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From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Steven Rostedt <rostedt@goodmis.org>,
	Petr Mladek <pmladek@suse.com>,
	Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	Alexander Potapenko <glider@google.com>,
	Dmitriy Vyukov <dvyukov@google.com>,
	kbuild test robot <fengguang.wu@intel.com>,
	syzkaller <syzkaller@googlegroups.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Subject: Re: [PATCH] printk: inject caller information into the body of message
Date: Mon, 1 Oct 2018 14:52:36 +0900
Message-ID: <20181001055236.GA386@jagdpanzerIV> (raw)
In-Reply-To: <20180929111317.GB1392@tigerII.localdomain>

On (09/29/18 20:13), Sergey Senozhatsky wrote:
> We used to flush "incomplete" cont lines (fragments) from console_unlock().
> 
> void console_unlock(void)
> {
> ...
>         /* flush buffered message fragment immediately to console */
>         console_cont_flush(text, sizeof(text));
> again:
>         for (;;) {
> 	...
> 	}
> ...
> }
> 
> Unless I'm missing something, we don't anymore.
> Since 5c2992ee7fd8a29d04125dc0aa3522784c5fa5eb.
> Now we print only log_buf entries. So we either wait for a \n to flush
> a complete cont buffer, or for a race to preliminary flush cont buffer.

BTW, it just crossed my mind:

Previously, we would do console_cont_flush() for each pr_cont(),
so console_unlock() would 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 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

console_lock()/console_unlock() makes sense only when we flush cont
buffer.

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

So we can console_lock()/console_unlock()/wake_up_klogd() only when we
know that we log_stor()-ed a message.

A quick-n-dirty patch (I can send a formal one) which compares log_next_seq
before and after vprintk_store(). log_next_seq is getting incremented each
time we log_store() a new log_buf message:

---

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 308497194bd4..53d9134f02a6 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1931,6 +1931,7 @@ asmlinkage int vprintk_emit(int facility, int level,
 	int printed_len;
 	bool in_sched = false;
 	unsigned long flags;
+	u64 curr_log_seq;
 
 	if (level == LOGLEVEL_SCHED) {
 		level = LOGLEVEL_DEFAULT;
@@ -1942,11 +1943,12 @@ 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);
 	logbuf_unlock_irqrestore(flags);
 
 	/* If called from the scheduler, we can not call up(). */
-	if (!in_sched) {
+	if (!in_sched && (curr_log_seq != log_next_seq)) {
 		/*
 		 * Disable preemption to avoid being preempted while holding
 		 * console_sem which would prevent anyone from printing to
@@ -1963,7 +1965,8 @@ asmlinkage int vprintk_emit(int facility, int level,
 		preempt_enable();
 	}
 
-	wake_up_klogd();
+	if (curr_log_seq != log_next_seq)
+		wake_up_klogd();
 	return printed_len;
 }
 EXPORT_SYMBOL(vprintk_emit);

---

	-ss

  parent reply index

Thread overview: 94+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <201804232233.CIC65675.OJSOMFQOFFHVtL@I-love.SAKURA.ne.jp>
     [not found] ` <CACT4Y+boyw_Qy=y-iTnsKZrtTgF0Hk3nHN_xtqUdX4etgiYDQw@mail.gmail.com>
2018-04-24  1:33   ` printk feature for syzbot? Sergey Senozhatsky
2018-04-24 14:40     ` Steven Rostedt
2018-04-26 10:06     ` Petr Mladek
2018-05-10  4:22       ` Sergey Senozhatsky
2018-05-10 11:30         ` Petr Mladek
2018-05-10 12:11           ` Sergey Senozhatsky
2018-05-10 14:22             ` Steven Rostedt
2018-05-10 14:50         ` Tetsuo Handa
2018-05-11  1:45           ` Sergey Senozhatsky
     [not found]             ` <201805110238.w4B2cIGH079602@www262.sakura.ne.jp>
2018-05-11  6:21               ` Sergey Senozhatsky
2018-05-11  9:17                 ` Dmitry Vyukov
2018-05-11  9:50                   ` Sergey Senozhatsky
2018-05-11 11:58                     ` [PATCH] printk: inject caller information into the body of message Tetsuo Handa
2018-05-17 11:21                       ` Sergey Senozhatsky
2018-05-17 11:52                         ` Sergey Senozhatsky
2018-05-18 12:15                         ` Petr Mladek
2018-05-18 12:25                           ` Dmitry Vyukov
2018-05-18 12:54                             ` Petr Mladek
2018-05-18 13:08                               ` Dmitry Vyukov
2018-05-24  2:21                                 ` Sergey Senozhatsky
2018-05-23 10:19                           ` Tetsuo Handa
2018-05-24  2:14                           ` Sergey Senozhatsky
2018-05-26  6:36                             ` Dmitry Vyukov
2018-06-20  5:44                               ` Dmitry Vyukov
2018-06-20  8:31                                 ` Sergey Senozhatsky
2018-06-20  8:45                                   ` Dmitry Vyukov
2018-06-20  9:06                                     ` Sergey Senozhatsky
2018-06-20  9:18                                       ` Sergey Senozhatsky
2018-06-20  9:31                                         ` Dmitry Vyukov
2018-06-20 11:07                                           ` Sergey Senozhatsky
2018-06-20 11:32                                             ` Dmitry Vyukov
2018-06-20 13:06                                               ` Sergey Senozhatsky
2018-06-22 13:06                                                 ` Tetsuo Handa
2018-06-25  1:41                                                   ` Sergey Senozhatsky
2018-06-25  9:36                                                     ` Dmitry Vyukov
2018-06-27 10:29                                                       ` Tetsuo Handa
2018-09-10 11:20                                                 ` Alexander Potapenko
2018-09-12  6:53                                                   ` Sergey Senozhatsky
2018-09-12 16:05                                                     ` Steven Rostedt
2018-09-13  7:12                                                       ` Sergey Senozhatsky
2018-09-13 12:26                                                         ` Petr Mladek
2018-09-13 14:28                                                           ` Sergey Senozhatsky
2018-09-14  1:22                                                             ` Steven Rostedt
2018-09-14  2:15                                                               ` Sergey Senozhatsky
2018-09-14  6:57                                                             ` Sergey Senozhatsky
2018-09-14 10:37                                                               ` Tetsuo Handa
2018-09-14 11:50                                                                 ` Sergey Senozhatsky
2018-09-14 12:03                                                                   ` Tetsuo Handa
2018-09-14 12:22                                                                     ` Sergey Senozhatsky
2018-09-19 11:02                                                                       ` Tetsuo Handa
2018-09-24  8:11                                                                         ` Tetsuo Handa
2018-09-27 16:10                                                                           ` Tetsuo Handa
2018-09-28  9:02                                                                             ` Sergey Senozhatsky
2018-09-28  9:09                                                                           ` Sergey Senozhatsky
2018-09-28 11:01                                                                             ` Tetsuo Handa
2018-09-29 10:51                                                                               ` Sergey Senozhatsky
2018-09-29 11:15                                                                                 ` Tetsuo Handa
2018-10-01  2:37                                                                                   ` Sergey Senozhatsky
2018-10-01  2:58                                                                                     ` Sergey Senozhatsky
2018-10-01 11:21                                                                                     ` Tetsuo Handa
2018-10-02  6:38                                                                                       ` Sergey Senozhatsky
2018-10-08 10:31                                                                                         ` Tetsuo Handa
2018-10-08 16:03                                                                                           ` Petr Mladek
2018-10-08 20:48                                                                                             ` Tetsuo Handa
2018-10-09 14:52                                                                                               ` Petr Mladek
2018-10-09 21:19                                                                                                 ` Tetsuo Handa
2018-10-10 10:14                                                                                                   ` Tetsuo Handa
2018-10-11 10:20                                                                                                     ` Tetsuo Handa
2018-10-11 13:47                                                                                                       ` Steven Rostedt
2018-10-08 15:43                                                                                         ` Petr Mladek
2018-09-28  8:56                                                                         ` Sergey Senozhatsky
2018-09-28 11:21                                                                           ` Tetsuo Handa
2018-09-29 11:13                                                                             ` Sergey Senozhatsky
2018-09-29 11:39                                                                               ` Tetsuo Handa
2018-10-01  5:52                                                                               ` Sergey Senozhatsky [this message]
2018-10-01  8:37                                                                                 ` Sergey Senozhatsky
2018-10-01 18:06                                                                               ` Steven Rostedt
2018-09-14  1:12                                                         ` Steven Rostedt
2018-09-14  1:55                                                           ` Sergey Senozhatsky
2018-06-21  8:29                                               ` Sergey Senozhatsky
2018-06-20  9:30                                       ` Dmitry Vyukov
2018-06-20 11:19                                         ` Sergey Senozhatsky
2018-06-20 11:25                                           ` Dmitry Vyukov
2018-06-20 11:37                                         ` Fengguang Wu
2018-06-20 12:31                                           ` Dmitry Vyukov
2018-06-20 12:41                                             ` Fengguang Wu
2018-06-20 12:45                                               ` Dmitry Vyukov
2018-06-20 12:48                                                 ` Fengguang Wu
2018-05-11 13:37                     ` printk feature for syzbot? Steven Rostedt
2018-05-15  5:20                       ` Sergey Senozhatsky
2018-05-15 14:39                         ` Steven Rostedt
2018-05-11 11:02                 ` [PATCH] printk: fix possible reuse of va_list variable Tetsuo Handa
2018-05-11 11:27                   ` Sergey Senozhatsky
2018-05-17 11:57                   ` Petr Mladek

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