Messages in this thread Patch in this message | | | From | Uros Bizjak <> | Subject | [PATCH v3 2/4] locking/atomic/x86: Introduce arch_atomic64_read_nonatomic to x86_32 | Date | Wed, 10 Apr 2024 08:29:34 +0200 |
| |
Introduce arch_atomic64_read_nonatomic for 32-bit targets to load the value from atomic64_t location in a non-atomic way. This function is intended to be used in cases where a subsequent atomic operation will handle the torn value, and can be used to prime the first iteration of unconditional try_cmpxchg() loops.
Suggested-by: Mark Rutland <mark.rutland@arm.com> Signed-off-by: Uros Bizjak <ubizjak@gmail.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Ingo Molnar <mingo@kernel.org> Cc: Borislav Petkov <bp@alien8.de> Cc: Dave Hansen <dave.hansen@linux.intel.com> Cc: "H. Peter Anvin" <hpa@zytor.com> Cc: Peter Zijlstra <peterz@infradead.org> --- v2: New patch. v3: Expand function comment. --- arch/x86/include/asm/atomic64_32.h | 26 ++++++++++++++++++++++++++ 1 file changed, 26 insertions(+)
diff --git a/arch/x86/include/asm/atomic64_32.h b/arch/x86/include/asm/atomic64_32.h index 11e817dab44a..bc76a88ae481 100644 --- a/arch/x86/include/asm/atomic64_32.h +++ b/arch/x86/include/asm/atomic64_32.h @@ -14,6 +14,32 @@ typedef struct { #define ATOMIC64_INIT(val) { (val) } +/* + * Read an atomic64_t non-atomically. + * + * This is intended to be used in cases where a subsequent atomic operation + * will handle the torn value, and can be used to prime the first iteration + * of unconditional try_cmpxchg() loops, e.g.: + * + * s64 val = arch_atomic64_read_nonatomic(v); + * do { } while (!arch_atomic64_try_cmpxchg(v, &val, val OP i); + * + * This is NOT safe to use where the value is not always checked by a + * subsequent atomic operation, such as in conditional try_cmpxchg() loops + * that can break before the atomic operation, e.g.: + * + * s64 val = arch_atomic64_read_nonatomic(v); + * do { + * if (condition(val)) + * break; + * } while (!arch_atomic64_try_cmpxchg(v, &val, val OP i); + */ +static __always_inline s64 arch_atomic64_read_nonatomic(const atomic64_t *v) +{ + /* See comment in arch_atomic_read(). */ + return __READ_ONCE(v->counter); +} + #define __ATOMIC64_DECL(sym) void atomic64_##sym(atomic64_t *, ...) #ifndef ATOMIC64_EXPORT #define ATOMIC64_DECL_ONE __ATOMIC64_DECL -- 2.44.0
| |