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SubjectRe: [PATCH RT] printk: Shortcut out of waiter spinning on PREEMPT_RT
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

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