linux-kernel - [RFC][PATCHv5 11/13] printk: always offload printing from user-space processes

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Message-Id: <20170815025625.1977-12-sergey.senozhatsky@gmail.com>
Date:   Tue, 15 Aug 2017 11:56:23 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Eric Biederman <ebiederm@...ssion.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, Pavel Machek <pavel@....cz>,
        Andreas Mohr <andi@...as.de>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: [RFC][PATCHv5 11/13] printk: always offload printing from user-space processes

console_lock() and, thus, console_unlock() can be executed by
user-space processes doing sys calls (e.g. tty, drm, etc.) so we
may end up in a situation when systemd loops in console_unlock()
printing kernel messages. Always offload printing duty from
user-space process (if we can safely wake up printk_kthread,
of course).

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
---
 kernel/printk/printk.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 6da4e21c3b45..0ffbaa33280f 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -592,6 +592,13 @@ static inline bool console_offload_printing(void)
 	if (!is_printk_offloading_safe())
 		return false;
 
+	/*
+	 * Always offload from user-space processes, we've got printk kthread
+	 * for printing.
+	 */
+	if (!(current->flags & PF_KTHREAD))
+		goto offload;
+
 	/* A new task - reset the counters. */
 	if (printing_task != current) {
 		printing_start_ts = local_clock() >> 30LL;
@@ -634,6 +641,7 @@ static inline bool console_offload_printing(void)
 		return true;
 	}
 
+offload:
 	/*
 	 * We try to set `printk_kthread' CPU affinity to any online CPU
 	 * except for this_cpu. Because otherwise `printk_kthread' may be
-- 
2.14.1