96 lines
3.3 KiB
Diff
96 lines
3.3 KiB
Diff
From ca6770a982bcd5497cd057b5e0a8126c2e589a14 Mon Sep 17 00:00:00 2001
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From: John Ogness <john.ogness@linutronix.de>
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Date: Fri, 4 Feb 2022 16:01:17 +0106
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Subject: [PATCH 22/62] printk: avoid preempt_disable() for PREEMPT_RT
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During non-normal operation, printk() calls will attempt to
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write the messages directly to the consoles. This involves
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using console_trylock() to acquire @console_sem.
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Preemption is disabled while directly printing to the consoles
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in order to ensure that the printing task is not scheduled away
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while holding @console_sem, thus blocking all other printers
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and causing delays in printing.
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Commit fd5f7cde1b85 ("printk: Never set console_may_schedule in
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console_trylock()") specifically reverted a previous attempt at
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allowing preemption while printing.
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However, on PREEMPT_RT systems, disabling preemption while
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printing is not allowed because console drivers typically
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acquire a spin lock (which under PREEMPT_RT is an rtmutex).
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Since direct printing is only used during early boot and
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non-panic dumps, the risks of delayed print output for these
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scenarios will be accepted under PREEMPT_RT.
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Signed-off-by: John Ogness <john.ogness@linutronix.de>
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Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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---
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kernel/printk/printk.c | 21 ++++++++++++++++++++-
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1 file changed, 20 insertions(+), 1 deletion(-)
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diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
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index 73b1727087c7..3d0ff49cca29 100644
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--- a/kernel/printk/printk.c
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+++ b/kernel/printk/printk.c
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@@ -1981,6 +1981,7 @@ static int console_lock_spinning_disable_and_check(void)
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return 1;
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}
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+#if !IS_ENABLED(CONFIG_PREEMPT_RT)
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/**
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* console_trylock_spinning - try to get console_lock by busy waiting
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*
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@@ -2054,6 +2055,7 @@ static int console_trylock_spinning(void)
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return 1;
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}
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+#endif /* CONFIG_PREEMPT_RT */
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/*
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* Call the specified console driver, asking it to write out the specified
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@@ -2393,6 +2395,18 @@ asmlinkage int vprintk_emit(int facility, int level,
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/* If called from the scheduler, we can not call up(). */
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if (!in_sched && allow_direct_printing()) {
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+#if IS_ENABLED(CONFIG_PREEMPT_RT)
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+ /*
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+ * Use the non-spinning trylock since PREEMPT_RT does not
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+ * support console lock handovers.
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+ *
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+ * Direct printing will most likely involve taking spinlocks.
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+ * For PREEMPT_RT, this is only allowed if in a preemptible
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+ * context.
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+ */
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+ if (preemptible() && console_trylock())
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+ console_unlock();
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+#else
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/*
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* The caller may be holding system-critical or
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* timing-sensitive locks. Disable preemption during direct
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@@ -2410,6 +2424,7 @@ asmlinkage int vprintk_emit(int facility, int level,
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if (console_trylock_spinning())
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console_unlock();
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preempt_enable();
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+#endif
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}
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if (in_sched)
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@@ -3119,8 +3134,12 @@ static bool console_emit_next_record_transferable(struct console *con, char *tex
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/*
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* Handovers are only supported if threaded printers are atomically
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* blocked. The context taking over the console_lock may be atomic.
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+ *
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+ * PREEMPT_RT also does not support handovers because the spinning
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+ * waiter can cause large latencies.
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*/
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- if (!console_kthreads_atomically_blocked()) {
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+ if (!console_kthreads_atomically_blocked() ||
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+ IS_ENABLED(CONFIG_PREEMPT_RT)) {
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*handover = false;
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handover = NULL;
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}
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--
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2.43.0
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