On Mon 2018-02-26 15:37:35, Sergey Senozhatsky wrote:
> On (02/19/18 17:01), Petr Mladek wrote:
> [..]
> > - raw_spin_lock(&logbuf_lock);
> > retry = console_seq != log_next_seq;
> > + /*
> > + * Check whether userland needs notification. Do this only when really
> > + * leaving to avoid race with console_trylock_spinning().
> > + */
> > + if (seen_seq != log_next_seq && !retry) {
> > + wake_klogd = true;
> > + seen_seq = log_next_seq;
> > + }
>
> Let's add the "why" part. This "!retry" might be hard to understand. We
> are looking at
>
> - CPUa is about to leave console_unlock()
> - printk on CPUb appends a new message
> - CPUa detects that `console_seq != log_next_seq', updates `seen_seq'
> - printk on CPUb is getting preempted
> - CPUa re-takes the console_sem via retry path
> - printk CPUb is becoming TASK_RUNNING again - it now spins for console_sem,
> since we have an active console_sem owner
> - CPUa detects that there is a console_sem waiter, so it offloads the
> printing task, without ever waking up klogd
Yes, this is the scenario. I agree that it is very tricky.
> Either we can have that complex "seen_seq != log_next_seq && !retry"
> check - or we simply can add
>
> if (console_lock_spinning_disable_and_check()) {
> printk_safe_exit_irqrestore(flags);
> if (wake_klogd)
> wake_up_klogd();
> }
>
> to the offloading return path.
>
> The later is *may be* simpler to follow. The rule is: every
> !console_suspend and !cant-use-consoles return path from console_unlock()
> must wake_up_klogd() [if needed].
Yes, this looks more straighforward. It might cause earlier and more
often wakeup but we want to go this way anyway. I do not see any
real problem with it.
Okay, what about the following patch for 4.16-rc4?
>From 7450da0e7e0d7ee5926f09c8bbbaa44c67ae0b48 Mon Sep 17 00:00:00 2001
From: Petr Mladek <[email protected]>
Date: Mon, 26 Feb 2018 15:44:20 +0100
Subject: [PATCH] printk: Wake klogd when passing console_lock owner
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")
Suggested-by: Sergey Senozhatsky <[email protected]>
Signed-off-by: Petr Mladek <[email protected]>
---
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 fc1123583fa6..f274fbef821d 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2397,7 +2397,7 @@ void console_unlock(void)
if (console_lock_spinning_disable_and_check()) {
printk_safe_exit_irqrestore(flags);
- return;
+ goto out;
}
printk_safe_exit_irqrestore(flags);
@@ -2430,6 +2430,7 @@ void console_unlock(void)
if (retry && console_trylock())
goto again;
+out:
if (wake_klogd)
wake_up_klogd();
}
--
2.13.6
