linux-kernel - Re: Regression in next with use printk

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Date:   Wed, 15 Feb 2017 01:56:45 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Tony Lindgren <tony@...mide.com>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>, linux-pm@...r.kernel.org,
        John Stultz <john.stultz@...aro.org>
Subject: Re: Regression in next with use printk_safe buffers in printk

On (02/14/17 17:18), Peter Zijlstra wrote:
> On Wed, Feb 15, 2017 at 01:01:40AM +0900, Sergey Senozhatsky wrote:
> > 
> > but I'm a bit confused by rt_b->rt_runtime_lock in this unsafe lock
> > scenario (so it's not ABBA, but ABAD)
> > 
> > >   lock(hrtimer_bases.lock);
> > >                                lock(&rt_b->rt_runtime_lock);
> > >                                lock(hrtimer_bases.lock);
> > >   lock(tk_core);
> > >
> > >
> > > Chain exists of:
> > >
> > >	tk_core --> &rt_b->rt_runtime_lock --> hrtimer_bases.lock
> > 
> > 
> > I'm lacking some knowledge here, sorry. where does the tk_core --> &rt_b->rt_runtime_lock
> > come from?
> 
> rt_b->rt_runtime_lock is one of the scheduler locks, since we do
> printk() under tk_core, which does semaphore muck, which then includes
> the entire scheduler chain of locks.

thanks, Peter.

that crossed my mind, but I kinda assumed that we do printk() from
under tk_core using sched fair, and rt_runtime_lock is from sched rt.


so something like below, perhaps. would be helpful if Tony can test it.

(I'll send out this patch 'in a proper way' tomorrow, after some sleep,
it's 2am here).

8< ====

>From e1755b0bf7f8a0be5fdf4dd7303bf4cd150d9d20 Mon Sep 17 00:00:00 2001
From: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Date: Wed, 15 Feb 2017 01:42:18 +0900
Subject: [PATCH] time/timekeeping_debug: use printk_deferred()

Do not call printk() from tk_debug_account_sleep_time(), because
tk_debug_account_sleep_time() is called under tk_core seq lock.
It's not safe to call printk() under tk_core, because console_sem
invokes scheduled (via wake_up_process()->activate_task()), which,
in turn, can call timekeeping code again, for instance, via
get_time()->ktime_get(). This may result in infinite loop on
tk_core.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
---
 kernel/time/timekeeping_debug.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/time/timekeeping_debug.c b/kernel/time/timekeeping_debug.c
index ca9fb800336b..b8f7146c3538 100644
--- a/kernel/time/timekeeping_debug.c
+++ b/kernel/time/timekeeping_debug.c
@@ -75,7 +75,8 @@ void tk_debug_account_sleep_time(struct timespec64 *t)
 	int bin = min(fls(t->tv_sec), NUM_BINS-1);
 
 	sleep_time_bin[bin]++;
-	pr_info("Suspended for %lld.%03lu seconds\n", (s64)t->tv_sec,
+	printk_deferred(KERN_INFO "Suspended for %lld.%03lu seconds\n",
+			(s64)t->tv_sec,
 			t->tv_nsec / NSEC_PER_MSEC);
 }
 
-- 
2.11.1