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Date:   Tue, 15 Jan 2019 19:54:33 +0100
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-rt-users <linux-rt-users@vger.kernel.org>,
        Petr Mladek <pmladek@suse.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Subject: Re: [PATCH RT] printk: Shortcut out of waiter spinning on PREEMPT_RT
Message-ID: <20190115185433.e7hmlpmjvpyqdyhc@linutronix.de>
References: <20190115123910.2b18df54@gandalf.local.home>
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On 2019-01-15 12:39:10 [-0500], Steven Rostedt wrote:
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1742,6 +1742,13 @@ static int console_trylock_spinning(void)
>  	if (console_trylock())
>  		return 1;
>  
> +	/*
> +	 * The consoles are preemptable in PREEMPT_RT, which can cause
> +	 * spinning to deadlock.
> +	 */
> +	if (IS_ENABLED(CONFIG_PREEMPT_RT_FULL))
> +		return 0;
> +
>  	printk_safe_enter_irqsave(flags);
>  
>  	raw_spin_lock(&console_owner_lock);

So my ("printk: Make rt aware") patch does:

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 6553508ff3889..d983c509f74a2 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1617,6 +1617,7 @@ SYSCALL_DEFINE3(syslog, int, type, char __user *, buf, int, len)
        return do_syslog(type, buf, len, SYSLOG_FROM_READER);
 }
 
+#ifndef CONFIG_PREEMPT_RT_FULL
 /*
  * Special console_lock variants that help to reduce the risk of soft-lockups.
  * They allow to pass console_lock to another printk() call using a busy wait.
@@ -1757,6 +1758,15 @@ static int console_trylock_spinning(void)
        return 1;
 }
 
+#else
+
+static int console_trylock_spinning(void)
+{
+       return console_trylock();
+}
+
+#endif
+
 /*
  * Call the console drivers, asking them to write out
  * log_buf[start] to log_buf[end - 1].


So it never gets into the codepath where you try to avoid spinning.
Right?

Sebastian