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printk: nbcon: Introduce KDB helpers

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These helpers will be used when calling console->write_atomic on
KDB code in the next patch. It's basically the same implementation
as nbcon_device_try_acquire, but using NBCON_PRIO_EMERGENCY when
acquiring the context.

If the acquire succeeds, the message and message length are assigned to
nbcon_write_context so ->write_atomic can print the message.

After release try to flush the console since there may be a backlog of
messages in the ringbuffer. The kthread console printers do not get a
chance to run while kdb is active.

Reviewed-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
Link: https://patch.msgid.link/20251016-nbcon-kgdboc-v6-2-866aac60a80e@suse.com
Signed-off-by: Petr Mladek <pmladek@suse.com>
This commit is contained in:
Marcos Paulo de Souza 2025-10-16 11:47:55 -03:00 committed by Petr Mladek
parent 4da42aaa82
commit 49f7d3054e
2 changed files with 67 additions and 0 deletions
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6
include/linux/console.h
View File

@ -606,6 +606,9 @@ extern bool nbcon_can_proceed(struct nbcon_write_context *wctxt);
extern bool nbcon_enter_unsafe(struct nbcon_write_context *wctxt);
extern bool nbcon_exit_unsafe(struct nbcon_write_context *wctxt);
extern void nbcon_reacquire_nobuf(struct nbcon_write_context *wctxt);
extern bool nbcon_kdb_try_acquire(struct console *con,
struct nbcon_write_context *wctxt);
extern void nbcon_kdb_release(struct nbcon_write_context *wctxt);
/*
* Check if the given console is currently capable and allowed to print
@ -655,6 +658,9 @@ static inline bool nbcon_can_proceed(struct nbcon_write_context *wctxt) { return
static inline bool nbcon_enter_unsafe(struct nbcon_write_context *wctxt) { return false; }
static inline bool nbcon_exit_unsafe(struct nbcon_write_context *wctxt) { return false; }
static inline void nbcon_reacquire_nobuf(struct nbcon_write_context *wctxt) { }
static inline bool nbcon_kdb_try_acquire(struct console *con,
struct nbcon_write_context *wctxt) { return false; }
static inline void nbcon_kdb_release(struct console *con) { }
static inline bool console_is_usable(struct console *con, short flags,
bool use_atomic) { return false; }
#endif

61
kernel/printk/nbcon.c
View File

@ -1855,3 +1855,64 @@ void nbcon_device_release(struct console *con)
console_srcu_read_unlock(cookie);
}
EXPORT_SYMBOL_GPL(nbcon_device_release);
/**
* nbcon_kdb_try_acquire - Try to acquire nbcon console and enter unsafe
* section
* @con: The nbcon console to acquire
* @wctxt: The nbcon write context to be used on success
*
* Context: Under console_srcu_read_lock() for emitting a single kdb message
* using the given con->write_atomic() callback. Can be called
* only when the console is usable at the moment.
*
* Return: True if the console was acquired. False otherwise.
*
* kdb emits messages on consoles registered for printk() without
* storing them into the ring buffer. It has to acquire the console
* ownerhip so that it could call con->write_atomic() callback a safe way.
*
* This function acquires the nbcon console using priority NBCON_PRIO_EMERGENCY
* and marks it unsafe for handover/takeover.
*/
bool nbcon_kdb_try_acquire(struct console *con,
struct nbcon_write_context *wctxt)
{
struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
memset(ctxt, 0, sizeof(*ctxt));
ctxt->console = con;
ctxt->prio = NBCON_PRIO_EMERGENCY;
if (!nbcon_context_try_acquire(ctxt, false))
return false;
if (!nbcon_context_enter_unsafe(ctxt))
return false;
return true;
}
/**
* nbcon_kdb_release - Exit unsafe section and release the nbcon console
*
* @wctxt: The nbcon write context initialized by a successful
* nbcon_kdb_try_acquire()
*/
void nbcon_kdb_release(struct nbcon_write_context *wctxt)
{
struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
if (!nbcon_context_exit_unsafe(ctxt))
return;
nbcon_context_release(ctxt);
/*
* Flush any new printk() messages added when the console was blocked.
* Only the console used by the given write context was blocked.
* The console was locked only when the write_atomic() callback
* was usable.
*/
__nbcon_atomic_flush_pending_con(ctxt->console, prb_next_reserve_seq(prb), false);
}