Messages in this thread | | | Date | Mon, 2 Mar 2020 10:49:07 +0100 | From | Petr Mladek <> | Subject | Re: Regression in v4.19.106 breaking waking up of readers of /proc/kmsg and /dev/kmsg |
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On Sun 2020-03-01 14:22:19, Sergey Senozhatsky wrote: > On (20/02/29 18:47), Steven Rostedt wrote: > [..] > > > > What do folks think? > > > > > > Well, my 5 cents, there is nothing that prevents "too-early" > > > printk_deferred() calls in the future. From that POV I'd probably > > > prefer to "forbid" printk_deffered() to touch per-CPU deferred > > > machinery until it's not "too early" anymore. Similar to what we > > > do in printk_safe::queue_flush_work(). > > > > I agree that printk_deferred() should handle being called too early. > > But the issue is with per_cpu variables correct? Not the irq_work? > > Correct. printk_deferred() and printk_safe()/printk_nmi() irq_works > are per-CPU. We use "a special" flag in printk_safe()/printk_nmi() to > tell if it's too early to modify per-CPU irq_work or not. > > I believe that we need to use that flag for all printk-safe/nmi > per-CPU data, including buffers, not only for irq_work. Just in > case if printk_safe or printk_nmi, somehow, are being called too > early. > > > We could add a flag in init/main.c after setup_per_cpu_areas() and then > > just have printk_deferred() act like a normal printk(). At that point, > > there shouldn't be an issue in calling printk() directly, is there? > > Sure, this will work. I believe we introduced a "work around" approach > in printk-safe because noone would ACK a global init/main.c flag for > printk(). If we can land a "per_cpu_areas_ready" flag (I've some doubts > here), then yes (!), let's use it and let's remove printk-safe workaround.
A compromise might be to set a flag in setup_log_buf(). It is called much earlier but it seems to be safe enough.
mm_init() is called close after setup_log_buf(). And it seems to be using per-cpu variables when creating caches, see:
+ mm_init() + kmem_cache_init() + create_boot_cache() + __kmem_cache_create() + setup_cpu_cache()
It is just a detail. But I would make the flag independent on the existing printk_safe stuff. printk_safe will get removed with the lockless printk buffer. While the irq_work() will still be needed for the wakeup functions.
Sergey, do you agree and want to update your patch accordingly?
Best Regards, Petr
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