From: John Ogness Date: Mon, 11 Dec 2023 09:34:16 +0000 Subject: [PATCH 50/50] printk: Avoid false positive lockdep report for legacy driver. Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.7/older/patches-6.7-rt6.tar.xz printk may invoke the legacy console driver from atomic context. This leads to a lockdep splat because the console driver will acquire a sleeping lock and the caller may also hold a spinning lock. This is noticed by lockdep on !PREEMPT_RT configurations because it will also lead to a problem on PREEMPT_RT. On PREEMPT_RT the atomic path is always avoided and the console driver is always invoked from a dedicated thread. Thus the lockdep splat is a false positive. Override the lock-context before invoking the console driver. Signed-off-by: John Ogness Signed-off-by: Sebastian Andrzej Siewior --- kernel/printk/printk.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -2824,6 +2824,8 @@ static void __console_unlock(void) up_console_sem(); } +static DEFINE_WAIT_OVERRIDE_MAP(printk_legacy_map, LD_WAIT_SLEEP); + #ifdef CONFIG_PRINTK /* @@ -2992,7 +2994,7 @@ static bool console_emit_next_record(str /* * On PREEMPT_RT this function is either in a thread or * panic context. So there is no need for concern about - * printk reentrance or handovers. + * printk reentrance, handovers, or lockdep complaints. */ con->write(con, outbuf, pmsg.outbuf_len); @@ -3014,7 +3016,9 @@ static bool console_emit_next_record(str /* Do not trace print latency. */ stop_critical_timings(); + lock_map_acquire_try(&printk_legacy_map); con->write(con, outbuf, pmsg.outbuf_len); + lock_map_release(&printk_legacy_map); start_critical_timings(); @@ -3091,7 +3095,10 @@ static bool console_flush_all(bool do_co any_usable = true; if (flags & CON_NBCON) { + + lock_map_acquire_try(&printk_legacy_map); progress = nbcon_atomic_emit_next_record(con, handover, cookie); + lock_map_release(&printk_legacy_map); printk_seq = nbcon_seq_read(con); } else {