From: John Ogness <john.ogness@linutronix.de> Date: Fri, 20 Oct 2023 10:03:42 +0000 Subject: [PATCH 24/48] printk: nbcon: Add unsafe flushing on panic Add nbcon_atomic_flush_unsafe() to flush all nbcon consoles using the write_atomic() callback and allowing unsafe hostile takeovers. Call this at the end of panic() as a final attempt to flush any pending messages. Note that legacy consoles use unsafe methods for flushing from the beginning of panic (see bust_spinlocks()). Therefore, systems using both legacy and nbcon consoles may still fail to see panic messages due to unsafe legacy console usage. Signed-off-by: John Ogness <john.ogness@linutronix.de> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> --- include/linux/printk.h | 5 +++++ kernel/panic.c | 1 + kernel/printk/nbcon.c | 26 +++++++++++++++++++++----- 3 files changed, 27 insertions(+), 5 deletions(-) Index: linux-6.8.2-rt10/include/linux/printk.h =================================================================== @ linux-6.8.2-rt10/include/linux/printk.h:200 @ extern asmlinkage void dump_stack(void) void printk_trigger_flush(void); extern void nbcon_driver_acquire(struct console *con); extern void nbcon_driver_release(struct console *con); +void nbcon_atomic_flush_unsafe(void); #else static inline __printf(1, 0) int vprintk(const char *s, va_list args) @ linux-6.8.2-rt10/include/linux/printk.h:289 @ static inline void nbcon_driver_release( { } +static inline void nbcon_atomic_flush_unsafe(void) +{ +} + #endif bool this_cpu_in_panic(void); Index: linux-6.8.2-rt10/kernel/panic.c =================================================================== --- linux-6.8.2-rt10.orig/kernel/panic.c +++ linux-6.8.2-rt10/kernel/panic.c @ linux-6.8.2-rt10/include/linux/printk.h:456 @ void panic(const char *fmt, ...) * Explicitly flush the kernel log buffer one last time. */ console_flush_on_panic(CONSOLE_FLUSH_PENDING); + nbcon_atomic_flush_unsafe(); local_irq_enable(); for (i = 0; ; i += PANIC_TIMER_STEP) { Index: linux-6.8.2-rt10/kernel/printk/nbcon.c =================================================================== --- linux-6.8.2-rt10.orig/kernel/printk/nbcon.c +++ linux-6.8.2-rt10/kernel/printk/nbcon.c @ linux-6.8.2-rt10/include/linux/printk.h:1036 @ bool nbcon_legacy_emit_next_record(struc * write_atomic() callback * @con: The nbcon console to flush * @stop_seq: Flush up until this record + * @allow_unsafe_takeover: True, to allow unsafe hostile takeovers * * Return: True if taken over while printing. Otherwise false. * @ linux-6.8.2-rt10/include/linux/printk.h:1045 @ bool nbcon_legacy_emit_next_record(struc * there are no more records available to read or this context is not allowed * to acquire the console. */ -static bool __nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq) +static bool __nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq, + bool allow_unsafe_takeover) { struct nbcon_write_context wctxt = { }; struct nbcon_context *ctxt = &ACCESS_PRIVATE(&wctxt, ctxt); @ linux-6.8.2-rt10/include/linux/printk.h:1054 @ static bool __nbcon_atomic_flush_pending ctxt->console = con; ctxt->spinwait_max_us = 2000; ctxt->prio = nbcon_get_default_prio(); + ctxt->allow_unsafe_takeover = allow_unsafe_takeover; if (!nbcon_context_try_acquire(ctxt)) return false; @ linux-6.8.2-rt10/include/linux/printk.h:1081 @ static bool __nbcon_atomic_flush_pending * __nbcon_atomic_flush_pending - Flush all nbcon consoles using their * write_atomic() callback * @stop_seq: Flush up until this record + * @allow_unsafe_takeover: True, to allow unsafe hostile takeovers */ -static void __nbcon_atomic_flush_pending(u64 stop_seq) +static void __nbcon_atomic_flush_pending(u64 stop_seq, bool allow_unsafe_takeover) { struct console *con; bool should_retry; @ linux-6.8.2-rt10/include/linux/printk.h:1116 @ static void __nbcon_atomic_flush_pending */ local_irq_save(irq_flags); - should_retry |= __nbcon_atomic_flush_pending_con(con, stop_seq); - + should_retry |= __nbcon_atomic_flush_pending_con(con, stop_seq, + allow_unsafe_takeover); local_irq_restore(irq_flags); } console_srcu_read_unlock(cookie); @ linux-6.8.2-rt10/include/linux/printk.h:1134 @ static void __nbcon_atomic_flush_pending */ void nbcon_atomic_flush_pending(void) { - __nbcon_atomic_flush_pending(prb_next_reserve_seq(prb)); + __nbcon_atomic_flush_pending(prb_next_reserve_seq(prb), false); +} + +/** + * nbcon_atomic_flush_unsafe - Flush all nbcon consoles using their + * write_atomic() callback and allowing unsafe hostile takeovers + * + * Flush the backlog up through the currently newest record. Unsafe hostile + * takeovers will be performed, if necessary. + */ +void nbcon_atomic_flush_unsafe(void) +{ + __nbcon_atomic_flush_pending(prb_next_reserve_seq(prb), true); } /**