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Diffstat (limited to '')
| -rw-r--r-- | debian/patches-rt/0022-printk-avoid-preempt_disable-for-PREEMPT_RT.patch | 96 |
1 files changed, 96 insertions, 0 deletions
diff --git a/debian/patches-rt/0022-printk-avoid-preempt_disable-for-PREEMPT_RT.patch b/debian/patches-rt/0022-printk-avoid-preempt_disable-for-PREEMPT_RT.patch new file mode 100644 index 000000000..ff28f79d9 --- /dev/null +++ b/debian/patches-rt/0022-printk-avoid-preempt_disable-for-PREEMPT_RT.patch @@ -0,0 +1,96 @@ +From a549c0dff2a1a3b11b40abc969358d8cb367871d Mon Sep 17 00:00:00 2001 +From: John Ogness <john.ogness@linutronix.de> +Date: Fri, 4 Feb 2022 16:01:17 +0106 +Subject: [PATCH 22/62] printk: avoid preempt_disable() for PREEMPT_RT +Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.1/older/patches-6.1.69-rt21.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(-) + +diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c +index 73b1727087c7..3d0ff49cca29 100644 +--- a/kernel/printk/printk.c ++++ b/kernel/printk/printk.c +@@ -1981,6 +1981,7 @@ static int console_lock_spinning_disable_and_check(void) + return 1; + } + ++#if !IS_ENABLED(CONFIG_PREEMPT_RT) + /** + * console_trylock_spinning - try to get console_lock by busy waiting + * +@@ -2054,6 +2055,7 @@ static int console_trylock_spinning(void) + + return 1; + } ++#endif /* CONFIG_PREEMPT_RT */ + + /* + * Call the specified console driver, asking it to write out the specified +@@ -2393,6 +2395,18 @@ asmlinkage int vprintk_emit(int facility, int level, + + /* 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 +@@ -2410,6 +2424,7 @@ asmlinkage int vprintk_emit(int facility, int level, + if (console_trylock_spinning()) + console_unlock(); + preempt_enable(); ++#endif + } + + if (in_sched) +@@ -3119,8 +3134,12 @@ static bool console_emit_next_record_transferable(struct console *con, char *tex + /* + * 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; + } +-- +2.43.0 + |