From: John Ogness Date: Fri, 20 Oct 2023 10:03:42 +0000 Subject: [PATCH 28/50] printk: nbcon: Add unsafe flushing on panic Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.7/older/patches-6.7-rt6.tar.xz 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 Signed-off-by: Sebastian Andrzej Siewior --- include/linux/printk.h | 5 +++++ kernel/panic.c | 1 + kernel/printk/nbcon.c | 18 ++++++++++++++++-- 3 files changed, 22 insertions(+), 2 deletions(-) --- a/include/linux/printk.h +++ b/include/linux/printk.h @@ -199,6 +199,7 @@ extern asmlinkage void dump_stack(void) void printk_trigger_flush(void); extern void nbcon_acquire(struct uart_port *up); extern void nbcon_release(struct uart_port *up); +void nbcon_atomic_flush_unsafe(void); #else static inline __printf(1, 0) int vprintk(const char *s, va_list args) @@ -287,6 +288,10 @@ static inline void nbcon_release(struct { } +static inline void nbcon_atomic_flush_unsafe(void) +{ +} + #endif #ifdef CONFIG_SMP --- a/kernel/panic.c +++ b/kernel/panic.c @@ -453,6 +453,7 @@ 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) { --- a/kernel/printk/nbcon.c +++ b/kernel/printk/nbcon.c @@ -1030,8 +1030,9 @@ bool nbcon_atomic_emit_next_record(struc * __nbcon_atomic_flush_all - 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_all(u64 stop_seq) +static void __nbcon_atomic_flush_all(u64 stop_seq, bool allow_unsafe_takeover) { struct nbcon_write_context wctxt = { }; struct nbcon_context *ctxt = &ACCESS_PRIVATE(&wctxt, ctxt); @@ -1059,6 +1060,7 @@ static void __nbcon_atomic_flush_all(u64 memset(ctxt, 0, sizeof(*ctxt)); ctxt->console = con; ctxt->spinwait_max_us = 2000; + ctxt->allow_unsafe_takeover = allow_unsafe_takeover; /* * Atomic flushing does not use console driver @@ -1093,7 +1095,19 @@ static void __nbcon_atomic_flush_all(u64 */ void nbcon_atomic_flush_all(void) { - __nbcon_atomic_flush_all(prb_next_reserve_seq(prb)); + __nbcon_atomic_flush_all(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_all(prb_next_reserve_seq(prb), true); } /**