Add smp_cond_load_relaxed_timewait(), which extends
smp_cond_load_relaxed() to allow waiting for a finite 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_TIMEWAIT_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@arndb.de>
Cc: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-arch@vger.kernel.org
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---
 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..c87d6fd8746f 100644
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -273,6 +273,41 @@ do {									\
 })
 #endif
 
+#ifndef SMP_TIMEWAIT_SPIN_COUNT
+#define SMP_TIMEWAIT_SPIN_COUNT		200
+#endif
+
+/**
+ * smp_cond_load_relaxed_timewait() - (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_timewait
+#define smp_cond_load_relaxed_timewait(ptr, cond_expr, time_check_expr)	\
+({									\
+	typeof(ptr) __PTR = (ptr);					\
+	__unqual_scalar_typeof(*ptr) VAL;				\
+	u32 __n = 0, __spin = SMP_TIMEWAIT_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
-- 
2.31.1

On Fri, Aug 29, 2025 at 01:07:31AM -0700, Ankur Arora wrote:
> Add smp_cond_load_relaxed_timewait(), which extends
> smp_cond_load_relaxed() to allow waiting for a finite 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_TIMEWAIT_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@arndb.de>
> Cc: Will Deacon <will@kernel.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: linux-arch@vger.kernel.org
> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>

Apart from the name, this looks fine (I'd have preferred the "timeout"
suffix).

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

Catalin Marinas <catalin.marinas@arm.com> writes:

> On Fri, Aug 29, 2025 at 01:07:31AM -0700, Ankur Arora wrote:
>> Add smp_cond_load_relaxed_timewait(), which extends
>> smp_cond_load_relaxed() to allow waiting for a finite 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_TIMEWAIT_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@arndb.de>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Cc: linux-arch@vger.kernel.org
>> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
>
> Apart from the name, this looks fine (I'd have preferred the "timeout"
> suffix).

So, the suffix "timewait" made sense to me because I was trying to
differentiate with spinwait etc.

Given that that issue is no longer meaningful, "timeout" makes more sense.

Will change.

> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

Thanks for this and all the reviews.

--
ankur