Commit daeed159 authored by John Ogness's avatar John Ogness Committed by Petr Mladek
Browse files

printk: Avoid false positive lockdep report for legacy printing 



Legacy console printing from printk() caller context may invoke
the console driver from atomic context. This leads to a lockdep
splat because the console driver will acquire a sleeping lock
and the caller may already hold a spinning lock. This is noticed
by lockdep on !PREEMPT_RT configurations because it will lead to
a problem on PREEMPT_RT.

However, on PREEMPT_RT the printing path from atomic context is
always avoided and the console driver is always invoked from a
dedicated thread. Thus the lockdep splat on !PREEMPT_RT is a
false positive.

For !PREEMPT_RT override the lock-context before invoking the
console driver to avoid the false positive.

Do not override the lock-context for PREEMPT_RT in order to
allow lockdep to catch any real locking context issues related
to the write callback usage.

Signed-off-by: default avatarJohn Ogness <john.ogness@linutronix.de>
Reviewed-by: default avatarPetr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240904120536.115780-18-john.ogness@linutronix.de


Signed-off-by: default avatarPetr Mladek <pmladek@suse.com>
parent 1529bbb6

kernel/printk/printk.c

+63 −20
Original line number Diff line number Diff line
@@ -2981,6 +2981,34 @@ bool printk_get_next_message(struct printk_message *pmsg, u64 seq, 
	return true;

}



/*

 * Legacy console printing from printk() caller context does not respect

 * raw_spinlock/spinlock nesting. For !PREEMPT_RT the lockdep warning is a

 * false positive. For PREEMPT_RT the false positive condition does not

 * occur.

 *

 * This map is used to temporarily establish LD_WAIT_SLEEP context for the

 * console write() callback when legacy printing to avoid false positive

 * lockdep complaints, thus allowing lockdep to continue to function for

 * real issues.

 */

#ifdef CONFIG_PREEMPT_RT

static inline void printk_legacy_allow_spinlock_enter(void) { }

static inline void printk_legacy_allow_spinlock_exit(void) { }

#else

static DEFINE_WAIT_OVERRIDE_MAP(printk_legacy_map, LD_WAIT_SLEEP);



static inline void printk_legacy_allow_spinlock_enter(void)

{

	lock_map_acquire_try(&printk_legacy_map);

}



static inline void printk_legacy_allow_spinlock_exit(void)

{

	lock_map_release(&printk_legacy_map);

}

#endif /* CONFIG_PREEMPT_RT */



/*

 * Used as the printk buffers for non-panic, serialized console printing.

 * This is for legacy (!CON_NBCON) as well as all boot (CON_BOOT) consoles.
@@ -3030,6 +3058,19 @@ static bool console_emit_next_record(struct console *con, bool *handover, int co 
		con->dropped = 0;

	}



	/* Write everything out to the hardware. */



	if (force_legacy_kthread() && !panic_in_progress()) {

		/*

		 * With forced threading this function is in a task context

		 * (either legacy kthread or get_init_console_seq()). There

		 * is no need for concern about printk reentrance, handovers,

		 * or lockdep complaints.

		 */



		con->write(con, outbuf, pmsg.outbuf_len);

		con->seq = pmsg.seq + 1;

	} else {

		/*

		 * While actively printing out messages, if another printk()

		 * were to occur on another CPU, it may wait for this one to
@@ -3046,8 +3087,9 @@ static bool console_emit_next_record(struct console *con, bool *handover, int co 
		/* Do not trace print latency. */

		stop_critical_timings();



	/* Write everything out to the hardware. */

		printk_legacy_allow_spinlock_enter();

		con->write(con, outbuf, pmsg.outbuf_len);

		printk_legacy_allow_spinlock_exit();



		start_critical_timings();
@@ -3055,6 +3097,7 @@ static bool console_emit_next_record(struct console *con, bool *handover, int co 


		*handover = console_lock_spinning_disable_and_check(cookie);

		printk_safe_exit_irqrestore(flags);

	}

skip:

	return true;

}