Date: Wed, 10 Jun 2015 17:32:53 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Petr Mladek <pmladek@suse.cz>
Cc: Steven Rostedt <rostedt@goodmis.org>, linux-kernel@vger.kernel.org,
        jkosina@suse.cz, paulmck@linux.vnet.ibm.com,
        Ingo Molnar <mingo@kernel.org>, Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [RFC][PATCH] printk: Fixup the nmi printk mess
Message-ID: <20150610153253.GJ3644@twins.programming.kicks-ass.net>
References: <20150610125509.GO19282@twins.programming.kicks-ass.net>
 <20150610143155.GD9409@pathway.suse.cz>
 <20150610145737.GE9409@pathway.suse.cz>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20150610145737.GE9409@pathway.suse.cz>
User-Agent: Mutt/1.5.21 (2012-12-30)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1328
Lines: 34


You are aware that you can delete bits of the email that are not
relevant, right?

On Wed, Jun 10, 2015 at 04:57:37PM +0200, Petr Mladek wrote:

> Also note that show_regs() calls many separate printk()s, the irqwork
> is scheduled by the first one => it is quite likely that some
> backtrace will get messed.

The irq_work is only ever called when we're inside an NMI, the irq_work
will only ever execute once that NMI is done -- on that CPU.

How will that miss anything?

> Anothrer problem is that __printk_nmi_flush() is per-CPU => more
> instances might be running in parallel. I haven't tested this but
> I quess that it will mix backtraces from different CPUs in
> the main ring buffer.

Correct, such is the life of printk().

> Note that the backtraces used to be serialized via
> static arch_spinlock_t lock = __ARCH_SPIN_LOCK_UNLOCKED
> in the past. See the commit
> a9edc8809328 ("x86/nmi: Perform a safe NMI stack trace on all CPUs")

We could easily add a static raw_spinlock_t to __printk_nmi_flush() and
serialize its invocations if people think that is important.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/