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Subject: Re: [PATCH v4 07/16] locking/rwsem: Implement lock handoff to prevent
 lock starvation
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        Davidlohr Bueso <dave@stgolabs.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        huang ying <huang.ying.caritas@gmail.com>
References: <20190413172259.2740-1-longman@redhat.com>
 <20190413172259.2740-8-longman@redhat.com>
 <20190416154937.GL12232@hirez.programming.kicks-ass.net>
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From:   Waiman Long <longman@redhat.com>
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Organization: Red Hat
Message-ID: <d199371e-b232-d34d-a4e7-452a1d929ce6@redhat.com>
Date:   Tue, 16 Apr 2019 14:41:37 -0400
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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