From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Petr Mladek <pmladek@suse.com>, Steven Rostedt <rostedt@goodmis.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
linux-kernel@vger.kernel.org,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Subject: [PATCHv2] printk: CON_PRINTBUFFER console registration is a bit racy
Date: Fri, 28 Sep 2018 18:53:04 +0900 [thread overview]
Message-ID: <20180928095304.9972-1-sergey.senozhatsky@gmail.com> (raw)
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
next reply other threads:[~2018-09-28 9:53 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-09-28 9:53 Sergey Senozhatsky [this message]
2018-10-03 9:23 ` [PATCHv2] printk: CON_PRINTBUFFER console registration is a bit racy Petr Mladek
2018-10-03 12:27 ` Sergey Senozhatsky
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20180928095304.9972-1-sergey.senozhatsky@gmail.com \
--to=sergey.senozhatsky.work@gmail.com \
--cc=linux-kernel@vger.kernel.org \
--cc=pmladek@suse.com \
--cc=rostedt@goodmis.org \
--cc=sergey.senozhatsky@gmail.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).