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Commit 32a76006 authored by Ingo Molnar's avatar Ingo Molnar
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printk: make printk more robust by not allowing recursion 



make printk more robust by allowing recursion only if there's a crash
going on. Also add recursion detection.

I've tested it with an artificially injected printk recursion - instead
of a lockup or spontaneous reboot or other crash, the output was a well
controlled:

[   41.057335] SysRq : <2>BUG: recent printk recursion!
[   41.057335] loglevel0-8 reBoot Crashdump show-all-locks(D) tErm Full kIll saK showMem Nice powerOff showPc show-all-timers(Q) unRaw Sync showTasks Unmount shoW-blocked-tasks

also do all this printk-debug logic with irqs disabled.

Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
Reviewed-by: default avatarNick Piggin <npiggin@suse.de>
parent b47711bf

kernel/printk.c

+38 −10
Original line number Original line Diff line number Diff line
@@ -628,30 +628,57 @@ asmlinkage int printk(const char *fmt, ...) 
/* cpu currently holding logbuf_lock */

/* cpu currently holding logbuf_lock */

static volatile unsigned int printk_cpu = UINT_MAX;

static volatile unsigned int printk_cpu = UINT_MAX;





const char printk_recursion_bug_msg [] =

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

static int printk_recursion_bug;



asmlinkage int vprintk(const char *fmt, va_list args)

asmlinkage int vprintk(const char *fmt, va_list args)

{

{

	static int log_level_unknown = 1;

	static char printk_buf[1024];



	unsigned long flags;

	unsigned long flags;

	int printed_len;

	int printed_len = 0;

	int this_cpu;

	char *p;

	char *p;

	static char printk_buf[1024];

	static int log_level_unknown = 1;





	boot_delay_msec();

	boot_delay_msec();





	preempt_disable();

	preempt_disable();

	if (unlikely(oops_in_progress) && printk_cpu == smp_processor_id())

		/* If a crash is occurring during printk() on this CPU,

		 * make sure we can't deadlock */

		zap_locks();



	/* This stops the holder of console_sem just where we want him */

	/* This stops the holder of console_sem just where we want him */

	raw_local_irq_save(flags);

	raw_local_irq_save(flags);

	this_cpu = smp_processor_id();



	/*

	 * Ouch, printk recursed into itself!

	 */

	if (unlikely(printk_cpu == this_cpu)) {

		/*

		 * If a crash is occurring during printk() on this CPU,

		 * then try to get the crash message out but make sure

		 * we can't deadlock. Otherwise just return to avoid the

		 * recursion and return - but flag the recursion so that

		 * it can be printed at the next appropriate moment:

		 */

		if (!oops_in_progress) {

			printk_recursion_bug = 1;

			goto out_restore_irqs;

		}

		zap_locks();

	}



	lockdep_off();

	lockdep_off();

	spin_lock(&logbuf_lock);

	spin_lock(&logbuf_lock);

	printk_cpu = smp_processor_id();

	printk_cpu = this_cpu;





	if (printk_recursion_bug) {

		printk_recursion_bug = 0;

		strcpy(printk_buf, printk_recursion_bug_msg);

		printed_len = sizeof(printk_recursion_bug_msg);

	}

	/* Emit the output into the temporary buffer */

	/* Emit the output into the temporary buffer */

	printed_len = vscnprintf(printk_buf, sizeof(printk_buf), fmt, args);

	printed_len += vscnprintf(printk_buf + printed_len,

				  sizeof(printk_buf), fmt, args);





	/*

	/*

	 * Copy the output into log_buf.  If the caller didn't provide

	 * Copy the output into log_buf.  If the caller didn't provide
@@ -744,6 +771,7 @@ asmlinkage int vprintk(const char *fmt, va_list args) 
		printk_cpu = UINT_MAX;

		printk_cpu = UINT_MAX;

		spin_unlock(&logbuf_lock);

		spin_unlock(&logbuf_lock);

		lockdep_on();

		lockdep_on();

out_restore_irqs:

		raw_local_irq_restore(flags);

		raw_local_irq_restore(flags);

	}

	}