From a14d39d7ac45eebe45ed0772d1ed837f15b15af4 Mon Sep 17 00:00:00 2001
From: Kent Overstreet <kent.overstreet@gmail.com>
Date: Mon, 26 Apr 2021 19:40:09 -0400
Subject: [PATCH] bit_spinlocks now use futexes

Spinlocks aren't a good idea in userspace, where we can't actually
disable preemption.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
---
 include/linux/bit_spinlock.h | 44 +++++++++++++++++++-----------------
 include/linux/rhashtable.h   |  1 +
 2 files changed, 24 insertions(+), 21 deletions(-)

diff --git a/include/linux/bit_spinlock.h b/include/linux/bit_spinlock.h
index 0e88820a..ed47cc63 100644
--- a/include/linux/bit_spinlock.h
+++ b/include/linux/bit_spinlock.h
@@ -3,38 +3,40 @@
 
 #include <linux/kernel.h>
 #include <linux/preempt.h>
-#include <linux/atomic.h>
-#include <linux/bug.h>
+#include <linux/futex.h>
 
-static inline void bit_spin_lock(int bitnum, unsigned long *addr)
+static inline void bit_spin_lock(int nr, unsigned long *_addr)
 {
-	while (unlikely(test_and_set_bit_lock(bitnum, addr))) {
-		do {
-			cpu_relax();
-		} while (test_bit(bitnum, addr));
+	u32 mask, *addr = ((u32 *) _addr) + (nr / 32), v;
+
+	nr &= 31;
+	mask = 1U << nr;
+
+	while (1) {
+		v = __atomic_fetch_or(addr, mask, __ATOMIC_ACQUIRE);
+		if (!(v & mask))
+			break;
+
+		futex(addr, FUTEX_WAIT|FUTEX_PRIVATE_FLAG, v, NULL, NULL, 0);
 	}
 }
 
-static inline int bit_spin_trylock(int bitnum, unsigned long *addr)
+static inline void bit_spin_wake(int nr, unsigned long *_addr)
 {
-	return !test_and_set_bit_lock(bitnum, addr);
+	u32 *addr = ((u32 *) _addr) + (nr / 32);
+
+	futex(addr, FUTEX_WAKE|FUTEX_PRIVATE_FLAG, INT_MAX, NULL, NULL, 0);
 }
 
-static inline void bit_spin_unlock(int bitnum, unsigned long *addr)
+static inline void bit_spin_unlock(int nr, unsigned long *_addr)
 {
-	BUG_ON(!test_bit(bitnum, addr));
+	u32 mask, *addr = ((u32 *) _addr) + (nr / 32);
 
-	clear_bit_unlock(bitnum, addr);
-}
+	nr &= 31;
+	mask = 1U << nr;
 
-static inline void __bit_spin_unlock(int bitnum, unsigned long *addr)
-{
-	bit_spin_unlock(bitnum, addr);
-}
-
-static inline int bit_spin_is_locked(int bitnum, unsigned long *addr)
-{
-	return test_bit(bitnum, addr);
+	__atomic_and_fetch(addr, ~mask, __ATOMIC_RELEASE);
+	futex(addr, FUTEX_WAKE|FUTEX_PRIVATE_FLAG, INT_MAX, NULL, NULL, 0);
 }
 
 #endif /* __LINUX_BIT_SPINLOCK_H */
diff --git a/include/linux/rhashtable.h b/include/linux/rhashtable.h
index 6cf8c257..c5e717bf 100644
--- a/include/linux/rhashtable.h
+++ b/include/linux/rhashtable.h
@@ -395,6 +395,7 @@ static inline void rht_assign_unlock(struct bucket_table *tbl,
 	rcu_assign_pointer(*bkt, (void *)obj);
 	preempt_enable();
 	__release(bitlock);
+	bit_spin_wake(0, (unsigned long *) bkt);
 }
 
 /**