linux-kernel - [RFC PATCH] locking/barriers: Introduce smp_cond_load_mask

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Date:   Sun, 25 Dec 2022 06:55:29 -0500
From:   guoren@...nel.org
To:     peterz@...radead.org, longman@...hat.com
Cc:     linux-kernel@...r.kernel.org, guoren@...nel.org,
        Guo Ren <guoren@...ux.alibaba.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...hat.com>
Subject: [RFC PATCH] locking/barriers: Introduce smp_cond_load_mask_relaxed & acquire

From: Guo Ren <guoren@...ux.alibaba.com>

The current cond_load primitive contains two parts (condition expression
and load value), but the usage of cond_load may require the sub-size
condition expression of the load size. That means hardware could utilize
a mask argument to optimize the wait condition. If the mask argument
size is less than the hardware minimum wait size, the hardware uses its
minimum size.

The patch contains a qspinlock example: When it is at the head of the
waitqueue, it waits for the owner & pending to go away. The forward
progress condition only cares locked_pending part, but it needs to load
the 32-bit lock value as a return.

That also means WFE-liked instruction would need a mask argument of the
load reservation set.

Signed-off-by: Guo Ren <guoren@...ux.alibaba.com>
Signed-off-by: Guo Ren <guoren@...nel.org>
Cc: Waiman Long <longman@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Boqun Feng <boqun.feng@...il.com>
Cc: Will Deacon <will@...nel.org>
Cc: Ingo Molnar <mingo@...hat.com>
---
 include/asm-generic/barrier.h | 22 ++++++++++++++++++++++
 include/linux/atomic.h        |  4 ++++
 kernel/locking/qspinlock.c    |  3 ++-
 3 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
index 961f4d88f9ef..fec61629f769 100644
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -275,6 +275,28 @@ do {									\
 })
 #endif
 
+/**
+ * smp_cond_load_mask_relaxed() - (Spin) wait for cond with ACQUIRE ordering
+ * @ptr: pointer to the variable to wait on
+ * @cond: boolean expression to wait for
+ * @mask: mask *ptr to wait for (effect of 0 is the same with -1)
+ */
+#ifndef smp_cond_load_mask_relaxed
+#define smp_cond_load_mask_relaxed(ptr, cond_expr, mask)	\
+	smp_cond_load_relaxed(ptr, cond_expr)
+#endif
+
+/**
+ * smp_cond_load_mask_acquire() - (Spin) wait for cond with ACQUIRE ordering
+ * @ptr: pointer to the variable to wait on
+ * @cond: boolean expression to wait for
+ * @mask: mask *ptr to wait for (effect of 0 is the same with -1)
+ */
+#ifndef smp_cond_load_mask_acquire
+#define smp_cond_load_mask_acquire(ptr, cond_expr, mask)	\
+	smp_cond_load_acquire(ptr, cond_expr)
+#endif
+
 /*
  * pmem_wmb() ensures that all stores for which the modification
  * are written to persistent storage by preceding instructions have
diff --git a/include/linux/atomic.h b/include/linux/atomic.h
index 8dd57c3a99e9..dc7351945f27 100644
--- a/include/linux/atomic.h
+++ b/include/linux/atomic.h
@@ -27,9 +27,13 @@
 
 #define atomic_cond_read_acquire(v, c) smp_cond_load_acquire(&(v)->counter, (c))
 #define atomic_cond_read_relaxed(v, c) smp_cond_load_relaxed(&(v)->counter, (c))
+#define atomic_cond_read_mask_acquire(v, c, m) smp_cond_load_mask_acquire(&(v)->counter, (c), (m))
+#define atomic_cond_read_mask_relaxed(v, c, m) smp_cond_load_mask_relaxed(&(v)->counter, (c), (m))
 
 #define atomic64_cond_read_acquire(v, c) smp_cond_load_acquire(&(v)->counter, (c))
 #define atomic64_cond_read_relaxed(v, c) smp_cond_load_relaxed(&(v)->counter, (c))
+#define atomic64_cond_read_mask_acquire(v, c, m) smp_cond_load_mask_acquire(&(v)->counter, (c), (m))
+#define atomic64_cond_read_mask_relaxed(v, c, m) smp_cond_load_mask_relaxed(&(v)->counter, (c), (m))
 
 /*
  * The idea here is to build acquire/release variants by adding explicit
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index ebe6b8ec7cb3..14fdd2ee752c 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -511,7 +511,8 @@ void __lockfunc queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
 	if ((val = pv_wait_head_or_lock(lock, node)))
 		goto locked;
 
-	val = atomic_cond_read_acquire(&lock->val, !(VAL & _Q_LOCKED_PENDING_MASK));
+	val = atomic_cond_read_mask_acquire(&lock->val, !(VAL & _Q_LOCKED_PENDING_MASK),
+					    _Q_LOCKED_PENDING_MASK);
 
 locked:
 	/*
-- 
2.36.1