4.14-stable review patch. If anyone has any objections, please let me know.
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[ 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@pathway.suse.cz Cc: Steven Rostedt rostedt@goodmis.org Cc: linux-kernel@vger.kernel.org Cc: Tejun Heo tj@kernel.org Suggested-by: Sergey Senozhatsky sergey.senozhatsky@gmail.com Reviewed-by: Sergey Senozhatsky sergey.senozhatsky@gmail.com Signed-off-by: Petr Mladek pmladek@suse.com Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/printk/printk.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
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(); }