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Date: Wed, 19 Oct 2016 16:41:40 +0200
From: Petr Mladek <pmladek@suse.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>, Jan Kara <jack@suse.cz>,
        Tejun Heo <tj@kernel.org>, Calvin Owens <calvinowens@fb.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Mel Gorman <mgorman@techsingularity.net>,
        Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@redhat.com>,
        linux-kernel@vger.kernel.org,
        Jason Wessel <jason.wessel@windriver.com>
Subject: Re: [PATCH 1/3] printk: Fix kdb_trap_printk placement
Message-ID: <20161019144140.GG11071@pathway.suse.cz>
References: <20161018170830.405990950@infradead.org>
 <20161018171513.585962054@infradead.org>
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On Tue 2016-10-18 19:08:31, Peter Zijlstra wrote:
> Some people figured vprintk_emit() makes for a nice API and exported
> it, bypassing the kdb trap.
> 
> This still leaves vprintk_nmi() outside of the kbd reach, should that
> be fixed too?

Good question! vkdb_printf() tries to avoid a deadlock but the code is racy:

int vkdb_printf(enum kdb_msgsrc src, const char *fmt, va_list ap)
{
[...]
	/* Serialize kdb_printf if multiple cpus try to write at once.
	 * But if any cpu goes recursive in kdb, just print the output,
	 * even if it is interleaved with any other text.
	 */
	if (!KDB_STATE(PRINTF_LOCK)) {
		KDB_STATE_SET(PRINTF_LOCK);
		spin_lock_irqsave(&kdb_printf_lock, flags);
		got_printf_lock = 1;
		atomic_inc(&kdb_event);
	} else {
		__acquire(kdb_printf_lock);
	}


Let's have the following situation:

CPU1					CPU2

if (!KDB_STATE(PRINTF_LOCK)) {
	KDB_STATE_SET(PRINTF_LOCK);

					if (!KDB_STATE(PRINTF_LOCK)) {
					} else {
						__acquire(kdb_printf_lock);
					}

Now, both CPUs are in the critical section and happily writing over each
other, e.g. in

	vsnprintf(next_avail, size_avail, fmt, ap);

I quess that we want to fix this race. But I am not sure if it will
be done an NMI-safe way. I am going to send a patch for this.

Well, vkdb_printf() is called later when the messages are pushed
to the main logbuffer by printk_nmi_flush_line(). It is not perfect
but...


> Cc: Jason Wessel <jason.wessel@windriver.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Otherwise, your patch makes sense:

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr