Commit 286b113d authored by Marcos Paulo de Souza's avatar Marcos Paulo de Souza Committed by Petr Mladek
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printk: nbcon: Allow KDB to acquire the NBCON context 



KDB can interrupt any console to execute the "mirrored printing" at any
time, so add an exception to nbcon_context_try_acquire_direct to allow
to get the context if the current CPU is the same as kdb_printf_cpu.

This change will be necessary for the next patch, which fixes
kdb_msg_write to work with NBCON consoles by calling ->write_atomic on
such consoles. But to print it first needs to acquire the ownership of
the console, so nbcon_context_try_acquire_direct is fixed here.

Reviewed-by: default avatarJohn Ogness <john.ogness@linutronix.de>
Signed-off-by: default avatarMarcos Paulo de Souza <mpdesouza@suse.com>
Reviewed-by: default avatarPetr Mladek <pmladek@suse.com>
Link: https://patch.msgid.link/20251016-nbcon-kgdboc-v6-3-866aac60a80e@suse.com


[pmladek@suse.com: Fix compilation with !CONFIG_KGDB_KDB.]
Signed-off-by: default avatarPetr Mladek <pmladek@suse.com>
parent 49f7d305

include/linux/kdb.h

+16 −0
Original line number Diff line number Diff line
@@ -14,6 +14,7 @@ 
 */



#include <linux/list.h>

#include <linux/smp.h>



/* Shifted versions of the command enable bits are be used if the command

 * has no arguments (see kdb_check_flags). This allows commands, such as
@@ -207,11 +208,26 @@ static inline const char *kdb_walk_kallsyms(loff_t *pos) 
/* Dynamic kdb shell command registration */

extern int kdb_register(kdbtab_t *cmd);

extern void kdb_unregister(kdbtab_t *cmd);



/* Return true when KDB as locked for printing a message on this CPU. */

static inline

bool kdb_printf_on_this_cpu(void)

{

	/*

	 * We can use raw_smp_processor_id() here because the task could

	 * not get migrated when KDB has locked for printing on this CPU.

	 */

	return unlikely(READ_ONCE(kdb_printf_cpu) == raw_smp_processor_id());

}



#else /* ! CONFIG_KGDB_KDB */

static inline __printf(1, 2) int kdb_printf(const char *fmt, ...) { return 0; }

static inline void kdb_init(int level) {}

static inline int kdb_register(kdbtab_t *cmd) { return 0; }

static inline void kdb_unregister(kdbtab_t *cmd) {}



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



#endif	/* CONFIG_KGDB_KDB */

enum {

	KDB_NOT_INITIALIZED,

kernel/printk/nbcon.c

+5 −1
Original line number Diff line number Diff line
@@ -10,6 +10,7 @@ 
#include <linux/export.h>

#include <linux/init.h>

#include <linux/irqflags.h>

#include <linux/kdb.h>

#include <linux/kthread.h>

#include <linux/minmax.h>

#include <linux/panic.h>
@@ -249,13 +250,16 @@ static int nbcon_context_try_acquire_direct(struct nbcon_context *ctxt, 
		 * since all non-panic CPUs are stopped during panic(), it

		 * is safer to have them avoid gaining console ownership.

		 *

		 * If this acquire is a reacquire (and an unsafe takeover

		 * One exception is when kdb has locked for printing on this CPU.

		 *

		 * Second exception is a reacquire (and an unsafe takeover

		 * has not previously occurred) then it is allowed to attempt

		 * a direct acquire in panic. This gives console drivers an

		 * opportunity to perform any necessary cleanup if they were

		 * interrupted by the panic CPU while printing.

		 */

		if (panic_on_other_cpu() &&

		    !kdb_printf_on_this_cpu() &&

		    (!is_reacquire || cur->unsafe_takeover)) {

			return -EPERM;

		}