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From:   Stephen Brennan <stephen.s.brennan@oracle.com>
To:     Sergey Senozhatsky <senozhatsky@chromium.org>,
        Petr Mladek <pmladek@suse.com>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Stephen Brennan <stephen.s.brennan@oracle.com>,
        Sebastian Reichel <sre@kernel.org>,
        John Ogness <john.ogness@linutronix.de>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Luis Chamberlain <mcgrof@kernel.org>,
        linux-kernel@vger.kernel.org
Subject: [PATCH v2 3/4] printk: Avoid livelock with heavy printk during panic
Date:   Wed, 26 Jan 2022 15:02:35 -0800
Message-Id: <20220126230236.750229-4-stephen.s.brennan@oracle.com>
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During panic(), if another CPU is writing heavily the kernel log (e.g.
via /dev/kmsg), then the panic CPU may livelock writing out its messages
to the console. Note when too many messages are dropped during panic and
suppress further printk, except from the panic CPU. This could result in
some important messages being dropped. However, messages are already
being dropped, so this approach at least prevents a livelock.

Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>
---

Notes:
    v2: Add pr_warn when we suppress printk on non-panic CPU

 kernel/printk/printk.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 20b4b71a1a07..18107db118d4 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -93,6 +93,12 @@ EXPORT_SYMBOL_GPL(console_drivers);
  */
 int __read_mostly suppress_printk;
 
+/*
+ * During panic, heavy printk by other CPUs can delay the
+ * panic and risk deadlock on console resources.
+ */
+int __read_mostly suppress_panic_printk;
+
 #ifdef CONFIG_LOCKDEP
 static struct lockdep_map console_lock_dep_map = {
 	.name = "console_lock"
@@ -2228,6 +2234,10 @@ asmlinkage int vprintk_emit(int facility, int level,
 	if (unlikely(suppress_printk))
 		return 0;
 
+	if (unlikely(suppress_panic_printk) &&
+	    atomic_read(&panic_cpu) != raw_smp_processor_id())
+		return 0;
+
 	if (level == LOGLEVEL_SCHED) {
 		level = LOGLEVEL_DEFAULT;
 		in_sched = true;
@@ -2613,6 +2623,7 @@ void console_unlock(void)
 {
 	static char ext_text[CONSOLE_EXT_LOG_MAX];
 	static char text[CONSOLE_LOG_MAX];
+	static int panic_console_dropped;
 	unsigned long flags;
 	bool do_cond_resched, retry;
 	struct printk_info info;
@@ -2667,6 +2678,11 @@ void console_unlock(void)
 		if (console_seq != r.info->seq) {
 			console_dropped += r.info->seq - console_seq;
 			console_seq = r.info->seq;
+			if (panic_in_progress() && panic_console_dropped++ > 10) {
+				suppress_panic_printk = 1;
+				pr_warn("Too many dropped messages. "
+				        "Supress messages on non-panic CPUs to prevent livelock.\n");
+			}
 		}
 
 		if (suppress_message_printing(r.info->level)) {
-- 
2.30.2