deadlock occurs due to printk being used while having the current > > > CPU's runqueue locked, and the underlying framebuffer console attempting to lock > > > the same runqueue when printk tries to flush the log buffer. > > > > Yes, that's a known 'feature' of some consoles. printk() is in the > > process of being reworked to not call con->write() from the printk() > > calling context, which would go a long way towards fixing this. > > And it's getting worse by the year: > - direct scanout displays (i.e. just a few mmio writes and it will show > up) are on the way out at least in laptops, everyone gets self-refresh > (dp psr) under software control, so without being able to kick a kthread > off nothing shows up except more oopses printk() tries to avoid this problem by waking processes via irq_vork(). For example, it is used for waking userspace log daemons, see wake_up_klogd(). There is one hidden wake_up_process() in up() of @console_sem. It is "less" critical because it calls the wake_up() only when there is another process waiting for console_lock(). It must be a non-printk caller because printk() uses trylock and does not add waiters. So, there is a non-trivial chance to avoid the deadlock and the messages. I wonder if fbdev might use the same trick. > - because of the impendence mismatch between fbdev and drm-kms we even go > ever more this direction for dumb framebuffers, including the firmware > boot-up framebuffer simpledrm. This could perhaps be fixed with a new > dedicate console driver directly on top of drm-kms, but that's on the > wishlist for years and I don't see anyone typing that. > > So yeah for fbcon at least I think we really should throw out direct > con->write from printk completely. Yup, that would be great. Well, I guess that it would be available only for a panic mode when it will not race with other users? Best Regards, Petr
