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2024-01-05 071106ecf68c401173c58808b1cf5f68cc50d390

 kernel/arch/parisc/include/asm/spinlock.h
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@@ -10,13 +10,21 @@
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 static inline int arch_spin_is_locked(arch_spinlock_t *x)
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 {
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 	volatile unsigned int *a = __ldcw_align(x);
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-	return *a == 0;
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+	return READ_ONCE(*a) == 0;
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 }
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15
 
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-#define arch_spin_lock(lock) arch_spin_lock_flags(lock, 0)
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+static inline void arch_spin_lock(arch_spinlock_t *x)
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+{
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+	volatile unsigned int *a;
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+
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+	a = __ldcw_align(x);
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+	while (__ldcw(a) == 0)
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+		while (*a == 0)
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+			continue;
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+}
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25
 
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 static inline void arch_spin_lock_flags(arch_spinlock_t *x,
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-					 unsigned long flags)
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+					unsigned long flags)
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 {
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 	volatile unsigned int *a;
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..
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@@ -25,10 +33,8 @@
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 		while (*a == 0)
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 			if (flags & PSW_SM_I) {
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 				local_irq_enable();
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-				cpu_relax();
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 				local_irq_disable();
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-			} else
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-				cpu_relax();
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+			}
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 }
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 #define arch_spin_lock_flags arch_spin_lock_flags
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40
 
..
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@@ -37,133 +43,107 @@
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 	volatile unsigned int *a;
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44
 
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 	a = __ldcw_align(x);
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-	mb();
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-	*a = 1;
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+	/* Release with ordered store. */
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+	__asm__ __volatile__("stw,ma %0,0(%1)" : : "r"(1), "r"(a) : "memory");
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 }
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 static inline int arch_spin_trylock(arch_spinlock_t *x)
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 {
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 	volatile unsigned int *a;
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-	int ret;
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53
 
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 	a = __ldcw_align(x);
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-        ret = __ldcw(a) != 0;
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-
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-	return ret;
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+	return __ldcw(a) != 0;
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 }
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57
 
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 /*
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  * Read-write spinlocks, allowing multiple readers but only one writer.
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- * Linux rwlocks are unfair to writers; they can be starved for an indefinite
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- * time by readers.  With care, they can also be taken in interrupt context.
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+ * Unfair locking as Writers could be starved indefinitely by Reader(s)
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  *
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- * In the PA-RISC implementation, we have a spinlock and a counter.
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- * Readers use the lock to serialise their access to the counter (which
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- * records how many readers currently hold the lock).
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- * Writers hold the spinlock, preventing any readers or other writers from
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- * grabbing the rwlock.
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+ * The spinlock itself is contained in @counter and access to it is
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+ * serialized with @lock_mutex.
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  */
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65
 
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-/* Note that we have to ensure interrupts are disabled in case we're
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- * interrupted by some other code that wants to grab the same read lock */
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-static  __inline__ void arch_read_lock(arch_rwlock_t *rw)
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+/* 1 - lock taken successfully */
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+static inline int arch_read_trylock(arch_rwlock_t *rw)
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 {
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+	int ret = 0;
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 	unsigned long flags;
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-	local_irq_save(flags);
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-	arch_spin_lock_flags(&rw->lock, flags);
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-	rw->counter++;
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-	arch_spin_unlock(&rw->lock);
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-	local_irq_restore(flags);
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-}
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71
 
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-/* Note that we have to ensure interrupts are disabled in case we're
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- * interrupted by some other code that wants to grab the same read lock */
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-static  __inline__ void arch_read_unlock(arch_rwlock_t *rw)
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-{
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-	unsigned long flags;
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 	local_irq_save(flags);
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-	arch_spin_lock_flags(&rw->lock, flags);
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-	rw->counter--;
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-	arch_spin_unlock(&rw->lock);
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-	local_irq_restore(flags);
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-}
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+	arch_spin_lock(&(rw->lock_mutex));
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74
 
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-/* Note that we have to ensure interrupts are disabled in case we're
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- * interrupted by some other code that wants to grab the same read lock */
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-static __inline__ int arch_read_trylock(arch_rwlock_t *rw)
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-{
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-	unsigned long flags;
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- retry:
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-	local_irq_save(flags);
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-	if (arch_spin_trylock(&rw->lock)) {
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-		rw->counter++;
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-		arch_spin_unlock(&rw->lock);
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-		local_irq_restore(flags);
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-		return 1;
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+	/*
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+	 * zero means writer holds the lock exclusively, deny Reader.
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+	 * Otherwise grant lock to first/subseq reader
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+	 */
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+	if (rw->counter > 0) {
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+		rw->counter--;
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+		ret = 1;
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 	}
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83
 
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+	arch_spin_unlock(&(rw->lock_mutex));
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 	local_irq_restore(flags);
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-	/* If write-locked, we fail to acquire the lock */
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-	if (rw->counter < 0)
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-		return 0;
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86
 
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-	/* Wait until we have a realistic chance at the lock */
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-	while (arch_spin_is_locked(&rw->lock) && rw->counter >= 0)
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+	return ret;
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+}
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+
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+/* 1 - lock taken successfully */
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+static inline int arch_write_trylock(arch_rwlock_t *rw)
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+{
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+	int ret = 0;
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+	unsigned long flags;
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+
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+	local_irq_save(flags);
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+	arch_spin_lock(&(rw->lock_mutex));
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+
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+	/*
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+	 * If reader(s) hold lock (lock < __ARCH_RW_LOCK_UNLOCKED__),
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+	 * deny writer. Otherwise if unlocked grant to writer
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+	 * Hence the claim that Linux rwlocks are unfair to writers.
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+	 * (can be starved for an indefinite time by readers).
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+	 */
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+	if (rw->counter == __ARCH_RW_LOCK_UNLOCKED__) {
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+		rw->counter = 0;
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+		ret = 1;
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+	}
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+	arch_spin_unlock(&(rw->lock_mutex));
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+	local_irq_restore(flags);
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+
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+	return ret;
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+}
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+
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+static inline void arch_read_lock(arch_rwlock_t *rw)
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+{
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+	while (!arch_read_trylock(rw))
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 		cpu_relax();
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-
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-	goto retry;
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 }
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120
 
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-/* Note that we have to ensure interrupts are disabled in case we're
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- * interrupted by some other code that wants to read_trylock() this lock */
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-static __inline__ void arch_write_lock(arch_rwlock_t *rw)
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+static inline void arch_write_lock(arch_rwlock_t *rw)
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+{
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+	while (!arch_write_trylock(rw))
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+		cpu_relax();
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+}
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+
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+static inline void arch_read_unlock(arch_rwlock_t *rw)
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 {
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 	unsigned long flags;
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-retry:
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+
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 	local_irq_save(flags);
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-	arch_spin_lock_flags(&rw->lock, flags);
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-
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-	if (rw->counter != 0) {
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-		arch_spin_unlock(&rw->lock);
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-		local_irq_restore(flags);
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-
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-		while (rw->counter != 0)
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-			cpu_relax();
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-
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-		goto retry;
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-	}
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-
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-	rw->counter = -1; /* mark as write-locked */
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-	mb();
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+	arch_spin_lock(&(rw->lock_mutex));
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+	rw->counter++;
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+	arch_spin_unlock(&(rw->lock_mutex));
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 	local_irq_restore(flags);
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 }
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137
 
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-static __inline__ void arch_write_unlock(arch_rwlock_t *rw)
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-{
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-	rw->counter = 0;
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-	arch_spin_unlock(&rw->lock);
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-}
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-
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-/* Note that we have to ensure interrupts are disabled in case we're
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- * interrupted by some other code that wants to read_trylock() this lock */
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-static __inline__ int arch_write_trylock(arch_rwlock_t *rw)
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+static inline void arch_write_unlock(arch_rwlock_t *rw)
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 {
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 	unsigned long flags;
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-	int result = 0;
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141
 
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 	local_irq_save(flags);
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-	if (arch_spin_trylock(&rw->lock)) {
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-		if (rw->counter == 0) {
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-			rw->counter = -1;
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-			result = 1;
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-		} else {
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-			/* Read-locked.  Oh well. */
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-			arch_spin_unlock(&rw->lock);
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-		}
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-	}
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+	arch_spin_lock(&(rw->lock_mutex));
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+	rw->counter = __ARCH_RW_LOCK_UNLOCKED__;
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+	arch_spin_unlock(&(rw->lock_mutex));
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 	local_irq_restore(flags);
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-
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-	return result;
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 }
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 #endif /* __ASM_SPINLOCK_H */