Commit 0161e2d6 authored by John Ogness's avatar John Ogness Committed by Petr Mladek
Browse files

printk: Defer legacy printing when holding printk_cpu_sync 



The documentation of printk_cpu_sync_get() clearly states
that the owner must never perform any activities where it waits
for a CPU. For legacy printing there can be spinning on the
console_lock and on the port lock. Therefore legacy printing
must be deferred when holding the printk_cpu_sync.

Note that in the case of emergency states, atomic consoles
are not prevented from printing when printk is deferred. This
is appropriate because they do not spin-wait indefinitely for
other CPUs.

Reported-by: default avatarRik van Riel <riel@surriel.com>
Closes: https://lore.kernel.org/r/20240715232052.73eb7fb1@imladris.surriel.com


Signed-off-by: default avatarJohn Ogness <john.ogness@linutronix.de>
Fixes: 55d6af1d ("lib/nmi_backtrace: explicitly serialize banner and regs")
Reviewed-by: default avatarPetr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20241209111746.192559-3-john.ogness@linutronix.de


Signed-off-by: default avatarPetr Mladek <pmladek@suse.com>
parent f1c21cf4

kernel/printk/internal.h

+6 −0
Original line number Diff line number Diff line
@@ -338,3 +338,9 @@ bool printk_get_next_message(struct printk_message *pmsg, u64 seq, 
void console_prepend_dropped(struct printk_message *pmsg, unsigned long dropped);

void console_prepend_replay(struct printk_message *pmsg);

#endif



#ifdef CONFIG_SMP

bool is_printk_cpu_sync_owner(void);

#else

static inline bool is_printk_cpu_sync_owner(void) { return false; }

#endif

kernel/printk/printk.c

+5 −0
Original line number Diff line number Diff line
@@ -4922,6 +4922,11 @@ void console_try_replay_all(void) 
static atomic_t printk_cpu_sync_owner = ATOMIC_INIT(-1);

static atomic_t printk_cpu_sync_nested = ATOMIC_INIT(0);



bool is_printk_cpu_sync_owner(void)

{

	return (atomic_read(&printk_cpu_sync_owner) == raw_smp_processor_id());

}



/**

 * __printk_cpu_sync_wait() - Busy wait until the printk cpu-reentrant

 *                            spinning lock is not owned by any CPU.

kernel/printk/printk_safe.c

+6 −1
Original line number Diff line number Diff line
@@ -61,10 +61,15 @@ bool is_printk_legacy_deferred(void) 
	/*

	 * The per-CPU variable @printk_context can be read safely in any

	 * context. CPU migration is always disabled when set.

	 *

	 * A context holding the printk_cpu_sync must not spin waiting for

	 * another CPU. For legacy printing, it could be the console_lock

	 * or the port lock.

	 */

	return (force_legacy_kthread() ||

		this_cpu_read(printk_context) ||

		in_nmi());

		in_nmi() ||

		is_printk_cpu_sync_owner());

}



asmlinkage int vprintk(const char *fmt, va_list args)