Implementing busy wait loops with cpu_relax() in callers poses
some difficulties for powerpc.
First, we want to put our SMT thread into a low priority mode for the
duration of the loop, but then return to normal priority after exiting
the loop. Dependong on the CPU design, 'HMT_low() ; HMT_medium();' as
cpu_relax() does may have HMT_medium take effect before HMT_low made
any (or much) difference.
Second, it can be beneficial for some implementations to spin on the
exit condition with a statically predicted-not-taken branch (i.e.,
always predict the loop will exit).
This is a quick RFC with a couple of users converted to see what
people think. I don't use a C branch with hints, because we don't want
the compiler moving the loop body out of line, which makes it a bit
messy unfortunately. If there's a better way to do it, I'm all ears.
I would not propose to switch all callers immediately, just some
core synchronisation primitives.
---
arch/powerpc/include/asm/processor.h | 22 ++++++++++++++++++++++
include/asm-generic/barrier.h | 7 ++-----
include/linux/bit_spinlock.h | 5 ++---
include/linux/cgroup.h | 7 ++-----
include/linux/seqlock.h | 10 ++++------
5 files changed, 32 insertions(+), 19 deletions(-)
diff --git a/arch/powerpc/include/asm/processor.h b/arch/powerpc/include/asm/processor.h
index 68e3bf5..e10aee2 100644
--- a/arch/powerpc/include/asm/processor.h
+++ b/arch/powerpc/include/asm/processor.h
@@ -402,6 +402,28 @@ static inline unsigned long __pack_fe01(unsigned int fpmode)
#ifdef CONFIG_PPC64
#define cpu_relax() do { HMT_low(); HMT_medium(); barrier(); } while (0)
+
+#define spin_do \
+do { \
+ HMT_low(); \
+ __asm__ __volatile__ ( "1010:");
+
+#define spin_while(cond) \
+ barrier(); \
+ __asm__ __volatile__ ( "cmpdi %0,0 \n\t" \
+ "beq- 1010b \n\t" \
+ : : "r" (cond)); \
+ HMT_medium(); \
+} while (0)
+
+#define spin_until(cond) \
+ barrier(); \
+ __asm__ __volatile__ ( "cmpdi %0,0 \n\t" \
+ "bne- 1010b \n\t" \
+ : : "r" (cond)); \
+ HMT_medium(); \
+} while (0)
+
#else
#define cpu_relax() barrier()
#endif
diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
index fe297b5..4c53b3a 100644
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -235,12 +235,9 @@ do { \
#define smp_cond_load_acquire(ptr, cond_expr) ({ \
typeof(ptr) __PTR = (ptr); \
typeof(*ptr) VAL; \
- for (;;) { \
+ spin_do { \
VAL = READ_ONCE(*__PTR); \
- if (cond_expr) \
- break; \
- cpu_relax(); \
- } \
+ } spin_until (cond_expr); \
smp_acquire__after_ctrl_dep(); \
VAL; \
})
diff --git a/include/linux/bit_spinlock.h b/include/linux/bit_spinlock.h
index 3b5bafc..695743c 100644
--- a/include/linux/bit_spinlock.h
+++ b/include/linux/bit_spinlock.h
@@ -25,9 +25,8 @@ static inline void bit_spin_lock(int bitnum, unsigned long *addr)
#if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)
while (unlikely(test_and_set_bit_lock(bitnum, addr))) {
preempt_enable();
- do {
- cpu_relax();
- } while (test_bit(bitnum, addr));
+ spin_do {
+ } spin_while (test_bit(bitnum, addr));
preempt_disable();
}
#endif
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 984f73b..e7d395f 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -450,12 +450,9 @@ task_get_css(struct task_struct *task, int subsys_id)
struct cgroup_subsys_state *css;
rcu_read_lock();
- while (true) {
+ spin_do {
css = task_css(task, subsys_id);
- if (likely(css_tryget_online(css)))
- break;
- cpu_relax();
- }
+ } spin_until (likely(css_tryget_online(css)));
rcu_read_unlock();
return css;
}
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index ead9765..93ed609 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -108,12 +108,10 @@ static inline unsigned __read_seqcount_begin(const seqcount_t *s)
{
unsigned ret;
-repeat:
- ret = READ_ONCE(s->sequence);
- if (unlikely(ret & 1)) {
- cpu_relax();
- goto repeat;
- }
+ spin_do {
+ ret = READ_ONCE(s->sequence);
+ } spin_while (unlikely(ret & 1));
+
return ret;
}
--
2.9.3
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