[PATCH 5/6] kernel/locking: Prevent slowpath writers getting held up by fastpath

[Waiman Long]

On 10/05/2017 09:56 AM, Peter Zijlstra wrote:
> On Thu, Oct 05, 2017 at 01:54:56PM +0100, Will Deacon wrote:
>> When a prospective writer takes the qrwlock locking slowpath due to the
>> lock being held, it attempts to cmpxchg the wmode field from 0 to
>> _QW_WAITING so that concurrent lockers also take the slowpath and queue
>> on the spinlock accordingly, allowing the lockers to drain.
>>
>> Unfortunately, this isn't fair, because a fastpath writer that comes in
>> after the lock is made available but before the _QW_WAITING flag is set
>> can effectively jump the queue. If there is a steady stream of prospective
>> writers, then the waiter will be held off indefinitely.
>>
>> This patch restores fairness by separating _QW_WAITING and _QW_LOCKED
>> into two bits in the wmode byte and having the waiter set _QW_WAITING
>> unconditionally. This then forces the slow-path for concurrent lockers,
>> but requires that a writer unlock operation performs an
>> atomic_sub_release instead of a store_release so that the waiting status
>> is preserved.
>> diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
>> index 02c0a768e6b0..8b7edef500e5 100644
>> --- a/include/asm-generic/qrwlock.h
>> +++ b/include/asm-generic/qrwlock.h
>> @@ -41,7 +41,7 @@
>>   *       +----+----+----+----+
>>   */
>>  #define	_QW_WAITING	1		/* A writer is waiting	   */
>> -#define	_QW_LOCKED	0xff		/* A writer holds the lock */
>> +#define	_QW_LOCKED	2		/* A writer holds the lock */
>>  #define	_QW_WMASK	0xff		/* Writer mask		   */
>>  #define	_QR_SHIFT	8		/* Reader count shift	   */
>>  #define _QR_BIAS	(1U << _QR_SHIFT)
>> @@ -134,7 +134,7 @@ static inline void queued_read_unlock(struct qrwlock *lock)
>>   */
>>  static inline void queued_write_unlock(struct qrwlock *lock)
>>  {
>> -	smp_store_release(&lock->wmode, 0);
>> +	(void)atomic_sub_return_release(_QW_LOCKED, &lock->cnts);
>>  }
> That is a fairly painful hit on x86. Changes a regular store into an
> "LOCK XADD" +20 cycles right there.
>
> Can't we steal one of the reader bits for waiting?
>
> diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
> index 7d026bf27713..5524801a02a5 100644
> --- a/include/asm-generic/qrwlock.h
> +++ b/include/asm-generic/qrwlock.h
> @@ -40,10 +40,10 @@
>   *       |      rd      | wr |
>   *       +----+----+----+----+
>   */
> -#define	_QW_WAITING	1		/* A writer is waiting	   */
> -#define	_QW_LOCKED	0xff		/* A writer holds the lock */
> -#define	_QW_WMASK	0xff		/* Writer mask		   */
> -#define	_QR_SHIFT	8		/* Reader count shift	   */
> +#define	_QW_WAITING	0x100		/* A writer is waiting	   */
> +#define	_QW_LOCKED	0x0ff		/* A writer holds the lock */
> +#define	_QW_WMASK	0x1ff		/* Writer mask		   */
> +#define	_QR_SHIFT	9		/* Reader count shift	   */
>  #define _QR_BIAS	(1U << _QR_SHIFT)
>  
>  /*
> diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
> index 2655f26ec882..5f75caea97e0 100644
> --- a/kernel/locking/qrwlock.c
> +++ b/kernel/locking/qrwlock.c
> @@ -54,7 +54,7 @@ struct __qrwlock {
>  static __always_inline void
>  rspin_until_writer_unlock(struct qrwlock *lock, u32 cnts)
>  {
> -	while ((cnts & _QW_WMASK) == _QW_LOCKED) {
> +	while ((cnts & _QW_LOCKED)) {
>  		cpu_relax();
>  		cnts = atomic_read_acquire(&lock->cnts);
>  	}
> @@ -120,21 +120,10 @@ void queued_write_lock_slowpath(struct qrwlock *lock)
>  	    (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0))
>  		goto unlock;
>  
> -	/*
> -	 * Set the waiting flag to notify readers that a writer is pending,
> -	 * or wait for a previous writer to go away.
> -	 */
> -	for (;;) {
> -		struct __qrwlock *l = (struct __qrwlock *)lock;
> -
> -		if (!READ_ONCE(l->wmode) &&
> -		   (cmpxchg_relaxed(&l->wmode, 0, _QW_WAITING) == 0))
> -			break;
> -
> -		cpu_relax();
> -	}
> +	/* Set the waiting flag to notify readers that a writer is pending */
> +	atomic_add(_QW_WAITING, &lock->cnts);
>  
> -	/* When no more readers, set the locked flag */
> +	/* When no more readers or writers, set the locked flag */
>  	for (;;) {
>  		cnts = atomic_read(&lock->cnts);
>  		if ((cnts == _QW_WAITING) &&

I think this is the better of dealing with the fairness problem.

Acked-by: Waiman Long <longman at redhat.com>