linux-kernel - Re: [PATCH v4 07/16] locking/rwsem: Implement lock handoff to prevent lock starvation

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Date:   Tue, 16 Apr 2019 14:41:37 -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>,
        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 v4 07/16] locking/rwsem: Implement lock handoff to prevent
 lock starvation

On 04/16/2019 12:15 PM, Peter Zijlstra wrote:
> On Tue, Apr 16, 2019 at 05:49:37PM +0200, Peter Zijlstra wrote:
>> See, if you first write that function in the form:
>>
>> 	long new;
>>
>> 	do {
>> 		new = count | RWSEM_WRITER_LOCKED;
>>
>> 		if (count & RWSEM_LOCK_MASK)
>> 			return false;
>>
>> 		if (list_is_singular(&sem->wait_list))
>> 			new &= ~RWSEM_FLAG_WAITERS;
>>
>> 	} while (atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));
>>
>> 	rwsem_set_owner(sem);
>> 	return true;
>>
>> And then add the HANDOFF bits like:
>>
>> 	long new;
>>
>> 	do {
>> +		bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
>>
>> +		new = (count | RWSEM_WRITER_LOCKED) & ~RWSEM_FLAG_HANDOFF;
>>
>> 		if (count & RWSEM_LOCK_MASK) {
>> +			if (has_handoff && wstate != WRITER_HANDOFF)
>> +				return false;
>> 			new |= RWSEM_FLAG_HANDOFF;
>> 		}
>>
>> +		if (has_handoff && wstate == WRITER_NOT_FIRST)
>> +			return false;
>>
>> 		if (list_is_singular(&sem->wait_list))
>> 			new &= ~RWSEM_FLAG_WAITERS;
>>
>> 	} while (atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));
> obviously that should be !
>

Right.

-Longman