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From: John Ogness <john.ogness@linutronix.de>
Date: Fri, 4 Feb 2022 16:01:17 +0106
Subject: [PATCH 18/18] printk: avoid preempt_disable() for PREEMPT_RT
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/5.19/older/patches-5.19-rc8-rt9.tar.xz
During non-normal operation, printk() calls will attempt to
write the messages directly to the consoles. This involves
using console_trylock() to acquire @console_sem.
Preemption is disabled while directly printing to the consoles
in order to ensure that the printing task is not scheduled away
while holding @console_sem, thus blocking all other printers
and causing delays in printing.
Commit fd5f7cde1b85 ("printk: Never set console_may_schedule in
console_trylock()") specifically reverted a previous attempt at
allowing preemption while printing.
However, on PREEMPT_RT systems, disabling preemption while
printing is not allowed because console drivers typically
acquire a spin lock (which under PREEMPT_RT is an rtmutex).
Since direct printing is only used during early boot and
non-panic dumps, the risks of delayed print output for these
scenarios will be accepted under PREEMPT_RT.
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
kernel/printk/printk.c | 21 ++++++++++++++++++++-
1 file changed, 20 insertions(+), 1 deletion(-)
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1984,6 +1984,7 @@ static int console_lock_spinning_disable
return 1;
}
+#if !IS_ENABLED(CONFIG_PREEMPT_RT)
/**
* console_trylock_spinning - try to get console_lock by busy waiting
*
@@ -2057,6 +2058,7 @@ static int console_trylock_spinning(void
return 1;
}
+#endif /* CONFIG_PREEMPT_RT */
/*
* Call the specified console driver, asking it to write out the specified
@@ -2396,6 +2398,18 @@ asmlinkage int vprintk_emit(int facility
/* If called from the scheduler, we can not call up(). */
if (!in_sched && allow_direct_printing()) {
+#if IS_ENABLED(CONFIG_PREEMPT_RT)
+ /*
+ * Use the non-spinning trylock since PREEMPT_RT does not
+ * support console lock handovers.
+ *
+ * Direct printing will most likely involve taking spinlocks.
+ * For PREEMPT_RT, this is only allowed if in a preemptible
+ * context.
+ */
+ if (preemptible() && console_trylock())
+ console_unlock();
+#else
/*
* The caller may be holding system-critical or
* timing-sensitive locks. Disable preemption during direct
@@ -2413,6 +2427,7 @@ asmlinkage int vprintk_emit(int facility
if (console_trylock_spinning())
console_unlock();
preempt_enable();
+#endif
}
wake_up_klogd();
@@ -3103,8 +3118,12 @@ static bool console_emit_next_record_tra
/*
* Handovers are only supported if threaded printers are atomically
* blocked. The context taking over the console_lock may be atomic.
+ *
+ * PREEMPT_RT also does not support handovers because the spinning
+ * waiter can cause large latencies.
*/
- if (!console_kthreads_atomically_blocked()) {
+ if (!console_kthreads_atomically_blocked() ||
+ IS_ENABLED(CONFIG_PREEMPT_RT)) {
*handover = false;
handover = NULL;
}
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