From d873008f09d48f130073312e6576b6964e4966a0 Mon Sep 17 00:00:00 2001 From: John Ogness Date: Fri, 4 Feb 2022 16:01:17 +0106 Subject: [PATCH 22/64] printk: avoid preempt_disable() for PREEMPT_RT 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 Signed-off-by: Sebastian Andrzej Siewior --- kernel/printk/printk.c | 21 ++++++++++++++++++++- 1 file changed, 20 insertions(+), 1 deletion(-) Index: linux-6.1.96-rt35/kernel/printk/printk.c =================================================================== --- linux-6.1.96-rt35.orig/kernel/printk/printk.c +++ linux-6.1.96-rt35/kernel/printk/printk.c @@ -2010,6 +2010,7 @@ 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 * @@ -2089,6 +2090,7 @@ static int console_trylock_spinning(void return 1; } +#endif /* CONFIG_PREEMPT_RT */ /* * Call the specified console driver, asking it to write out the specified @@ -2428,6 +2430,18 @@ 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 @@ -2445,6 +2459,7 @@ asmlinkage int vprintk_emit(int facility if (console_trylock_spinning()) console_unlock(); preempt_enable(); +#endif } if (in_sched) @@ -3154,8 +3169,12 @@ 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; }