On Thu 2018-02-01 11:46:47, Sergey Senozhatsky wrote: > On (01/30/18 13:23), Petr Mladek wrote: > [..] > > > If the system is in "big troubles" then what makes irq_work more > > > possible? Local IRQs can stay disabled, just like preemption. I > > > guess when the troubles are really big our strategy is the same > > > for both wq and irq_work solutions - we keep the printk_safe buffer > > > and wait for panic()->flush. > > > > But the patch still uses irq work because queue_work_on() could not > > be safely called from printk_safe(). By other words, it requires > > both irq_work and workqueues to be functional. > > Right, that's all true. The reason it's done this way is because buffers can > be big and we still flush under console_sem in console_unlock() loop, which > can in theory be problematic. In other words, I wanted to remove the root > cause - irq flush of printk_safe while we are still in printing > loop.
Good point! We know that we would eventually push non-trivial amount of messages and it would make sense to do it from non-atomic context. On the other hand, this does not solve the same problem with printk NMI buffer. And I guess that we do not want to risk offloading to workqueues for NMI messages. > > > I guess I'm OK with the wq dependency after all, but I may be mistaken. > > > printk_safe was never about "immediately flush the buffer", it was about > > > "avoid deadlocks", which was extended to "flush from any context which > > > will let us to avoid deadlock". It just happened that it inherited > > > irq_work dependency from printk_nmi. > > > > I see the point. But if I remember correctly, it was also designed > > before we started to be concerned about a sudden death and "get > > printks out ASAP" mantra. > > Can you elaborate a bit? The pull request with printk_safe was sent on February 22, 2017, see https://lkml.kernel.org/r/20170222114705.ga30...@linux.suse The printk softlockup was still being solved by an immediate offload from vprintk_emit() on March 29, 2017, see https://lkml.kernel.org/r/20170329092511.3958-3-sergey.senozhat...@gmail.com I believe that it was the mail from Pavel Machek that made us thinking about the sudden death. It was sent on April 7, 2017, see https://lkml.kernel.org/r/20170407120642.GB4756@amd The first version with the offload from console_unlock was sent on May 9, 2017, see https://lkml.kernel.org/r/20170509082859.854-3-sergey.senozhat...@gmail.com I am not exactly sure when the "get printks out ASAP" mantra started but I cannot forget the mail from June 30, 2017, see https://lkml.kernel.org/r/20170630070131.GA474@jagdpanzerIV.localdomain Best Regards, Petr
