[PATCH 09/11] ARC: cmpxchg/xchg: rewrite as macros to make type safe

From: Vineet Gupta
Date: Wed Aug 04 2021 - 15:16:40 EST

Existing code forces/assume args to type "long" which won't work in LP64
regime, so prepare code for that

Interestingly this should be a non functional change but I do see
some codegen changes

| bloat-o-meter vmlinux-cmpxchg-A vmlinux-cmpxchg-B
| add/remove: 0/0 grow/shrink: 17/12 up/down: 218/-150 (68)
|
| Function old new delta
| rwsem_optimistic_spin 518 550 +32
| rwsem_down_write_slowpath 1244 1274 +30
| __do_sys_perf_event_open 2576 2600 +24
| down_read 192 200 +8
| __down_read 192 200 +8
...
| task_work_run 168 148 -20
| dma_fence_chain_walk.part 760 736 -24
| __genradix_ptr_alloc 674 646 -28

Total: Before=6187409, After=6187477, chg +0.00%

Signed-off-by: Vineet Gupta <vgupta@xxxxxxxxxxxx>
---
arch/arc/include/asm/cmpxchg.h | 209 ++++++++++++++++++---------------
1 file changed, 117 insertions(+), 92 deletions(-)

diff --git a/arch/arc/include/asm/cmpxchg.h b/arch/arc/include/asm/cmpxchg.h
index bac9b564a140..00deb076d6f6 100644
--- a/arch/arc/include/asm/cmpxchg.h
+++ b/arch/arc/include/asm/cmpxchg.h
@@ -6,6 +6,7 @@
#ifndef __ASM_ARC_CMPXCHG_H
#define __ASM_ARC_CMPXCHG_H

+#include <linux/build_bug.h>
#include <linux/types.h>

#include <asm/barrier.h>
@@ -13,62 +14,77 @@

#ifdef CONFIG_ARC_HAS_LLSC

-static inline unsigned long
-__cmpxchg(volatile void *ptr, unsigned long expected, unsigned long new)
-{
- unsigned long prev;
-
- /*
- * Explicit full memory barrier needed before/after as
- * LLOCK/SCOND themselves don't provide any such semantics
- */
- smp_mb();
-
- __asm__ __volatile__(
- "1: llock %0, [%1] \n"
- " brne %0, %2, 2f \n"
- " scond %3, [%1] \n"
- " bnz 1b \n"
- "2: \n"
- : "=&r"(prev) /* Early clobber, to prevent reg reuse */
- : "r"(ptr), /* Not "m": llock only supports reg direct addr mode */
- "ir"(expected),
- "r"(new) /* can't be "ir". scond can't take LIMM for "b" */
- : "cc", "memory"); /* so that gcc knows memory is being written here */
-
- smp_mb();
-
- return prev;
-}
-
-#else /* !CONFIG_ARC_HAS_LLSC */
-
-static inline unsigned long
-__cmpxchg(volatile void *ptr, unsigned long expected, unsigned long new)
-{
- unsigned long flags;
- int prev;
- volatile unsigned long *p = ptr;
-
- /*
- * spin lock/unlock provide the needed smp_mb() before/after
- */
- atomic_ops_lock(flags);
- prev = *p;
- if (prev == expected)
- *p = new;
- atomic_ops_unlock(flags);
- return prev;
-}
+/*
+ * if (*ptr == @old)
+ * *ptr = @new
+ */
+#define __cmpxchg(ptr, old, new) \
+({ \
+ __typeof__(*(ptr)) _prev; \
+ \
+ __asm__ __volatile__( \
+ "1: llock %0, [%1] \n" \
+ " brne %0, %2, 2f \n" \
+ " scond %3, [%1] \n" \
+ " bnz 1b \n" \
+ "2: \n" \
+ : "=&r"(_prev) /* Early clobber prevent reg reuse */ \
+ : "r"(ptr), /* Not "m": llock only supports reg */ \
+ "ir"(old), \
+ "r"(new) /* Not "ir": scond can't take LIMM */ \
+ : "cc", \
+ "memory"); /* gcc knows memory is clobbered */ \
+ \
+ _prev; \
+})

-#endif
+#define arch_cmpxchg(ptr, old, new) \
+({ \
+ __typeof__(ptr) _p_ = (ptr); \
+ __typeof__(*(ptr)) _o_ = (old); \
+ __typeof__(*(ptr)) _n_ = (new); \
+ __typeof__(*(ptr)) _prev_; \
+ \
+ switch(sizeof((_p_))) { \
+ case 4: \
+ /* \
+ * Explicit full memory barrier needed before/after \
+ */ \
+ smp_mb(); \
+ _prev_ = __cmpxchg(_p_, _o_, _n_); \
+ smp_mb(); \
+ break; \
+ default: \
+ BUILD_BUG(); \
+ } \
+ _prev_; \
+})

-#define arch_cmpxchg(ptr, o, n) ({ \
- (typeof(*(ptr)))__cmpxchg((ptr), \
- (unsigned long)(o), \
- (unsigned long)(n)); \
+#else
+
+#define arch_cmpxchg(ptr, old, new) \
+({ \
+ volatile __typeof__(ptr) _p_ = (ptr); \
+ __typeof__(*(ptr)) _o_ = (old); \
+ __typeof__(*(ptr)) _n_ = (new); \
+ __typeof__(*(ptr)) _prev_; \
+ unsigned long __flags; \
+ \
+ BUILD_BUG_ON(sizeof(_p_) != 4); \
+ \
+ /* \
+ * spin lock/unlock provide the needed smp_mb() before/after \
+ */ \
+ atomic_ops_lock(__flags); \
+ _prev_ = *_p_; \
+ if (_prev_ == _o_) \
+ *_p_ = _n_; \
+ atomic_ops_unlock(__flags); \
+ _prev_; \
})

+#endif
+
/*
* atomic_cmpxchg is same as cmpxchg
* LLSC: only different in data-type, semantics are exactly same
@@ -77,55 +93,64 @@ __cmpxchg(volatile void *ptr, unsigned long expected, unsigned long new)
*/
#define arch_atomic_cmpxchg(v, o, n) ((int)arch_cmpxchg(&((v)->counter), (o), (n)))

-
/*
- * xchg (reg with memory) based on "Native atomic" EX insn
+ * xchg
*/
-static inline unsigned long __xchg(unsigned long val, volatile void *ptr,
- int size)
-{
- extern unsigned long __xchg_bad_pointer(void);
-
- switch (size) {
- case 4:
- smp_mb();
-
- __asm__ __volatile__(
- " ex %0, [%1] \n"
- : "+r"(val)
- : "r"(ptr)
- : "memory");
+#ifdef CONFIG_ARC_HAS_LLSC

- smp_mb();
+#define __xchg(ptr, val) \
+({ \
+ __asm__ __volatile__( \
+ " ex %0, [%1] \n" /* set new value */ \
+ : "+r"(val) \
+ : "r"(ptr) \
+ : "memory"); \
+ _val_; /* get old value */ \
+})

- return val;
- }
- return __xchg_bad_pointer();
-}
+#define arch_xchg(ptr, val) \
+({ \
+ __typeof__(ptr) _p_ = (ptr); \
+ __typeof__(*(ptr)) _val_ = (val); \
+ \
+ switch(sizeof(*(_p_))) { \
+ case 4: \
+ smp_mb(); \
+ _val_ = __xchg(_p_, _val_); \
+ smp_mb(); \
+ break; \
+ default: \
+ BUILD_BUG(); \
+ } \
+ _val_; \
+})

-#define _xchg(ptr, with) ((typeof(*(ptr)))__xchg((unsigned long)(with), (ptr), \
- sizeof(*(ptr))))
+#else /* !CONFIG_ARC_HAS_LLSC */

/*
- * xchg() maps directly to ARC EX instruction which guarantees atomicity.
- * However in !LLSC config, it also needs to be use @atomic_ops_lock spinlock
- * due to a subtle reason:
- * - For !LLSC, cmpxchg() needs to use that lock (see above) and there is lot
- * of kernel code which calls xchg()/cmpxchg() on same data (see llist.h)
- * Hence xchg() needs to follow same locking rules.
+ * EX instructions is baseline and present in !LLSC too. But in this
+ * regime it still needs use @atomic_ops_lock spinlock to allow interop
+ * with cmpxchg() which uses spinlock in !LLSC
+ * (llist.h use xchg and cmpxchg on sama data)
*/

-#ifndef CONFIG_ARC_HAS_LLSC
-
-#define arch_xchg(ptr, with) \
-({ \
- unsigned long flags; \
- typeof(*(ptr)) old_val; \
- \
- atomic_ops_lock(flags); \
- old_val = _xchg(ptr, with); \
- atomic_ops_unlock(flags); \
- old_val; \
+#define arch_xchg(ptr, val) \
+({ \
+ __typeof__(ptr) _p_ = (ptr); \
+ __typeof__(*(ptr)) _val_ = (val); \
+ \
+ unsigned long __flags; \
+ \
+ atomic_ops_lock(__flags); \
+ \
+ __asm__ __volatile__( \
+ " ex %0, [%1] \n" \
+ : "+r"(_val_) \
+ : "r"(_p_) \
+ : "memory"); \
+ \
+ atomic_ops_unlock(__flags); \
+ _val_; \
})

#endif
--
2.25.1