From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Petr Mladek <pmladek@suse.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
Steven Rostedt <rostedt@goodmis.org>,
Andrew Morton <akpm@linux-foundation.org>,
Peter Zijlstra <peterz@infradead.org>,
Russell King <rmk+kernel@arm.linux.org.uk>,
Daniel Thompson <daniel.thompson@linaro.org>,
Jiri Kosina <jkosina@suse.com>, Ingo Molnar <mingo@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
Chris Metcalf <cmetcalf@ezchip.com>,
linux-kernel@vger.kernel.org, x86@kernel.org,
linux-arm-kernel@lists.infradead.org,
adi-buildroot-devel@lists.sourceforge.net,
linux-cris-kernel@axis.com, linux-mips@linux-mips.org,
linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org,
linux-sh@vger.kernel.org, sparclinux@vger.kernel.org,
Jan Kara <jack@suse.cz>, Ralf Baechle <ralf@linux-mips.org>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Martin Schwidefsky <schwidefsky@de.ibm.com>,
David Miller <davem@davemloft.net>
Subject: Re: [PATCH v5 1/4] printk/nmi: generic solution for safe printk in NMI
Date: Fri, 21 Apr 2017 10:57:25 +0900 [thread overview]
Message-ID: <20170421015724.GA586@jagdpanzerIV.localdomain> (raw)
In-Reply-To: <20170420131154.GL3452@pathway.suse.cz>
Hello,
On (04/20/17 15:11), Petr Mladek wrote:
[..]
> Good analyze. I would summarize it that we need to be careful of:
>
> + logbug_lock
> + PRINTK_SAFE_CONTEXT
> + locks used by console drivers
>
> The first two things are easy to check. Except that a check for logbuf_lock
> might produce false negatives. The last check is very hard.
>
> > so at the moment what I can think of is something like
> >
> > -- check this_cpu_read(printk_context) in NMI prink
> >
> > -- if we are NOT in printk_safe on this CPU, then do printk_deferred()
> > and bypass `nmi_print_seq' buffer
>
> I would add also a check for logbuf_lock.
> > -- if we are in printk_safe
> > -- well... bad luck... have a bigger buffer.
>
> Yup, we do the best effort while still trying to stay on the safe
> side.
>
> I have cooked up a patch based on this. It uses printk_deferred()
> in NMI when it is safe. Note that console_flush_on_panic() will
> try to get them on the console when a kdump is not generated.
> I believe that it will help Steven.
OK. I need to look more at the patch. It does more than I'd expected/imagined.
[..]
> void printk_nmi_enter(void)
> {
> - this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK);
> + /*
> + * The size of the extra per-CPU buffer is limited. Use it
> + * only when really needed.
> + */
> + if (this_cpu_read(printk_context) & PRINTK_SAFE_CONTEXT_MASK ||
> + raw_spin_is_locked(&logbuf_lock)) {
> + this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK);
> + } else {
> + this_cpu_or(printk_context, PRINTK_NMI_DEFERRED_CONTEXT_MASK);
> + }
> }
well... the logbuf_lock can temporarily be locked from another CPU. I'd say
that spin_is_locked() has better chances for false positive than
this_cpu_read(printk_context). because this_cpu_read(printk_context) depends
only on this CPU state, while spin_is_locked() depends on all CPUs. and the
idea with this_cpu_read(printk_context) was that we check if the logbuf_lock
was locked from this particular CPU.
I agree that this_cpu_read(printk_context) covers slightly more than
logbuf_lock scope, so we may get positive this_cpu_read(printk_context)
with unlocked logbuf_lock, but I don't tend to think that it's a big
problem.
wouldn't something as simple as below do the trick?
// absolutely and completely untested //
diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
index 033e50a7d706..c7477654c5b1 100644
--- a/kernel/printk/printk_safe.c
+++ b/kernel/printk/printk_safe.c
@@ -303,7 +303,10 @@ static int vprintk_nmi(const char *fmt, va_list args)
{
struct printk_safe_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
- return printk_safe_log_store(s, fmt, args);
+ if (this_cpu_read(printk_context) & PRINTK_SAFE_CONTEXT_MASK)
+ return printk_safe_log_store(s, fmt, args);
+
+ return vprintk_emit(0, LOGLEVEL_SCHED, NULL, 0, fmt, args);
}
-ss
next prev parent reply other threads:[~2017-04-21 1:57 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-21 11:48 [PATCH v5 0/4] Cleaning printk stuff in NMI context Petr Mladek
2016-04-21 11:48 ` [PATCH v5 1/4] printk/nmi: generic solution for safe printk in NMI Petr Mladek
2016-04-27 9:31 ` Russell King - ARM Linux
2017-04-19 17:13 ` Steven Rostedt
2017-04-19 17:21 ` Peter Zijlstra
2017-04-20 3:31 ` Sergey Senozhatsky
2017-04-20 13:11 ` Petr Mladek
2017-04-21 1:57 ` Sergey Senozhatsky [this message]
2017-04-21 12:06 ` Petr Mladek
2017-04-24 2:17 ` Sergey Senozhatsky
2017-04-27 13:38 ` Petr Mladek
2017-04-27 14:31 ` Steven Rostedt
2017-04-27 15:28 ` Petr Mladek
2017-04-27 15:42 ` Steven Rostedt
2017-04-28 9:02 ` Peter Zijlstra
2017-04-28 13:44 ` Petr Mladek
2017-04-28 13:58 ` Peter Zijlstra
2017-04-28 14:47 ` Steven Rostedt
2017-04-27 16:14 ` Steven Rostedt
2017-04-28 1:35 ` Sergey Senozhatsky
2017-04-28 12:57 ` Petr Mladek
2017-04-28 14:16 ` Steven Rostedt
2017-04-28 1:25 ` Sergey Senozhatsky
2017-04-28 12:38 ` Petr Mladek
2016-04-21 11:48 ` [PATCH v5 2/4] printk/nmi: warn when some message has been lost in NMI context Petr Mladek
2016-04-27 9:34 ` Russell King - ARM Linux
2016-04-21 11:48 ` [PATCH v5 3/4] printk/nmi: increase the size of NMI buffer and make it configurable Petr Mladek
2016-04-21 11:48 ` [PATCH v5 4/4] printk/nmi: flush NMI messages on the system panic Petr Mladek
2016-04-23 3:49 ` Sergey Senozhatsky
2016-04-26 14:21 ` Petr Mladek
2016-04-27 0:34 ` Sergey Senozhatsky
2016-04-27 0:36 ` [PATCH v5 0/4] Cleaning printk stuff in NMI context Sergey Senozhatsky
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