[PATCH v13 10/15] bpf/rqspinlock: Use smp_cond_load_acquire_timeout()

From: Ankur Arora <hidden>
Date: 2026-07-02 01:35:08
Also in: bpf, linux-arch, linux-pm, lkml
Subsystem: bpf [general] (safe dynamic programs and tools), the rest · Maintainers: Alexei Starovoitov, Daniel Borkmann, Andrii Nakryiko, Eduard Zingerman, Kumar Kartikeya Dwivedi, Linus Torvalds

Switch out the conditional load interfaces used by rqspinlock
to smp_cond_read_acquire_timeout() and its wrapper,
atomic_cond_read_acquire_timeout().

Both these handle the timeout and amortize as needed, so use the
non-amortized RES_CHECK_TIMEOUT.

RES_CHECK_TIMEOUT does double duty here -- presenting the current
clock value, the timeout/deadlock error from clock_deadlock() to
the cond-load and, returning the error value via ret.

For correctness, we need to ensure that the error case of the
cond-load interface always agrees with that in clock_deadlock().

For the most part, this is fine because there's no independent clock,
or double reads from the clock in cond-load -- either of which could
lead to its internal state going out of sync from that of
clock_deadlock().

There is, however, an edge case where clock_deadlock() checks for:

        if (time > ts->timeout_end)
                return -ETIMEDOUT;

while smp_cond_load_acquire_timeout() checks for:

        __time_now = (time_expr_ns);
        if (__time_now <= 0 || __time_now >= __time_end) {
                VAL = READ_ONCE(*__PTR);
                break;
        }

This runs into a problem when (__time_now == __time_end) since
clock_deadlock() does not treat it as a timeout condition but
the second clause in the conditional above does.
So, add an equality check in clock_deadlock().

Finally, redefine SMP_TIMEOUT_POLL_COUNT to be 16k to be similar to
the spin-count used in the amortized version. We only do this for
non-arm64 as that uses a waiting implementation.

Cc: bpf@vger.kernel.org
Cc: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Ankur Arora <redacted>
---
 kernel/bpf/rqspinlock.c | 40 +++++++++++++++++++++++-----------------
 1 file changed, 23 insertions(+), 17 deletions(-)

diff --git a/kernel/bpf/rqspinlock.c b/kernel/bpf/rqspinlock.c
index 0ec17ebb67c1..e5e27266b813 100644
--- a/kernel/bpf/rqspinlock.c
+++ b/kernel/bpf/rqspinlock.c

@@ -215,7 +215,7 @@ static noinline s64 clock_deadlock(rqspinlock_t *lock, u32 mask,
 	}
 
 	time = ktime_get_mono_fast_ns();
-	if (time > ts->timeout_end)
+	if (time >= ts->timeout_end)
 		return -ETIMEDOUT;
 
 	/*

@@ -235,11 +235,10 @@ static noinline s64 clock_deadlock(rqspinlock_t *lock, u32 mask,
 }
 
 /*
- * Do not amortize with spins when res_smp_cond_load_acquire is defined,
- * as the macro does internal amortization for us.
+ * Spin amortized version of RES_CHECK_TIMEOUT. Used when busy-waiting in
+ * atomic_try_cmpxchg().
  */
-#ifndef res_smp_cond_load_acquire
-#define RES_CHECK_TIMEOUT(ts, ret, mask)					\
+#define RES_CHECK_TIMEOUT_AMORTIZED(ts, ret, mask)				\
 	({									\
 		s64 __timeval_err = 0;						\
 		if (!(ts).spin++)						\

@@ -247,7 +246,7 @@ static noinline s64 clock_deadlock(rqspinlock_t *lock, u32 mask,
 		(ret) = __timeval_err < 0 ? __timeval_err : 0;			\
 		__timeval_err;							\
 	})
-#else
+
 #define RES_CHECK_TIMEOUT(ts, ret, mask)					\
 	({									\
 		s64 __timeval_err;						\

@@ -255,7 +254,6 @@ static noinline s64 clock_deadlock(rqspinlock_t *lock, u32 mask,
 		(ret) = __timeval_err < 0 ? __timeval_err : 0;			\
 		__timeval_err;							\
 	})
-#endif
 
 /*
  * Initialize the 'spin' member.

@@ -269,6 +267,17 @@ static noinline s64 clock_deadlock(rqspinlock_t *lock, u32 mask,
  */
 #define RES_RESET_TIMEOUT(ts, _duration) ({ (ts).timeout_end = 0; (ts).duration = _duration; })
 
+/*
+ * Limit how often we invoke clock_deadlock() while spin-waiting in
+ * smp_cond_load_acquire_timeout() or atomic_cond_read_acquire_timeout().
+ *
+ * We only override the default value not superceding ARM64's override.
+ */
+#ifndef CONFIG_ARM64
+#undef SMP_TIMEOUT_POLL_COUNT
+#define SMP_TIMEOUT_POLL_COUNT	(16*1024)
+#endif
+
 /*
  * Provide a test-and-set fallback for cases when queued spin lock support is
  * absent from the architecture.

@@ -296,7 +305,7 @@ int __lockfunc resilient_tas_spin_lock(rqspinlock_t *lock)
 	val = atomic_read(&lock->val);
 
 	if (val || !atomic_try_cmpxchg(&lock->val, &val, 1)) {
-		if (RES_CHECK_TIMEOUT(ts, ret, ~0u) < 0)
+		if (RES_CHECK_TIMEOUT_AMORTIZED(ts, ret, ~0u) < 0)
 			goto out;
 		cpu_relax();
 		goto retry;

@@ -319,12 +328,6 @@ EXPORT_SYMBOL_GPL(resilient_tas_spin_lock);
  */
 static DEFINE_PER_CPU_ALIGNED(struct qnode, rqnodes[_Q_MAX_NODES]);
 
-#ifndef res_smp_cond_load_acquire
-#define res_smp_cond_load_acquire(v, c) smp_cond_load_acquire(v, c)
-#endif
-
-#define res_atomic_cond_read_acquire(v, c) res_smp_cond_load_acquire(&(v)->counter, (c))
-
 /**
  * resilient_queued_spin_lock_slowpath - acquire the queued spinlock
  * @lock: Pointer to queued spinlock structure

@@ -421,7 +424,9 @@ int __lockfunc resilient_queued_spin_lock_slowpath(rqspinlock_t *lock, u32 val)
 	 */
 	if (val & _Q_LOCKED_MASK) {
 		RES_RESET_TIMEOUT(ts, RES_DEF_TIMEOUT);
-		res_smp_cond_load_acquire(&lock->locked, !VAL || RES_CHECK_TIMEOUT(ts, ret, _Q_LOCKED_MASK) < 0);
+		smp_cond_load_acquire_timeout(&lock->locked, !VAL,
+					      RES_CHECK_TIMEOUT(ts, ret, _Q_LOCKED_MASK),
+					      ts.duration);
 	}
 
 	if (ret) {

@@ -582,8 +587,9 @@ int __lockfunc resilient_queued_spin_lock_slowpath(rqspinlock_t *lock, u32 val)
 	 * us.
 	 */
 	RES_RESET_TIMEOUT(ts, RES_DEF_TIMEOUT * 2);
-	val = res_atomic_cond_read_acquire(&lock->val, !(VAL & _Q_LOCKED_PENDING_MASK) ||
-					   RES_CHECK_TIMEOUT(ts, ret, _Q_LOCKED_PENDING_MASK) < 0);
+	val = atomic_cond_read_acquire_timeout(&lock->val, !(VAL & _Q_LOCKED_PENDING_MASK),
+					       RES_CHECK_TIMEOUT(ts, ret, _Q_LOCKED_PENDING_MASK),
+					       ts.duration);
 
 	/* Disable queue destruction when we detect deadlocks. */
 	if (ret == -EDEADLK) {

-- 
2.43.7

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