1 files changed, 28 insertions, 55 deletions
diff --git a/arch/s390/include/asm/preempt.h b/arch/s390/include/asm/preempt.h
index 2c29bdf12127..6ccd033acfe5 100644
--- a/arch/s390/include/asm/preempt.h
+++ b/arch/s390/include/asm/preempt.h
@@ -8,12 +8,19 @@
 #include <asm/cmpxchg.h>
 #include <asm/march.h>
 
-#ifdef MARCH_HAS_Z196_FEATURES
-
 /* We use the MSB mostly because its available */
 #define PREEMPT_NEED_RESCHED	0x80000000
+
+/*
+ * We use the PREEMPT_NEED_RESCHED bit as an inverted NEED_RESCHED such
+ * that a decrement hitting 0 means we can and should reschedule.
+ */
 #define PREEMPT_ENABLED	(0 + PREEMPT_NEED_RESCHED)
 
+/*
+ * We mask the PREEMPT_NEED_RESCHED bit so as not to confuse all current users
+ * that think a non-zero value indicates we cannot preempt.
+ */
 static __always_inline int preempt_count(void)
 {
 	return READ_ONCE(get_lowcore()->preempt_count) & ~PREEMPT_NEED_RESCHED;
@@ -29,6 +36,15 @@ static __always_inline void preempt_count_set(int pc)
 	} while (!arch_try_cmpxchg(&get_lowcore()->preempt_count, &old, new));
 }
 
+/*
+ * We fold the NEED_RESCHED bit into the preempt count such that
+ * preempt_enable() can decrement and test for needing to reschedule with a
+ * short instruction sequence.
+ *
+ * We invert the actual bit, so that when the decrement hits 0 we know we both
+ * need to resched (the bit is cleared) and can resched (no preempt count).
+ */
+
 static __always_inline void set_preempt_need_resched(void)
 {
 	__atomic_and(~PREEMPT_NEED_RESCHED, &get_lowcore()->preempt_count);
@@ -64,67 +80,24 @@ static __always_inline void __preempt_count_sub(int val)
 	__preempt_count_add(-val);
 }
 
+/*
+ * Because we keep PREEMPT_NEED_RESCHED set when we do _not_ need to reschedule
+ * a decrement which hits zero means we have no preempt_count and should
+ * reschedule.
+ */
 static __always_inline bool __preempt_count_dec_and_test(void)
 {
-	return __atomic_add(-1, &get_lowcore()->preempt_count) == 1;
-}
-
-static __always_inline bool should_resched(int preempt_offset)
-{
-	return unlikely(READ_ONCE(get_lowcore()->preempt_count) ==
-			preempt_offset);
-}
-
-#else /* MARCH_HAS_Z196_FEATURES */
-
-#define PREEMPT_ENABLED	(0)
-
-static __always_inline int preempt_count(void)
-{
-	return READ_ONCE(get_lowcore()->preempt_count);
-}
-
-static __always_inline void preempt_count_set(int pc)
-{
-	get_lowcore()->preempt_count = pc;
-}
-
-static __always_inline void set_preempt_need_resched(void)
-{
-}
-
-static __always_inline void clear_preempt_need_resched(void)
-{
-}
-
-static __always_inline bool test_preempt_need_resched(void)
-{
-	return false;
-}
-
-static __always_inline void __preempt_count_add(int val)
-{
-	get_lowcore()->preempt_count += val;
-}
-
-static __always_inline void __preempt_count_sub(int val)
-{
-	get_lowcore()->preempt_count -= val;
-}
-
-static __always_inline bool __preempt_count_dec_and_test(void)
-{
-	return !--get_lowcore()->preempt_count && tif_need_resched();
+	return __atomic_add_const_and_test(-1, &get_lowcore()->preempt_count);
 }
 
+/*
+ * Returns true when we need to resched and can (barring IRQ state).
+ */
 static __always_inline bool should_resched(int preempt_offset)
 {
-	return unlikely(preempt_count() == preempt_offset &&
-			tif_need_resched());
+	return unlikely(READ_ONCE(get_lowcore()->preempt_count) == preempt_offset);
 }
 
-#endif /* MARCH_HAS_Z196_FEATURES */
-
 #define init_task_preempt_count(p)	do { } while (0)
 /* Deferred to CPU bringup time */
 #define init_idle_preempt_count(p, cpu)	do { } while (0)