* [PATCHv2] printk: CON_PRINTBUFFER console registration is a bit racy
@ 2018-09-28 9:53 Sergey Senozhatsky
2018-10-03 9:23 ` Petr Mladek
0 siblings, 1 reply; 3+ messages in thread
From: Sergey Senozhatsky @ 2018-09-28 9:53 UTC (permalink / raw)
To: Petr Mladek, Steven Rostedt
Cc: Sergey Senozhatsky, linux-kernel, Sergey Senozhatsky
CON_PRINTBUFFER console registration requires us to do several
preparation steps:
- Rollback console_seq to replay logbuf messages which were already
seen on other consoles;
- Set exclusive_console flag so console_unlock() will ->write() logbuf
messages only to the exclusive_console driver.
The way we do it, however, is a bit racy
logbuf_lock_irqsave(flags);
console_seq = syslog_seq;
console_idx = syslog_idx;
logbuf_unlock_irqrestore(flags);
<< preemption enabled
<< irqs enabled
exclusive_console = newcon;
console_unlock();
We rollback console_seq under logbuf_lock with IRQs disabled, but
we set exclusive_console with local IRQs enabled and logbuf unlocked.
If the system oops-es or panic-s before we set exclusive_console - and
given that we have IRQs and preemption enabled there is such a
possibility - we will re-play all logbuf messages to every registered
console, which may be a bit annoying and time consuming.
Move exclusive_console assignment to the same IRQs-disabled and
logbuf_lock-protected section where we rollback console_seq.
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---
v2: added comment (Petr)
kernel/printk/printk.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 308497194bd4..24a81ba9ec77 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2742,14 +2742,18 @@ void register_console(struct console *newcon)
logbuf_lock_irqsave(flags);
console_seq = syslog_seq;
console_idx = syslog_idx;
- logbuf_unlock_irqrestore(flags);
/*
* We're about to replay the log buffer. Only do this to the
* just-registered console to avoid excessive message spam to
* the already-registered consoles.
+ *
+ * Set exclusive_console with disabled interrupts to reduce
+ * race window with eventual console_flush_on_panic() that
+ * ignores console_lock.
*/
exclusive_console = newcon;
exclusive_console_stop_seq = console_seq;
+ logbuf_unlock_irqrestore(flags);
}
console_unlock();
console_sysfs_notify();
--
2.19.0
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCHv2] printk: CON_PRINTBUFFER console registration is a bit racy
2018-09-28 9:53 [PATCHv2] printk: CON_PRINTBUFFER console registration is a bit racy Sergey Senozhatsky
@ 2018-10-03 9:23 ` Petr Mladek
2018-10-03 12:27 ` Sergey Senozhatsky
0 siblings, 1 reply; 3+ messages in thread
From: Petr Mladek @ 2018-10-03 9:23 UTC (permalink / raw)
To: Sergey Senozhatsky; +Cc: Steven Rostedt, linux-kernel, Sergey Senozhatsky
On Fri 2018-09-28 18:53:04, Sergey Senozhatsky wrote:
> CON_PRINTBUFFER console registration requires us to do several
> preparation steps:
> - Rollback console_seq to replay logbuf messages which were already
> seen on other consoles;
> - Set exclusive_console flag so console_unlock() will ->write() logbuf
> messages only to the exclusive_console driver.
>
> The way we do it, however, is a bit racy
>
> logbuf_lock_irqsave(flags);
> console_seq = syslog_seq;
> console_idx = syslog_idx;
> logbuf_unlock_irqrestore(flags);
> << preemption enabled
> << irqs enabled
> exclusive_console = newcon;
> console_unlock();
>
> We rollback console_seq under logbuf_lock with IRQs disabled, but
> we set exclusive_console with local IRQs enabled and logbuf unlocked.
> If the system oops-es or panic-s before we set exclusive_console - and
> given that we have IRQs and preemption enabled there is such a
> possibility - we will re-play all logbuf messages to every registered
> console, which may be a bit annoying and time consuming.
>
> Move exclusive_console assignment to the same IRQs-disabled and
> logbuf_lock-protected section where we rollback console_seq.
>
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
JFYI, I have just pushed this into printk.git, for-4.20 branch.
Best Regards,
Petr
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCHv2] printk: CON_PRINTBUFFER console registration is a bit racy
2018-10-03 9:23 ` Petr Mladek
@ 2018-10-03 12:27 ` Sergey Senozhatsky
0 siblings, 0 replies; 3+ messages in thread
From: Sergey Senozhatsky @ 2018-10-03 12:27 UTC (permalink / raw)
To: Petr Mladek
Cc: Sergey Senozhatsky, Steven Rostedt, linux-kernel, Sergey Senozhatsky
On (10/03/18 11:23), Petr Mladek wrote:
> On Fri 2018-09-28 18:53:04, Sergey Senozhatsky wrote:
> > CON_PRINTBUFFER console registration requires us to do several
> > preparation steps:
> > - Rollback console_seq to replay logbuf messages which were already
> > seen on other consoles;
> > - Set exclusive_console flag so console_unlock() will ->write() logbuf
> > messages only to the exclusive_console driver.
> >
> > The way we do it, however, is a bit racy
> >
> > logbuf_lock_irqsave(flags);
> > console_seq = syslog_seq;
> > console_idx = syslog_idx;
> > logbuf_unlock_irqrestore(flags);
> > << preemption enabled
> > << irqs enabled
> > exclusive_console = newcon;
> > console_unlock();
> >
> > We rollback console_seq under logbuf_lock with IRQs disabled, but
> > we set exclusive_console with local IRQs enabled and logbuf unlocked.
> > If the system oops-es or panic-s before we set exclusive_console - and
> > given that we have IRQs and preemption enabled there is such a
> > possibility - we will re-play all logbuf messages to every registered
> > console, which may be a bit annoying and time consuming.
> >
> > Move exclusive_console assignment to the same IRQs-disabled and
> > logbuf_lock-protected section where we rollback console_seq.
> >
> > Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
>
> JFYI, I have just pushed this into printk.git, for-4.20 branch.
Thanks.
-ss
^ permalink raw reply [flat|nested] 3+ messages in thread
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