This is a note to let you know that I've just added the patch titled printk: Wake klogd when passing console_lock owner to the 4.14-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: printk-wake-klogd-when-passing-console_lock-owner.patch and it can be found in the queue-4.14 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. commit adcc7accbc8d54cd9ba446a87e44b85e7c5cfa06 Author: Petr Mladek <pmladek@xxxxxxxx> Date: Mon Feb 26 15:44:20 2018 +0100 printk: Wake klogd when passing console_lock owner [ Upstream commit c14376de3a1befa70d9811ca2872d47367b48767 ] wake_klogd is a local variable in console_unlock(). The information is lost when the console_lock owner using the busy wait added by the commit dbdda842fe96f8932 ("printk: Add console owner and waiter logic to load balance console writes"). The following race is possible: CPU0 CPU1 console_unlock() for (;;) /* calling console for last message */ printk() log_store() log_next_seq++; /* see new message */ if (seen_seq != log_next_seq) { wake_klogd = true; seen_seq = log_next_seq; } console_lock_spinning_enable(); if (console_trylock_spinning()) /* spinning */ if (console_lock_spinning_disable_and_check()) { printk_safe_exit_irqrestore(flags); return; console_unlock() if (seen_seq != log_next_seq) { /* already seen */ /* nothing to do */ Result: Nobody would wakeup klogd. One solution would be to make a global variable from wake_klogd. But then we would need to manipulate it under a lock or so. This patch wakes klogd also when console_lock is passed to the spinning waiter. It looks like the right way to go. Also userspace should have a chance to see and store any "flood" of messages. Note that the very late klogd wake up was a historic solution. It made sense on single CPU systems or when sys_syslog() operations were synchronized using the big kernel lock like in v2.1.113. But it is questionable these days. Fixes: dbdda842fe96f8932 ("printk: Add console owner and waiter logic to load balance console writes") Link: http://lkml.kernel.org/r/20180226155734.dzwg3aovqnwtvkoy@xxxxxxxxxxxxxxx Cc: Steven Rostedt <rostedt@xxxxxxxxxxx> Cc: linux-kernel@xxxxxxxxxxxxxxx Cc: Tejun Heo <tj@xxxxxxxxxx> Suggested-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx> Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx> Signed-off-by: Petr Mladek <pmladek@xxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 2f654a79f80b..2e2c86dd226f 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -2393,7 +2393,7 @@ void console_unlock(void) if (console_lock_spinning_disable_and_check()) { printk_safe_exit_irqrestore(flags); - return; + goto out; } printk_safe_exit_irqrestore(flags); @@ -2426,6 +2426,7 @@ void console_unlock(void) if (retry && console_trylock()) goto again; +out: if (wake_klogd) wake_up_klogd(); }