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);