linux-kernel - [PATCH v2] atomic: Fix bugs in 'fetch_or()' and rename it to 'xchg

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Date:	Tue, 15 Mar 2016 13:21:45 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH v2] atomic: Fix bugs in 'fetch_or()' and rename it to
 'xchg_or()'

Linus noticed a couple of problems with the fetch_or() implementation introduced
by 5fd7a09cfb8c ("atomic: Export fetch_or()"):

 - Sloppy macro implementation: 'mask' and 'ptr' is evaluated multiple times,
   which will break if arguments have side effects. Also, it uses 
   double-underscore temporary variables - which is dangerous as low level asm 
   ones are using those too and they may alias in unexpected ways.

 - Sloppy semantics: the naming and structure of the macro is ambigious, with
   no well-defined semantics. It's neither an atomic nor a cmpxchg() interface,
   but still it lives in include/linux/atomic.h.

Solve this by:

 - Extracting the arguments into helper variables once, and never
   using the original arguments from that point on. This makes it
   pretty clear that there can be no unwanted macro evaluation
   side effects.

 - Using single-underscore temporary variables.

 - Renaming fetch_or() to xchg_or(), recognizing that the semantics
   are xchg()-alike.

Also propagate the rename to the call sites.

Reported-by: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Chris Metcalf <cmetcalf@...hip.com>
Cc: Christoph Lameter <cl@...ux.com>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: Luiz Capitulino <lcapitulino@...hat.com>
Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Rik van Riel <riel@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Viresh Kumar <viresh.kumar@...aro.org>
Link: http://lkml.kernel.org/r/20160315093245.GA7943@gmail.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 include/linux/atomic.h   | 26 ++++++++++++++++----------
 kernel/sched/core.c      |  2 +-
 kernel/time/tick-sched.c |  4 ++--
 3 files changed, 19 insertions(+), 13 deletions(-)

diff --git a/include/linux/atomic.h b/include/linux/atomic.h
index 6c502cb13c95..7bc5297bcca8 100644
--- a/include/linux/atomic.h
+++ b/include/linux/atomic.h
@@ -549,22 +549,28 @@ static inline int atomic_dec_if_positive(atomic_t *v)
 #endif
 
 /**
- * fetch_or - perform *ptr |= mask and return old value of *ptr
- * @ptr: pointer to value
+ * xchg_or - perform *ptr |= mask atomically and return old value of *ptr
+ * @ptr: pointer to value (cmpxchg() compatible integer pointer type)
  * @mask: mask to OR on the value
  *
- * cmpxchg based fetch_or, macro so it works for different integer types
+ * cmpxchg() based, it's a macro so it works for different integer types.
  */
-#ifndef fetch_or
-#define fetch_or(ptr, mask)						\
-({	typeof(*(ptr)) __old, __val = *(ptr);				\
+#ifndef xchg_or
+# define xchg_or(ptr, mask)						\
+({									\
+	typeof(ptr)  _ptr  = (ptr);					\
+	typeof(mask) _mask = (mask);					\
+									\
+	typeof(*_ptr) _old, _val = *_ptr;				\
+									\
 	for (;;) {							\
-		__old = cmpxchg((ptr), __val, __val | (mask));		\
-		if (__old == __val)					\
+		_old = cmpxchg(_ptr, _val, _val | _mask);		\
+		if (_old == _val)					\
 			break;						\
-		__val = __old;						\
+		_val = _old;						\
 	}								\
-	__old;								\
+									\
+	_old;								\
 })
 #endif
 
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e5725b931bee..bbd347c04826 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -329,7 +329,7 @@ static inline void init_hrtick(void)
 static bool set_nr_and_not_polling(struct task_struct *p)
 {
 	struct thread_info *ti = task_thread_info(p);
-	return !(fetch_or(&ti->flags, _TIF_NEED_RESCHED) & _TIF_POLLING_NRFLAG);
+	return !(xchg_or(&ti->flags, _TIF_NEED_RESCHED) & _TIF_POLLING_NRFLAG);
 }
 
 /*
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 969e6704c3c9..851631899352 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -264,7 +264,7 @@ static void tick_nohz_dep_set_all(unsigned long *dep,
 {
 	unsigned long prev;
 
-	prev = fetch_or(dep, BIT_MASK(bit));
+	prev = xchg_or(dep, BIT_MASK(bit));
 	if (!prev)
 		tick_nohz_full_kick_all();
 }
@@ -294,7 +294,7 @@ void tick_nohz_dep_set_cpu(int cpu, enum tick_dep_bits bit)
 
 	ts = per_cpu_ptr(&tick_cpu_sched, cpu);
 
-	prev = fetch_or(&ts->tick_dep_mask, BIT_MASK(bit));
+	prev = xchg_or(&ts->tick_dep_mask, BIT_MASK(bit));
 	if (!prev) {
 		preempt_disable();
 		/* Perf needs local kick that is NMI safe */