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Date:   Wed, 17 Jan 2018 13:42:01 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Tejun Heo <tj@kernel.org>
Cc:     Petr Mladek <pmladek@suse.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        akpm@linux-foundation.org, linux-mm@kvack.org,
        Cong Wang <xiyou.wangcong@gmail.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Mel Gorman <mgorman@suse.de>, Michal Hocko <mhocko@kernel.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Jan Kara <jack@suse.cz>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
        rostedt@home.goodmis.org, Byungchul Park <byungchul.park@lge.com>,
        Pavel Machek <pavel@ucw.cz>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup
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On Wed, 17 Jan 2018 12:12:51 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> @@ -2393,15 +2451,20 @@ void console_unlock(void)
>  		 * waiter waiting to take over.
>  		 */
>  		console_lock_spinning_enable();
> +		offload = recursion_check_start();
>  
>  		stop_critical_timings();	/* don't trace print latency */
>  		call_console_drivers(ext_text, ext_len, text, len);
>  		start_critical_timings();
>  
> +		recursion_check_finish(offload);
> +
>  		if (console_lock_spinning_disable_and_check()) {
>  			printk_safe_exit_irqrestore(flags);
>  			return;
>  		}
> +		if (offload)
> +			kick_offload_thread();
>  

Ah, major flaw in this code. The recursion check needs to be in
printk() itself around the trylock.

-- Steve

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 9cb943c90d98..31df145cc4d7 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1826,6 +1826,63 @@ static size_t log_output(int facility, int level, enum log_flags lflags, const c
 	/* Store it in the record log */
 	return log_store(facility, level, lflags, 0, dict, dictlen, text, text_len);
 }
+/*
+ * Used for which context the printk is in.
+ *  NMI     = 0
+ *  IRQ     = 1
+ *  SOFTIRQ = 2
+ *  NORMAL  = 3
+ *
+ * Stack ordered, where the lower number can preempt
+ * the higher number: mask &= mask - 1, will only clear
+ * the lowerest set bit.
+ */
+enum {
+	CTX_NMI,
+	CTX_IRQ,
+	CTX_SOFTIRQ,
+	CTX_NORMAL,
+};
+
+static DEFINE_PER_CPU(int, recursion_bits);
+
+static bool recursion_check_start(void)
+{
+	unsigned long pc = preempt_count();
+	int val = this_cpu_read(recursion_bits);
+
+	if (!(pc & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET)))
+		bit = CTX_NORMAL;
+	else
+		bit = pc & NMI_MASK ? CTX_NMI :
+			pc & HARDIRQ_MASK ? CTX_IRQ : CTX_SOFTIRQ;
+
+	if (unlikely(val & (1 << bit)))
+		return true;
+
+	val |= (1 << bit);
+	this_cpu_write(recursion_bits, val);
+	return false;
+}
+
+static void recursion_check_finish(bool offload)
+{
+	int val = this_cpu_read(recursion_bits);
+
+	if (offload)
+		return;
+
+	val &= val - 1;
+	this_cpu_write(recursion_bits, val);
+}
+
+static void kick_offload_thread(void)
+{
+	/*
+	 * Consoles are triggering printks, offload the printks
+	 * to another CPU to hopefully avoid a lockup.
+	 */
+}
 
 asmlinkage int vprintk_emit(int facility, int level,
 			    const char *dict, size_t dictlen,
@@ -1895,12 +1952,14 @@ asmlinkage int vprintk_emit(int facility, int level,
 
 	/* If called from the scheduler, we can not call up(). */
 	if (!in_sched) {
+		bool offload;
 		/*
 		 * Disable preemption to avoid being preempted while holding
 		 * console_sem which would prevent anyone from printing to
 		 * console
 		 */
 		preempt_disable();
+		offload = recursion_check_start();
 		/*
 		 * Try to acquire and then immediately release the console
 		 * semaphore.  The release will print out buffers and wake up
@@ -1908,7 +1967,12 @@ asmlinkage int vprintk_emit(int facility, int level,
 		 */
 		if (console_trylock_spinning())
 			console_unlock();
+
+		recursion_check_finish(offload);
 		preempt_enable();
+
+		if (offload)
+			kick_offload_thread();
 	}
 
 	return printed_len;