linux-kernel - Re: [PATCH v3 0/6] powerpc: queued spinlocks and rwlocks

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Message-ID: <eaabf501-80fe-dd15-c03c-f75ce4f75877@redhat.com>
Date:   Tue, 21 Jul 2020 10:36:16 -0400
From:   Waiman Long <longman@...hat.com>
To:     Nicholas Piggin <npiggin@...il.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Anton Blanchard <anton@...abs.org>,
        Boqun Feng <boqun.feng@...il.com>, kvm-ppc@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, Ingo Molnar <mingo@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH v3 0/6] powerpc: queued spinlocks and rwlocks

On 7/21/20 7:08 AM, Nicholas Piggin wrote:
> diff --git a/arch/powerpc/include/asm/qspinlock.h b/arch/powerpc/include/asm/qspinlock.h
> index b752d34517b3..26d8766a1106 100644
> --- a/arch/powerpc/include/asm/qspinlock.h
> +++ b/arch/powerpc/include/asm/qspinlock.h
> @@ -31,16 +31,57 @@ static inline void queued_spin_unlock(struct qspinlock *lock)
>   
>   #else
>   extern void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val);
> +extern void queued_spin_lock_slowpath_queue(struct qspinlock *lock);
>   #endif
>   
>   static __always_inline void queued_spin_lock(struct qspinlock *lock)
>   {
> -	u32 val = 0;
> -
> -	if (likely(atomic_try_cmpxchg_lock(&lock->val, &val, _Q_LOCKED_VAL)))
> +	atomic_t *a = &lock->val;
> +	u32 val;
> +
> +again:
> +	asm volatile(
> +"1:\t"	PPC_LWARX(%0,0,%1,1) "	# queued_spin_lock			\n"
> +	: "=&r" (val)
> +	: "r" (&a->counter)
> +	: "memory");
> +
> +	if (likely(val == 0)) {
> +		asm_volatile_goto(
> +	"	stwcx.	%0,0,%1							\n"
> +	"	bne-	%l[again]						\n"
> +	"\t"	PPC_ACQUIRE_BARRIER "						\n"
> +		:
> +		: "r"(_Q_LOCKED_VAL), "r" (&a->counter)
> +		: "cr0", "memory"
> +		: again );
>   		return;
> -
> -	queued_spin_lock_slowpath(lock, val);
> +	}
> +
> +	if (likely(val == _Q_LOCKED_VAL)) {
> +		asm_volatile_goto(
> +	"	stwcx.	%0,0,%1							\n"
> +	"	bne-	%l[again]						\n"
> +		:
> +		: "r"(_Q_LOCKED_VAL | _Q_PENDING_VAL), "r" (&a->counter)
> +		: "cr0", "memory"
> +		: again );
> +
> +		atomic_cond_read_acquire(a, !(VAL & _Q_LOCKED_MASK));
> +//		clear_pending_set_locked(lock);
> +		WRITE_ONCE(lock->locked_pending, _Q_LOCKED_VAL);
> +//		lockevent_inc(lock_pending);
> +		return;
> +	}
> +
> +	if (val == _Q_PENDING_VAL) {
> +		int cnt = _Q_PENDING_LOOPS;
> +		val = atomic_cond_read_relaxed(a,
> +					       (VAL != _Q_PENDING_VAL) || !cnt--);
> +		if (!(val & ~_Q_LOCKED_MASK))
> +			goto again;
> +        }
> +	queued_spin_lock_slowpath_queue(lock);
>   }
>   #define queued_spin_lock queued_spin_lock
>   

I am fine with the arch code override some part of the generic code.


> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index b9515fcc9b29..ebcc6f5d99d5 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -287,10 +287,14 @@ static __always_inline u32  __pv_wait_head_or_lock(struct qspinlock *lock,
>   
>   #ifdef CONFIG_PARAVIRT_SPINLOCKS
>   #define queued_spin_lock_slowpath	native_queued_spin_lock_slowpath
> +#define queued_spin_lock_slowpath_queue	native_queued_spin_lock_slowpath_queue
>   #endif
>   
>   #endif /* _GEN_PV_LOCK_SLOWPATH */
>   
> +void queued_spin_lock_slowpath_queue(struct qspinlock *lock);
> +static void __queued_spin_lock_slowpath_queue(struct qspinlock *lock);
> +
>   /**
>    * queued_spin_lock_slowpath - acquire the queued spinlock
>    * @lock: Pointer to queued spinlock structure
> @@ -314,12 +318,6 @@ static __always_inline u32  __pv_wait_head_or_lock(struct qspinlock *lock,
>    */
>   void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>   {
> -	struct mcs_spinlock *prev, *next, *node;
> -	u32 old, tail;
> -	int idx;
> -
> -	BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS));
> -
>   	if (pv_enabled())
>   		goto pv_queue;
>   
> @@ -397,6 +395,26 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>   queue:
>   	lockevent_inc(lock_slowpath);
>   pv_queue:
> +	__queued_spin_lock_slowpath_queue(lock);
> +}
> +EXPORT_SYMBOL(queued_spin_lock_slowpath);
> +
> +void queued_spin_lock_slowpath_queue(struct qspinlock *lock)
> +{
> +	lockevent_inc(lock_slowpath);
> +	__queued_spin_lock_slowpath_queue(lock);
> +}
> +EXPORT_SYMBOL(queued_spin_lock_slowpath_queue);
> +
> +static void __queued_spin_lock_slowpath_queue(struct qspinlock *lock)
> +{
> +	struct mcs_spinlock *prev, *next, *node;
> +	u32 old, tail;
> +	u32 val;
> +	int idx;
> +
> +	BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS));
> +
>   	node = this_cpu_ptr(&qnodes[0].mcs);
>   	idx = node->count++;
>   	tail = encode_tail(smp_processor_id(), idx);
> @@ -559,7 +577,6 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>   	 */
>   	__this_cpu_dec(qnodes[0].mcs.count);
>   }
> -EXPORT_SYMBOL(queued_spin_lock_slowpath);
>   
>   /*
>    * Generate the paravirt code for queued_spin_unlock_slowpath().
>
I would prefer to extract out the pending bit handling code out into a 
separate helper function which can be overridden by the arch code 
instead of breaking the slowpath into 2 pieces.

Cheers,
Longman