linux-kernel - [RFC][PATCHv2 7/7] printk: remove zap

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Message-Id: <20160930151758.8965-8-sergey.senozhatsky@gmail.com>
Date:   Sat,  1 Oct 2016 00:17:58 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Petr Mladek <pmladek@...e.com>, Jan Kara <jack@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, Calvin Owens <calvinowens@...com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: [RFC][PATCHv2 7/7] printk: remove zap_locks() function

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

1) Since 08d78658f393 ('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 cf9b1106c81c ('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. May be we
still would want to do sema_init() in alt_printk_flush_on_panic(), just
in case.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
---
 kernel/printk/printk.c | 63 --------------------------------------------------
 1 file changed, 63 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index b99d6b3..fb2d26b 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1600,28 +1600,6 @@ 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)
@@ -1748,18 +1726,13 @@ 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;
 	int nmi_message_lost;
 	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;
@@ -1769,43 +1742,9 @@ asmlinkage int vprintk_emit(int facility, int level,
 	boot_delay_msec(level);
 	printk_delay();
 
-	local_irq_save(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;
-			local_irq_restore(flags);
-			return 0;
-		}
-		zap_locks();
-	}
-
 	alt_printk_enter();
 	/* 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));
-	}
 
 	nmi_message_lost = get_nmi_message_lost();
 	if (unlikely(nmi_message_lost)) {
@@ -1898,10 +1837,8 @@ asmlinkage int vprintk_emit(int facility, int level,
 						 dict, dictlen, text, text_len);
 	}
 
-	logbuf_cpu = UINT_MAX;
 	raw_spin_unlock(&logbuf_lock);
 	alt_printk_exit();
-	local_irq_restore(flags);
 
 	/* If called from the scheduler, we can not call up(). */
 	if (!in_sched) {
-- 
2.10.0.372.g6fe1b14