Add smp_cond_load_relaxed_timeout(), which extends
smp_cond_load_relaxed() to allow waiting for a duration.
The additional parameter allows for the timeout check.
The waiting is done via the usual cpu_relax() spin-wait around the
condition variable with periodic evaluation of the time-check.
The number of times we spin is defined by SMP_TIMEOUT_SPIN_COUNT
(chosen to be 200 by default) which, assuming each cpu_relax()
iteration takes around 20-30 cycles (measured on a variety of x86
platforms), amounts to around 4000-6000 cycles.
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: linux-arch@xxxxxxxxxxxxxxx
Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
Reviewed-by: Haris Okanovic <harisokn@xxxxxxxxxx>
Tested-by: Haris Okanovic <harisokn@xxxxxxxxxx>
Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
---
include/asm-generic/barrier.h | 35 +++++++++++++++++++++++++++++++++++
1 file changed, 35 insertions(+)
diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
index d4f581c1e21d..8483e139954f 100644
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -273,6 +273,41 @@ do { \
})
#endif
+#ifndef SMP_TIMEOUT_SPIN_COUNT
+#define SMP_TIMEOUT_SPIN_COUNT 200
+#endif
+
+/**
+ * smp_cond_load_relaxed_timeout() - (Spin) wait for cond with no ordering
+ * guarantees until a timeout expires.
+ * @ptr: pointer to the variable to wait on
+ * @cond: boolean expression to wait for
+ * @time_check_expr: expression to decide when to bail out
+ *
+ * Equivalent to using READ_ONCE() on the condition variable.
+ */
+#ifndef smp_cond_load_relaxed_timeout
+#define smp_cond_load_relaxed_timeout(ptr, cond_expr, time_check_expr) \
+({ \
+ typeof(ptr) __PTR = (ptr); \
+ __unqual_scalar_typeof(*ptr) VAL; \
+ u32 __n = 0, __spin = SMP_TIMEOUT_SPIN_COUNT; \
+ \
+ for (;;) { \
+ VAL = READ_ONCE(*__PTR); \
+ if (cond_expr) \
+ break; \
+ cpu_relax(); \
+ if (++__n < __spin) \
+ continue; \
+ if (time_check_expr) \
+ break; \
+ __n = 0; \
+ } \
+ (typeof(*ptr))VAL; \
+})
+#endif
+
/*
* pmem_wmb() ensures that all stores for which the modification
* are written to persistent storage by preceding instructions have
--
2.43.5
