linux-kernel - [PATCH] riscv: fix test_and_{set,clear}

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Message-Id: <20250311-riscv-fix-test-and-set-bit-comment-v1-1-8d2598e1e43b@iencinas.com>
Date: Tue, 11 Mar 2025 18:20:22 +0100
From: Ignacio Encinas <ignacio@...cinas.com>
To: Yury Norov <yury.norov@...il.com>, 
 Rasmus Villemoes <linux@...musvillemoes.dk>, 
 Paul Walmsley <paul.walmsley@...ive.com>, 
 Palmer Dabbelt <palmer@...belt.com>
Cc: linux-kernel-mentees@...ts.linux.dev, skhan@...uxfoundation.org, 
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, 
 Ignacio Encinas <ignacio@...cinas.com>
Subject: [PATCH] riscv: fix test_and_{set,clear}_bit ordering documentation

test_and_{set,clear}_bit are fully ordered as specified in
Documentation/atomic_bitops.txt. Fix incorrect comment stating otherwise.

Note that the implementation is correct since commit
9347ce54cd69 ("RISC-V: __test_and_op_bit_ord should be strongly ordered")
was introduced.

Signed-off-by: Ignacio Encinas <ignacio@...cinas.com>
---
This seems to be a leftover comment from the initial implementation
which assumed these operations were relaxed.

Documentation/atomic_bitops.txt states:

  [...]
  RMW atomic operations with return value:
  
    test_and_{set,clear,change}_bit()
    test_and_set_bit_lock()
  [...]

   - RMW operations that have a return value are fully ordered.

Similar comments can be found in
include/asm-generic/bitops/instrumented-atomic.h,
include/linux/atomic/atomic-long.h, etc...
---
 arch/riscv/include/asm/bitops.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/include/asm/bitops.h b/arch/riscv/include/asm/bitops.h
index c6bd3d8354a96b4e7bbef0e98a201da412301b57..49a0f48d93df5be4d38fe25b437378467e4ca433 100644
--- a/arch/riscv/include/asm/bitops.h
+++ b/arch/riscv/include/asm/bitops.h
@@ -226,7 +226,7 @@ static __always_inline int variable_fls(unsigned int x)
  * @nr: Bit to set
  * @addr: Address to count from
  *
- * This operation may be reordered on other architectures than x86.
+ * This is an atomic fully-ordered operation (implied full memory barrier).
  */
 static __always_inline int arch_test_and_set_bit(int nr, volatile unsigned long *addr)
 {
@@ -238,7 +238,7 @@ static __always_inline int arch_test_and_set_bit(int nr, volatile unsigned long
  * @nr: Bit to clear
  * @addr: Address to count from
  *
- * This operation can be reordered on other architectures other than x86.
+ * This is an atomic fully-ordered operation (implied full memory barrier).
  */
 static __always_inline int arch_test_and_clear_bit(int nr, volatile unsigned long *addr)
 {

---
base-commit: 2014c95afecee3e76ca4a56956a936e23283f05b
change-id: 20250311-riscv-fix-test-and-set-bit-comment-aa9081a27d61

Best regards,
-- 
Ignacio Encinas <ignacio@...cinas.com>