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Commit 266c2e0a authored by Linus Torvalds's avatar Linus Torvalds
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Make printk() console semaphore accesses sensible 



The printk() logic on when/how to get the console semaphore was
unreadable, this splits the code up into a few helper functions and
makes it easier to follow what is going on.

Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 5f7b703f

kernel/printk.c

+48 −35
Original line number Diff line number Diff line
@@ -616,6 +616,40 @@ asmlinkage int printk(const char *fmt, ...) 
/* cpu currently holding logbuf_lock */

static volatile unsigned int printk_cpu = UINT_MAX;



/*

 * Can we actually use the console at this time on this cpu?

 *

 * Console drivers may assume that per-cpu resources have

 * been allocated. So unless they're explicitly marked as

 * being able to cope (CON_ANYTIME) don't call them until

 * this CPU is officially up.

 */

static inline int can_use_console(unsigned int cpu)

{

	return cpu_online(cpu) || have_callable_console();

}



/*

 * Try to get console ownership to actually show the kernel

 * messages from a 'printk'. Return true (and with the

 * console_semaphore held, and 'console_locked' set) if it

 * is successful, false otherwise.

 *

 * This gets called with the 'logbuf_lock' spinlock held and

 * interrupts disabled. It should return with 'lockbuf_lock'

 * released but interrupts still disabled.

 */

static int acquire_console_semaphore_for_printk(unsigned int cpu)

{

	int retval = 0;



	if (can_use_console(cpu))

		retval = !try_acquire_console_sem();

	printk_cpu = UINT_MAX;

	spin_unlock(&logbuf_lock);

	return retval;

}



const char printk_recursion_bug_msg [] =

			KERN_CRIT "BUG: recent printk recursion!\n";

static int printk_recursion_bug;
@@ -725,43 +759,22 @@ asmlinkage int vprintk(const char *fmt, va_list args) 
			log_level_unknown = 1;

	}



	if (!down_trylock(&console_sem)) {

		/*

		 * We own the drivers.  We can drop the spinlock and

		 * let release_console_sem() print the text, maybe ...

		 */

		console_locked = 1;

		printk_cpu = UINT_MAX;

		spin_unlock(&logbuf_lock);



	/*

		 * Console drivers may assume that per-cpu resources have

		 * been allocated. So unless they're explicitly marked as

		 * being able to cope (CON_ANYTIME) don't call them until

		 * this CPU is officially up.

	 * Try to acquire and then immediately release the

	 * console semaphore. The release will do all the

	 * actual magic (print out buffers, wake up klogd,

	 * etc). 

	 *

	 * The acquire_console_semaphore_for_printk() function

	 * will release 'logbuf_lock' regardless of whether it

	 * actually gets the semaphore or not.

	 */

		if (cpu_online(smp_processor_id()) || have_callable_console()) {

			console_may_schedule = 0;

	if (acquire_console_semaphore_for_printk(this_cpu))

		release_console_sem();

		} else {

			/* Release by hand to avoid flushing the buffer. */

			console_locked = 0;

			up(&console_sem);

		}

		lockdep_on();

		raw_local_irq_restore(flags);

	} else {

		/*

		 * Someone else owns the drivers.  We drop the spinlock, which

		 * allows the semaphore holder to proceed and to call the

		 * console drivers with the output which we just produced.

		 */

		printk_cpu = UINT_MAX;

		spin_unlock(&logbuf_lock);



	lockdep_on();

out_restore_irqs:

	raw_local_irq_restore(flags);

	}



	preempt_enable();

	return printed_len;