linux-kernel - Re: [PATCH-tip v7 08/20] locking/rwsem: Implement lock handoff to prevent lock starvation

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Date:   Fri, 3 May 2019 09:57:44 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, Davidlohr Bueso <dave@...olabs.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        huang ying <huang.ying.caritas@...il.com>
Subject: Re: [PATCH-tip v7 08/20] locking/rwsem: Implement lock handoff to
 prevent lock starvation

On 5/3/19 9:10 AM, Peter Zijlstra wrote:
> On Sun, Apr 28, 2019 at 05:25:45PM -0400, Waiman Long wrote:
>> +static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem,
>> +					enum writer_wait_state wstate)
>>  {
>>  	long new;
>>  
>> +	lockdep_assert_held(&sem->wait_lock);
>> +	do {
>> +		bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
>>  
>> +		if (has_handoff && wstate == WRITER_NOT_FIRST)
>> +			return false;
>>  
>> +		if (count & RWSEM_LOCK_MASK) {
>> +			if (has_handoff || (wstate != WRITER_HANDOFF))
>> +				return false;
>> +			new = count | RWSEM_FLAG_HANDOFF;
>> +		} else {
>> +			new = (count | RWSEM_WRITER_LOCKED) &
>> +				~RWSEM_FLAG_HANDOFF;
>>  
>> +			if (list_is_singular(&sem->wait_list))
>> +				new &= ~RWSEM_FLAG_WAITERS;
>> +		}
>> +	} while (!atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));
>> +
>> +	/*
>> +	 * We have either acquired the lock with handoff bit cleared or
>> +	 * set the handoff bit.
>> +	 */
>> +	if (new & RWSEM_FLAG_HANDOFF)
>> +		return false;
>> +
>> +	rwsem_set_owner(sem);
>> +	return true;
>>  }
> I've changed that like so.
>
> --- a/kernel/locking/rwsem.c
> +++ b/kernel/locking/rwsem.c
> @@ -406,19 +406,23 @@ static inline bool rwsem_try_write_lock(
>  	long new;
>  
>  	lockdep_assert_held(&sem->wait_lock);
> +
>  	do {
>  		bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
>  
>  		if (has_handoff && wstate == WRITER_NOT_FIRST)
>  			return false;
>  
> +		new = count;
> +
>  		if (count & RWSEM_LOCK_MASK) {
>  			if (has_handoff || (wstate != WRITER_HANDOFF))
>  				return false;
> -			new = count | RWSEM_FLAG_HANDOFF;
> +
> +			new |= RWSEM_FLAG_HANDOFF;
>  		} else {
> -			new = (count | RWSEM_WRITER_LOCKED) &
> -				~RWSEM_FLAG_HANDOFF;
> +			new |= RWSEM_WRITER_LOCKED;
> +			new &= ~RWSEM_FLAG_HANDOFF;
>  
>  			if (list_is_singular(&sem->wait_list))
>  				new &= ~RWSEM_FLAG_WAITERS;

Will the apply the changes in the next version.

Thanks,
Longman