On 2023-07-03 09:02, Ruifeng Wang wrote:
-----Original Message-----
From: Mattias Rönnblom <hof...@lysator.liu.se>
Sent: Friday, June 30, 2023 3:44 AM
To: Ruifeng Wang <ruifeng.w...@arm.com>; tho...@monjalon.net;
david.march...@redhat.com
Cc: dev@dpdk.org; roret...@linux.microsoft.com; konstantin.v.anan...@yandex.ru;
Honnappa
Nagarahalli <honnappa.nagaraha...@arm.com>; nd <n...@arm.com>
Subject: Re: [PATCH v2] eal: add notes to SMP memory barrier APIs
On 2023-06-26 09:12, Ruifeng Wang wrote:
The rte_smp_xx() APIs are deprecated. But it is not mentioned in the
function header.
Added notes in function header for clarification.
Signed-off-by: Ruifeng Wang <ruifeng.w...@arm.com>
---
v2: Made the notes more specific.
lib/eal/include/generic/rte_atomic.h | 19 +++++++++++++++++++
1 file changed, 19 insertions(+)
diff --git a/lib/eal/include/generic/rte_atomic.h
b/lib/eal/include/generic/rte_atomic.h
index 58df843c54..35e0041ce6 100644
--- a/lib/eal/include/generic/rte_atomic.h
+++ b/lib/eal/include/generic/rte_atomic.h
@@ -55,6 +55,11 @@ static inline void rte_rmb(void);
* Guarantees that the LOAD and STORE operations that precede the
* rte_smp_mb() call are globally visible across the lcores
* before the LOAD and STORE operations that follows it.
+ *
+ * @note
+ * This function is deprecated. It provides fence synchronization
+ * primitive but doesn't take memory order parameter.
+ * rte_atomic_thread_fence() should be used instead.
I can't see why coding the memory model semantics into the name, rather than by
specification-by-means-of-a-parameter, could be the real issue.
Could you explain? Seems like just different syntax to me.
Yes, rte_smp_xx and rte_atomic_thread_fence have different syntaxes.
The compiler atomic builtins were accepted for memory ordering. It comprises
atomic arithmetic,
atomic load/store, and atomic fence. It is simpler and clearer to do memory
ordering by using
the atomic builtins whenever possible.
rte_smp_xx has functionality overlap with atomic fence builtins but with
different memory model
semantics and different syntaxes. Because of the differences, it will make
memory ordering a little
more complex if rte_smp_xx is kept aside atomic builtins suite.
I wasn't arguing for keeping Linux kernel-style barriers. It was just
the rationale that seemed flawed to me.
If Linux kernel-style memory barriers took a memory model parameter, we
would still prefer C11-style GCC barrier intrinsics (for this release).
The old <rte_atomic.h> atomic arithmetic and atomic load/store operations
suffered from
unspecified semantics in regards to any ordering they imposed on other memory
accesses. I
guess that shortcoming could be described as a "missing parameter", although
that too
would be misleading. Unclear semantics seems not be the case for the
kernel-style barriers
though.
*/
static inline void rte_smp_mb(void);
@@ -64,6 +69,11 @@ static inline void rte_smp_mb(void);
* Guarantees that the STORE operations that precede the
* rte_smp_wmb() call are globally visible across the lcores
* before the STORE operations that follows it.
+ *
+ * @note
+ * This function is deprecated. It provides fence synchronization
+ * primitive but doesn't take memory order parameter.
+ * rte_atomic_thread_fence() should be used instead.
*/
static inline void rte_smp_wmb(void);
@@ -73,6 +83,11 @@ static inline void rte_smp_wmb(void);
* Guarantees that the LOAD operations that precede the
* rte_smp_rmb() call are globally visible across the lcores
* before the LOAD operations that follows it.
+ *
+ * @note
+ * This function is deprecated. It provides fence synchronization
+ * primitive but doesn't take memory order parameter.
+ * rte_atomic_thread_fence() should be used instead.
*/
static inline void rte_smp_rmb(void);
///@}
@@ -122,6 +137,10 @@ static inline void rte_io_rmb(void);
/**
* Synchronization fence between threads based on the specified memory order.
+ *
+ * @param memorder
+ * The memory order defined by compiler atomic builtin at:
+ * https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html
*/
static inline void rte_atomic_thread_fence(int memorder);
