For hardware configurations lacking LLOCK/SCOND (say ARC750),
use a syscall to atomically do the cmpxchg.
This is costly and painful, but really the only way out.
Note that kenrel only guarantees this to work in a UP configuraion
Reported-by: Avinash Patil <avina...@quantenna.com>
Signed-off-by: Vineet Gupta <vgu...@synopsys.com>
---
libc/sysdeps/linux/arc/bits/atomic.h | 37 ++++++++++++++++++++++++++++++++++--
1 file changed, 35 insertions(+), 2 deletions(-)
diff --git a/libc/sysdeps/linux/arc/bits/atomic.h
b/libc/sysdeps/linux/arc/bits/atomic.h
index 48f37879c5c4..587860964744 100644
--- a/libc/sysdeps/linux/arc/bits/atomic.h
+++ b/libc/sysdeps/linux/arc/bits/atomic.h
@@ -38,6 +38,11 @@ void __arc_link_error (void);
#define __arch_compare_and_exchange_val_16_acq(mem, newval, oldval) \
({ __arc_link_error (); oldval; })
+#define __arch_compare_and_exchange_val_64_acq(mem, newval, oldval) \
+ ({ __arc_link_error (); oldval; })
+
+#ifdef __CONFIG_ARC_HAS_ATOMICS__
+
#define __arch_compare_and_exchange_val_32_acq(mem, newval, oldval) \
({ \
__typeof(oldval) prev; \
@@ -56,8 +61,36 @@ void __arc_link_error (void);
prev; \
})
-#define __arch_compare_and_exchange_val_64_acq(mem, newval, oldval) \
- ({ __arc_link_error (); oldval; })
+#else
+
+#ifndef __NR_arc_usr_cmpxchg
+#error "__NR_arc_usr_cmpxchg missing: Please upgrade to kernel 4.9+ headers"
+#endif
+
+/* With lack of hardware assist, use kernel to do the atomic operation
+ This will only work in a UP configuration
+ */
+#define __arch_compare_and_exchange_val_32_acq(mem, newval, oldval) \
+ ({ \
+ /* opecode INTERNAL_SYSCALL as it lacks cc clobber */ \
+ register int __ret __asm__("r0") = (int)(mem); \
+ register int __a1 __asm__("r1") = (int)(oldval); \
+ register int __a2 __asm__("r2") = (int)(newval); \
+ register int _sys_num __asm__("r8") = __NR_arc_usr_cmpxchg; \
+ \
+ __asm__ volatile ( \
+ ARC_TRAP_INSN \
+ : "+r" (__ret) \
+ : "r"(_sys_num), "r"(__ret), "r"(__a1), "r"(__a2) \
+ : "memory", "cc"); \
+ \
+ /* syscall returns previous value */ \
+ /* Z bit is set if cmpxchg succeeded (we don't use that yet) */ \
+ \
+ (__typeof(oldval)) __ret; \
+ })
+
+#endif
/* Store NEWVALUE in *MEM and return the old value.
Atomic EX is present in all configurations
--
2.7.4
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