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Date: Mon, 3 Aug 2015 15:33:06 +0530
From: Vineet Gupta <Vineet.Gupta1@...opsys.com>
To: "Peter Zijlstra (Intel)" <peterz@...radead.org>
CC: lkml <linux-kernel@...r.kernel.org>, <arc-linux-dev@...opsys.com>,
Vineet Gupta <Vineet.Gupta1@...opsys.com>
Subject: [PATCH 4/6] ARC: LLOCK/SCOND based rwlock
With LLOCK/SCOND, the rwlock counter can be atomically updated w/o need
for a guarding spin lock.
Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Vineet Gupta <vgupta@...opsys.com>
---
arch/arc/include/asm/spinlock.h | 187 ++++++++++++++++++++++++++++++++--
arch/arc/include/asm/spinlock_types.h | 2 +
2 files changed, 179 insertions(+), 10 deletions(-)
diff --git a/arch/arc/include/asm/spinlock.h b/arch/arc/include/asm/spinlock.h
index 4f6c90a0a68a..9a7a7293f127 100644
--- a/arch/arc/include/asm/spinlock.h
+++ b/arch/arc/include/asm/spinlock.h
@@ -75,6 +75,177 @@ static inline void arch_spin_unlock(arch_spinlock_t *lock)
smp_mb();
}
+/*
+ * Read-write spinlocks, allowing multiple readers but only one writer.
+ * Unfair locking as Writers could be starved indefinitely by Reader(s)
+ */
+
+static inline void arch_read_lock(arch_rwlock_t *rw)
+{
+ unsigned int val;
+
+ smp_mb();
+
+ /*
+ * zero means writer holds the lock exclusively, deny Reader.
+ * Otherwise grant lock to first/subseq reader
+ *
+ * if (rw->counter > 0) {
+ * rw->counter--;
+ * ret = 1;
+ * }
+ */
+
+ __asm__ __volatile__(
+ "1: llock %[val], [%[rwlock]] \n"
+ " brls %[val], %[WR_LOCKED], 1b\n" /* <= 0: spin while write locked */
+ " sub %[val], %[val], 1 \n" /* reader lock */
+ " scond %[val], [%[rwlock]] \n"
+ " bnz 1b \n"
+ " \n"
+ : [val] "=&r" (val)
+ : [rwlock] "r" (&(rw->counter)),
+ [WR_LOCKED] "ir" (0)
+ : "memory", "cc");
+
+ smp_mb();
+}
+
+/* 1 - lock taken successfully */
+static inline int arch_read_trylock(arch_rwlock_t *rw)
+{
+ unsigned int val, got_it = 0;
+
+ smp_mb();
+
+ __asm__ __volatile__(
+ "1: llock %[val], [%[rwlock]] \n"
+ " brls %[val], %[WR_LOCKED], 4f\n" /* <= 0: already write locked, bail */
+ " sub %[val], %[val], 1 \n" /* counter-- */
+ " scond %[val], [%[rwlock]] \n"
+ " bnz 1b \n" /* retry if collided with someone */
+ " mov %[got_it], 1 \n"
+ " \n"
+ "4: ; --- done --- \n"
+
+ : [val] "=&r" (val),
+ [got_it] "+&r" (got_it)
+ : [rwlock] "r" (&(rw->counter)),
+ [WR_LOCKED] "ir" (0)
+ : "memory", "cc");
+
+ smp_mb();
+
+ return got_it;
+}
+
+static inline void arch_write_lock(arch_rwlock_t *rw)
+{
+ unsigned int val;
+
+ smp_mb();
+
+ /*
+ * If reader(s) hold lock (lock < __ARCH_RW_LOCK_UNLOCKED__),
+ * deny writer. Otherwise if unlocked grant to writer
+ * Hence the claim that Linux rwlocks are unfair to writers.
+ * (can be starved for an indefinite time by readers).
+ *
+ * if (rw->counter == __ARCH_RW_LOCK_UNLOCKED__) {
+ * rw->counter = 0;
+ * ret = 1;
+ * }
+ */
+
+ __asm__ __volatile__(
+ "1: llock %[val], [%[rwlock]] \n"
+ " brne %[val], %[UNLOCKED], 1b \n" /* while !UNLOCKED spin */
+ " mov %[val], %[WR_LOCKED] \n"
+ " scond %[val], [%[rwlock]] \n"
+ " bnz 1b \n"
+ " \n"
+ : [val] "=&r" (val)
+ : [rwlock] "r" (&(rw->counter)),
+ [UNLOCKED] "ir" (__ARCH_RW_LOCK_UNLOCKED__),
+ [WR_LOCKED] "ir" (0)
+ : "memory", "cc");
+
+ smp_mb();
+}
+
+/* 1 - lock taken successfully */
+static inline int arch_write_trylock(arch_rwlock_t *rw)
+{
+ unsigned int val, got_it = 0;
+
+ smp_mb();
+
+ __asm__ __volatile__(
+ "1: llock %[val], [%[rwlock]] \n"
+ " brne %[val], %[UNLOCKED], 4f \n" /* !UNLOCKED, bail */
+ " mov %[val], %[WR_LOCKED] \n"
+ " scond %[val], [%[rwlock]] \n"
+ " bnz 1b \n" /* retry if collided with someone */
+ " mov %[got_it], 1 \n"
+ " \n"
+ "4: ; --- done --- \n"
+
+ : [val] "=&r" (val),
+ [got_it] "+&r" (got_it)
+ : [rwlock] "r" (&(rw->counter)),
+ [UNLOCKED] "ir" (__ARCH_RW_LOCK_UNLOCKED__),
+ [WR_LOCKED] "ir" (0)
+ : "memory", "cc");
+
+ smp_mb();
+
+ return got_it;
+}
+
+static inline void arch_read_unlock(arch_rwlock_t *rw)
+{
+ unsigned int val;
+
+ smp_mb();
+
+ /*
+ * rw->counter++;
+ */
+ __asm__ __volatile__(
+ "1: llock %[val], [%[rwlock]] \n"
+ " add %[val], %[val], 1 \n"
+ " scond %[val], [%[rwlock]] \n"
+ " bnz 1b \n"
+ " \n"
+ : [val] "=&r" (val)
+ : [rwlock] "r" (&(rw->counter))
+ : "memory", "cc");
+
+ smp_mb();
+}
+
+static inline void arch_write_unlock(arch_rwlock_t *rw)
+{
+ unsigned int val;
+
+ smp_mb();
+
+ /*
+ * rw->counter = __ARCH_RW_LOCK_UNLOCKED__;
+ */
+ __asm__ __volatile__(
+ "1: llock %[val], [%[rwlock]] \n"
+ " scond %[UNLOCKED], [%[rwlock]]\n"
+ " bnz 1b \n"
+ " \n"
+ : [val] "=&r" (val)
+ : [rwlock] "r" (&(rw->counter)),
+ [UNLOCKED] "r" (__ARCH_RW_LOCK_UNLOCKED__)
+ : "memory", "cc");
+
+ smp_mb();
+}
+
#else /* !CONFIG_ARC_HAS_LLSC */
static inline void arch_spin_lock(arch_spinlock_t *lock)
@@ -148,23 +319,14 @@ static inline void arch_spin_unlock(arch_spinlock_t *lock)
smp_mb();
}
-#endif
-
/*
* Read-write spinlocks, allowing multiple readers but only one writer.
+ * Unfair locking as Writers could be starved indefinitely by Reader(s)
*
* The spinlock itself is contained in @counter and access to it is
* serialized with @lock_mutex.
- *
- * Unfair locking as Writers could be starved indefinitely by Reader(s)
*/
-/* Would read_trylock() succeed? */
-#define arch_read_can_lock(x) ((x)->counter > 0)
-
-/* Would write_trylock() succeed? */
-#define arch_write_can_lock(x) ((x)->counter == __ARCH_RW_LOCK_UNLOCKED__)
-
/* 1 - lock taken successfully */
static inline int arch_read_trylock(arch_rwlock_t *rw)
{
@@ -235,6 +397,11 @@ static inline void arch_write_unlock(arch_rwlock_t *rw)
arch_spin_unlock(&(rw->lock_mutex));
}
+#endif
+
+#define arch_read_can_lock(x) ((x)->counter > 0)
+#define arch_write_can_lock(x) ((x)->counter == __ARCH_RW_LOCK_UNLOCKED__)
+
#define arch_read_lock_flags(lock, flags) arch_read_lock(lock)
#define arch_write_lock_flags(lock, flags) arch_write_lock(lock)
diff --git a/arch/arc/include/asm/spinlock_types.h b/arch/arc/include/asm/spinlock_types.h
index 662627ced4f2..4e1ef5f650c6 100644
--- a/arch/arc/include/asm/spinlock_types.h
+++ b/arch/arc/include/asm/spinlock_types.h
@@ -26,7 +26,9 @@ typedef struct {
*/
typedef struct {
volatile unsigned int counter;
+#ifndef CONFIG_ARC_HAS_LLSC
arch_spinlock_t lock_mutex;
+#endif
} arch_rwlock_t;
#define __ARCH_RW_LOCK_UNLOCKED__ 0x01000000
--
1.9.1
--
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