releases/4.9.47/locking-spinlock-debug-remove-spinlock-lockup-detection-code.patch

From bc88c10d7e6900916f5e1ba3829d66a9de92b633 Mon Sep 17 00:00:00 2001
From: Waiman Long <longman@redhat.com>
Date: Wed, 8 Feb 2017 14:46:48 -0500
Subject: locking/spinlock/debug: Remove spinlock lockup detection code

From: Waiman Long <longman@redhat.com>

commit bc88c10d7e6900916f5e1ba3829d66a9de92b633 upstream.

The current spinlock lockup detection code can sometimes produce false
positives because of the unfairness of the locking algorithm itself.

So the lockup detection code is now removed. Instead, we are relying
on the NMI watchdog to detect potential lockup. We won't have lockup
detection if the watchdog isn't running.

The commented-out read-write lock lockup detection code are also
removed.

Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1486583208-11038-1-git-send-email-longman@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 kernel/locking/spinlock_debug.c |   86 ++--------------------------------------
 1 file changed, 5 insertions(+), 81 deletions(-)

--- a/kernel/locking/spinlock_debug.c
+++ b/kernel/locking/spinlock_debug.c
@@ -103,38 +103,14 @@ static inline void debug_spin_unlock(raw
 	lock->owner_cpu = -1;
 }
 
-static void __spin_lock_debug(raw_spinlock_t *lock)
-{
-	u64 i;
-	u64 loops = loops_per_jiffy * HZ;
-
-	for (i = 0; i < loops; i++) {
-		if (arch_spin_trylock(&lock->raw_lock))
-			return;
-		__delay(1);
-	}
-	/* lockup suspected: */
-	spin_dump(lock, "lockup suspected");
-#ifdef CONFIG_SMP
-	trigger_all_cpu_backtrace();
-#endif
-
-	/*
-	 * The trylock above was causing a livelock.  Give the lower level arch
-	 * specific lock code a chance to acquire the lock. We have already
-	 * printed a warning/backtrace at this point. The non-debug arch
-	 * specific code might actually succeed in acquiring the lock.  If it is
-	 * not successful, the end-result is the same - there is no forward
-	 * progress.
-	 */
-	arch_spin_lock(&lock->raw_lock);
-}
-
+/*
+ * We are now relying on the NMI watchdog to detect lockup instead of doing
+ * the detection here with an unfair lock which can cause problem of its own.
+ */
 void do_raw_spin_lock(raw_spinlock_t *lock)
 {
 	debug_spin_lock_before(lock);
-	if (unlikely(!arch_spin_trylock(&lock->raw_lock)))
-		__spin_lock_debug(lock);
+	arch_spin_lock(&lock->raw_lock);
 	debug_spin_lock_after(lock);
 }
 
@@ -172,32 +148,6 @@ static void rwlock_bug(rwlock_t *lock, c
 
 #define RWLOCK_BUG_ON(cond, lock, msg) if (unlikely(cond)) rwlock_bug(lock, msg)
 
-#if 0		/* __write_lock_debug() can lock up - maybe this can too? */
-static void __read_lock_debug(rwlock_t *lock)
-{
-	u64 i;
-	u64 loops = loops_per_jiffy * HZ;
-	int print_once = 1;
-
-	for (;;) {
-		for (i = 0; i < loops; i++) {
-			if (arch_read_trylock(&lock->raw_lock))
-				return;
-			__delay(1);
-		}
-		/* lockup suspected: */
-		if (print_once) {
-			print_once = 0;
-			printk(KERN_EMERG "BUG: read-lock lockup on CPU#%d, "
-					"%s/%d, %p\n",
-				raw_smp_processor_id(), current->comm,
-				current->pid, lock);
-			dump_stack();
-		}
-	}
-}
-#endif
-
 void do_raw_read_lock(rwlock_t *lock)
 {
 	RWLOCK_BUG_ON(lock->magic != RWLOCK_MAGIC, lock, "bad magic");
@@ -247,32 +197,6 @@ static inline void debug_write_unlock(rw
 	lock->owner_cpu = -1;
 }
 
-#if 0		/* This can cause lockups */
-static void __write_lock_debug(rwlock_t *lock)
-{
-	u64 i;
-	u64 loops = loops_per_jiffy * HZ;
-	int print_once = 1;
-
-	for (;;) {
-		for (i = 0; i < loops; i++) {
-			if (arch_write_trylock(&lock->raw_lock))
-				return;
-			__delay(1);
-		}
-		/* lockup suspected: */
-		if (print_once) {
-			print_once = 0;
-			printk(KERN_EMERG "BUG: write-lock lockup on CPU#%d, "
-					"%s/%d, %p\n",
-				raw_smp_processor_id(), current->comm,
-				current->pid, lock);
-			dump_stack();
-		}
-	}
-}
-#endif
-
 void do_raw_write_lock(rwlock_t *lock)
 {
 	debug_write_lock_before(lock);