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Subject[PATCH] printk: CON_PRINTBUFFER console registration is a bit racy
Date
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 under the same IRQs-disabled and
logbuf_lock protected section where we rollback console_seq.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---
kernel/printk/printk.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 992bb6bb7ac2..c52a986a72b3 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2705,7 +2705,6 @@ 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
@@ -2713,6 +2712,7 @@ void register_console(struct console *newcon)
*/
exclusive_console = newcon;
exclusive_console_stop_seq = console_seq;
+ logbuf_unlock_irqrestore(flags);
}
console_unlock();
console_sysfs_notify();
--
2.19.0
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