Messages in this thread | | | Date | Tue, 23 Aug 2016 14:19:03 +0200 | From | Petr Mladek <> | Subject | Re: [PATCH v10 1/2] printk: Make printk() completely async |
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On Mon 2016-08-22 13:15:20, Sergey Senozhatsky wrote: > Hello, > > On (08/20/16 14:24), Sergey Senozhatsky wrote: > > On (08/19/16 21:00), Jan Kara wrote: > > > > > depending on .config BUG() may never return back -- passing control > > > > > to do_exit(), so printk_deferred_exit() won't be executed. thus we > > > > > probably need to have a per-cpu variable that would indicate that > > > > > we are in deferred_bug. hm... but do we really need deferred BUG() > > > > > in the first place? > > > > > > > > Good question. I am not aware of any BUG_ON() that would be called from > > > > wake_up_process() but it is hard to check everything. > > > > > > > > A conservative approach would be to force synchronous printk from > > > > BUG_ON(). > > > > > > Just a quick thought: Cannot we just do printk_deferred_enter() when we are > > > about to call into the scheduler from printk code and printk_deferred_exit() > > > when leaving it? That would look like the least error-prone way how > > > handling this kind of recursion... > > > > interesting idea. > > printk_deferred_enter() increments preempt count, so there may be additional > > obstacles and, as a result, ad-hocs, that scheduler people will sincerely hate. > > need to think more. > > the other thing I just thought of is doing something as follows > !!!not tested, will not compile, just an idea!!! > > --- > > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > index 6e260a0..bb8d719 100644 > --- a/kernel/printk/printk.c > +++ b/kernel/printk/printk.c > @@ -1789,6 +1789,7 @@ asmlinkage int vprintk_emit(int facility, int level, > printk_delay(); > > local_irq_save(flags); > + printk_nmi_enter(); > this_cpu = smp_processor_id();
Huh, this looks very interesting but I am afraid that it will not fly. The problem is that vprintk_nmi() is safe only when it is used exclusively in NMI.
The following could happen with your code:
/**** normar context ****/ vprintk_emit() printk_nmi_enter() ... wake_up_process() WARN() printk() vprintk_nmi() vsnprintf(..., "0123456789")
/* real NMI comes after writing "01234" */ /**** NMI context ****/ vprintk_nmi(); vsnprintf(..., "abcdefghijklmno");
/**** normal context ****/ /* we finish writing "56789" into the buffer */
=> part of the message from NMI gets broken "abcde56789klmno".
The lockless handling of the NMI per-CPU buffer already is not trivial. I would be afraid to add more hacks to make it writable in all contexts.
I am sorry about the bad news. This was so promising on the first look.
Best Regards, Petr
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