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printk: Avoid false positive lockdep report for legacy printing

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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: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240904120536.115780-18-john.ogness@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>
This commit is contained in:
John Ogness 2024-09-04 14:11:36 +02:06 committed by Petr Mladek
parent 1529bbb6e2
commit daeed1595b
1 changed files with 64 additions and 21 deletions
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85
kernel/printk/printk.c
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@ -2981,6 +2981,34 @@ out:
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.
@ -3030,31 +3058,46 @@ static bool console_emit_next_record(struct console *con, bool *handover, int co
con->dropped = 0;
}
/*
* While actively printing out messages, if another printk()
* were to occur on another CPU, it may wait for this one to
* finish. This task can not be preempted if there is a
* waiter waiting to take over.
*
* Interrupts are disabled because the hand over to a waiter
* must not be interrupted until the hand over is completed
* (@console_waiter is cleared).
*/
printk_safe_enter_irqsave(flags);
console_lock_spinning_enable();
/* Do not trace print latency. */
stop_critical_timings();
/* Write everything out to the hardware. */
con->write(con, outbuf, pmsg.outbuf_len);
start_critical_timings();
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->seq = pmsg.seq + 1;
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
* finish. This task can not be preempted if there is a
* waiter waiting to take over.
*
* Interrupts are disabled because the hand over to a waiter
* must not be interrupted until the hand over is completed
* (@console_waiter is cleared).
*/
printk_safe_enter_irqsave(flags);
console_lock_spinning_enable();
*handover = console_lock_spinning_disable_and_check(cookie);
printk_safe_exit_irqrestore(flags);
/* Do not trace print latency. */
stop_critical_timings();
printk_legacy_allow_spinlock_enter();
con->write(con, outbuf, pmsg.outbuf_len);
printk_legacy_allow_spinlock_exit();
start_critical_timings();
con->seq = pmsg.seq + 1;
*handover = console_lock_spinning_disable_and_check(cookie);
printk_safe_exit_irqrestore(flags);
}
skip:
return true;
}