Commit 8b1742c9 authored by Sergey Senozhatsky's avatar Sergey Senozhatsky Committed by Petr Mladek
Browse files

printk: remove zap_locks() function 

We use printk-safe now which makes printk-recursion detection code
in vprintk_emit() unreachable. The tricky thing here is that, apart
from detecting and reporting printk recursions, that code also used
to zap_locks() in case of panic() from the same CPU. However,
zap_locks() does not look to be needed anymore:

1) Since commit 08d78658 ("panic: release stale console lock to
   always get the logbuf printed out") panic flushing of `logbuf' to
   console ignores the state of `console_sem' by doing
   	panic()
		console_trylock();
		console_unlock();

2) Since commit cf9b1106 ("printk/nmi: flush NMI messages on the
   system panic") panic attempts to zap the `logbuf_lock' spin_lock to
   successfully flush nmi messages to `logbuf'.

Basically, it seems that we either already do what zap_locks() used to
do but in other places or we ignore the state of the lock. The only
reaming difference is that we don't re-init the console semaphore in
printk_safe_flush_on_panic(), but this is not necessary because we
don't call console drivers from printk_safe_flush_on_panic() due to
the fact that we are using a deferred printk() version (as was
suggested by Petr Mladek).

Link: http://lkml.kernel.org/r/20161227141611.940-8-sergey.senozhatsky@gmail.com


Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Tejun Heo <tj@kernel.org>
Cc: Calvin Owens <calvinowens@fb.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: default avatarSergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: default avatarPetr Mladek <pmladek@suse.com>
parent f975237b

kernel/printk/printk.c

+0 −61
Original line number Diff line number Diff line
@@ -1557,28 +1557,6 @@ static void call_console_drivers(int level, 
	}

}



/*

 * Zap console related locks when oopsing.

 * To leave time for slow consoles to print a full oops,

 * only zap at most once every 30 seconds.

 */

static void zap_locks(void)

{

	static unsigned long oops_timestamp;



	if (time_after_eq(jiffies, oops_timestamp) &&

	    !time_after(jiffies, oops_timestamp + 30 * HZ))

		return;



	oops_timestamp = jiffies;



	debug_locks_off();

	/* If a crash is occurring, make sure we can't deadlock */

	raw_spin_lock_init(&logbuf_lock);

	/* And make sure that we print immediately */

	sema_init(&console_sem, 1);

}



int printk_delay_msec __read_mostly;



static inline void printk_delay(void)
@@ -1688,17 +1666,13 @@ asmlinkage int vprintk_emit(int facility, int level, 
			    const char *dict, size_t dictlen,

			    const char *fmt, va_list args)

{

	static bool recursion_bug;

	static char textbuf[LOG_LINE_MAX];

	char *text = textbuf;

	size_t text_len = 0;

	enum log_flags lflags = 0;

	unsigned long flags;

	int this_cpu;

	int printed_len = 0;

	bool in_sched = false;

	/* cpu currently holding logbuf_lock in this function */

	static unsigned int logbuf_cpu = UINT_MAX;



	if (level == LOGLEVEL_SCHED) {

		level = LOGLEVEL_DEFAULT;
@@ -1709,42 +1683,8 @@ asmlinkage int vprintk_emit(int facility, int level, 
	printk_delay();



	printk_safe_enter_irqsave(flags);

	this_cpu = smp_processor_id();



	/*

	 * Ouch, printk recursed into itself!

	 */

	if (unlikely(logbuf_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 && !lockdep_recursing(current)) {

			recursion_bug = true;

			printk_safe_exit_irqrestore(flags);

			return 0;

		}

		zap_locks();

	}



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

	raw_spin_lock(&logbuf_lock);

	logbuf_cpu = this_cpu;



	if (unlikely(recursion_bug)) {

		static const char recursion_msg[] =

			"BUG: recent printk recursion!";



		recursion_bug = false;

		/* emit KERN_CRIT message */

		printed_len += log_store(0, 2, LOG_PREFIX|LOG_NEWLINE, 0,

					 NULL, 0, recursion_msg,

					 strlen(recursion_msg));

	}



	/*

	 * The printf needs to come first; we need the syslog

	 * prefix which might be passed-in as a parameter.
@@ -1787,7 +1727,6 @@ asmlinkage int vprintk_emit(int facility, int level, 


	printed_len += log_output(facility, level, lflags, dict, dictlen, text, text_len);



	logbuf_cpu = UINT_MAX;

	raw_spin_unlock(&logbuf_lock);

	printk_safe_exit_irqrestore(flags);

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