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[PATCH 0/2] locking/spinlock_debug: Change it to a mostly fair lock

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  [PATCH 0/2] locking/spinlock_debug: Change it to a mostly fair lock Waiman Long <longman@redhat.com> - 2017-02-01 19:30 +0100
    [PATCH 1/2] locking/spinlock_debug: Reduce lockup suspected message clutter Waiman Long <longman@redhat.com> - 2017-02-01 19:30 +0100

#1571879 — [PATCH 0/2] locking/spinlock_debug: Change it to a mostly fair lock

The current debug spinlock implementation is a TATAS unfair lock. This
can occasionally lead to system lockup with a debug kernel because
of the unfairness of the lock rather than inherent locking problem.

This patch set changes the debug spinlock implementation to a
mostly fair spinlock based on the MCS lock similar to what is done
in qspinlock.

Waiman Long (2):
  locking/spinlock_debug: Reduce lockup suspected message clutter
  locking/spinlock_debug: Reduce lock cacheline contention

 include/linux/spinlock_types.h  |  8 +++--
 kernel/locking/spinlock_debug.c | 73 ++++++++++++++++++++++++++++++-----------
 2 files changed, 58 insertions(+), 23 deletions(-)

-- 
1.8.3.1

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#1571881 — [PATCH 1/2] locking/spinlock_debug: Reduce lockup suspected message clutter

When the debug spinlock code detects a lockup, it will print out an
error messages as well as the backtraces of all the CPUs. However, if
more than one CPUs are waiting on that lock, multiple lockup messages
will be printed leading to garbled output.

To reduce clutter in the console log, now only one of the lock waiters
will be allowed to print out the CPU backtraces.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 include/linux/spinlock_types.h  |  3 ++-
 kernel/locking/spinlock_debug.c | 26 +++++++++++++++++++++-----
 2 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/include/linux/spinlock_types.h b/include/linux/spinlock_types.h
index 73548eb..ef28ce5 100644
--- a/include/linux/spinlock_types.h
+++ b/include/linux/spinlock_types.h
@@ -23,7 +23,7 @@
 	unsigned int break_lock;
 #endif
 #ifdef CONFIG_DEBUG_SPINLOCK
-	unsigned int magic, owner_cpu;
+	unsigned int magic, owner_cpu, lockup;
 	void *owner;
 #endif
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
@@ -45,6 +45,7 @@
 # define SPIN_DEBUG_INIT(lockname)		\
 	.magic = SPINLOCK_MAGIC,		\
 	.owner_cpu = -1,			\
+	.lockup = 0,				\
 	.owner = SPINLOCK_OWNER_INIT,
 #else
 # define SPIN_DEBUG_INIT(lockname)
diff --git a/kernel/locking/spinlock_debug.c b/kernel/locking/spinlock_debug.c
index 0374a59..0f880a8 100644
--- a/kernel/locking/spinlock_debug.c
+++ b/kernel/locking/spinlock_debug.c
@@ -27,6 +27,7 @@ void __raw_spin_lock_init(raw_spinlock_t *lock, const char *name,
 	lock->magic = SPINLOCK_MAGIC;
 	lock->owner = SPINLOCK_OWNER_INIT;
 	lock->owner_cpu = -1;
+	lock->lockup = 0;
 }
 
 EXPORT_SYMBOL(__raw_spin_lock_init);
@@ -101,6 +102,24 @@ static inline void debug_spin_unlock(raw_spinlock_t *lock)
 							lock, "wrong CPU");
 	lock->owner = SPINLOCK_OWNER_INIT;
 	lock->owner_cpu = -1;
+	lock->lockup = 0;
+}
+
+static inline void __spin_lockup(raw_spinlock_t *lock)
+{
+	/*
+	 * lockup suspected:
+	 *
+	 * Only one of the lock waiters will be allowed to print the lockup
+	 * message in order to avoid an avalanche of lockup and backtrace
+	 * messages from different lock waiters of the same lock.
+	 */
+	if (!xchg(&lock->lockup, 1)) {
+		spin_dump(lock, "lockup suspected");
+#ifdef CONFIG_SMP
+		trigger_all_cpu_backtrace();
+#endif
+	}
 }
 
 static void __spin_lock_debug(raw_spinlock_t *lock)
@@ -113,11 +132,8 @@ static void __spin_lock_debug(raw_spinlock_t *lock)
 			return;
 		__delay(1);
 	}
-	/* lockup suspected: */
-	spin_dump(lock, "lockup suspected");
-#ifdef CONFIG_SMP
-	trigger_all_cpu_backtrace();
-#endif
+
+	__spin_lockup(lock);
 
 	/*
 	 * The trylock above was causing a livelock.  Give the lower level arch
-- 
1.8.3.1

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