linux-kernel - Re: [PATCH v10 1/2] printk: Make printk() completely async

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Date:	Thu, 18 Aug 2016 18:51:44 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Petr Mladek <pmladek@...e.com>
Cc:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Jan Kara <jack@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Jan Kara <jack@...e.com>, Tejun Heo <tj@...nel.org>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Byungchul Park <byungchul.park@....com>, vlevenetz@...sol.com,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v10 1/2] printk: Make printk() completely async

On (08/18/16 11:33), Petr Mladek wrote:
[..]
> > > 2. Force synchronous printk inside WARN()/BUG() macros.
> > 
> > will it help? semaphore up() calls wake_up_process() regardless the context.
> > not to mention that we still may have spin_dump() enabled.
> 
> Good point. That changes my preferences :-)
> 
> > 
> > > 3. Force printk_deferred() inside WARN()/BUG() macros via the per-CPU
> > >    printk_func.
> > > 
> > >    It might be elegant. But we do not want this outside the scheduler
> > >    code. Therefore we would need special variants of  WARN_*_SCHED()
> > >    BUG_*_SCHED() macros.
> 
> Also we need to make sure that everything will be done on a single CPU
> as the printk_func is per-CPU variable.

lib/dump_stack.c dump_stack() disables local IRQs, so we will stay
on the same CPU.

schematically dump_stack_deferred() looks like the one below. I don't
dump registers, that will require one more *_deferred() function.
and as you can see I've replaced show_stack() with save_stack_trace().
it's not entirely universal (as of now), but worked for my needs at
that time.

---

asmlinkage __visible void dump_stack_deferred(void)
{
       unsigned long flags;
       unsigned long stack_trace[16];
       int was_locked;
       int old, cpu;
       int i;

       struct stack_trace trace = {
               .nr_entries = 0,
               .max_entries = ARRAY_SIZE(stack_trace),
               .entries = stack_trace,
               .skip = 0,
       };

       local_irq_save(flags);
retry:
       cpu = smp_processor_id();
       old = atomic_cmpxchg(&dump_lock, -1, cpu);
       if (old == -1) {
               was_locked = 0;
       } else if (old == cpu) {
               was_locked = 1;
       } else {
               cpu_relax();
               goto retry;
       }

       save_stack_trace(&trace);

       for (i = 0; i < trace.nr_entries; i++) {
               printk_deferred("[%p] %pS\n",
                               (void *)stack_trace[i],
                               (void *)stack_trace[i]);
       }

       if (!was_locked)
               atomic_set(&dump_lock, -1);

       local_irq_restore(flags);
}

---

	-ss