[PATCH AUTOSEL 4.19 003/237] printk: lock/unlock console only for new logbuf entries

Sasha Levin
2019-11-16T16:28:16Z

From: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>

[ Upstream commit 3ac37a93fa9217e576bebfd4ba3e80edaaeb2289 ]

Prior to commit 5c2992ee7fd8a29 ("printk: remove console flushing special
cases for partial buffered lines") we would do console_cont_flush()
for each pr_cont() to print cont fragments, so console_unlock() would
actually print data:

pr_cont();
console_lock();
console_unlock()
console_cont_flush(); // print cont fragment
...
pr_cont();
console_lock();
console_unlock()
console_cont_flush(); // print cont fragment

We don't do console_cont_flush() anymore, so when we do pr_cont()
console_unlock() does nothing (unless we flushed the cont buffer):

pr_cont();
console_lock();
console_unlock(); // noop
...
pr_cont();
console_lock();
console_unlock(); // noop
...
pr_cont();
cont_flush();
console_lock();
console_unlock(); // print data

We also wakeup klogd purposelessly for pr_cont() output - un-flushed
cont buffer is not stored in log_buf; there is nothing to pull.

Thus we can console_lock()/console_unlock()/wake_up_klogd() only when
we know that we log_store()-ed a message and there is something to
print to the consoles/syslog.

Link: http://lkml.kernel.org/r/20181002023836.4487-3-sergey.senozhatsky@gmail.com
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dmitriy Vyukov <dvyukov@google.com>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Tejun Heo <tj@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: LKML <linux-kernel@vger.kernel.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Signed-off-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 | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index d0d03223b45b1..9ee6016a19fc8 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1894,8 +1894,9 @@ asmlinkage int vprintk_emit(int facility, int level,
const char *fmt, va_list args)
{
int printed_len;
- bool in_sched = false;
+ bool in_sched = false, pending_output;
unsigned long flags;
+ u64 curr_log_seq;

if (level == LOGLEVEL_SCHED) {
level = LOGLEVEL_DEFAULT;
@@ -1907,11 +1908,13 @@ asmlinkage int vprintk_emit(int facility, int level,

/* This stops the holder of console_sem just where we want him */
logbuf_lock_irqsave(flags);
+ curr_log_seq = log_next_seq;
printed_len = vprintk_store(facility, level, dict, dictlen, fmt, args);
+ pending_output = (curr_log_seq != log_next_seq);
logbuf_unlock_irqrestore(flags);

/* If called from the scheduler, we can not call up(). */
- if (!in_sched) {
+ if (!in_sched && pending_output) {
/*
* Disable preemption to avoid being preempted while holding
* console_sem which would prevent anyone from printing to
@@ -1928,7 +1931,8 @@ asmlinkage int vprintk_emit(int facility, int level,
preempt_enable();
}

- wake_up_klogd();
+ if (pending_output)
+ wake_up_klogd();
return printed_len;
}
EXPORT_SYMBOL(vprintk_emit);
--
2.20.1