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Date:   Tue, 16 Apr 2019 18:15:22 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Waiman Long <longman@redhat.com>
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>
Subject: Re: [PATCH v4 07/16] locking/rwsem: Implement lock handoff to
 prevent lock starvation
Message-ID: <20190416161522.GR14281@hirez.programming.kicks-ass.net>
References: <20190413172259.2740-1-longman@redhat.com>
 <20190413172259.2740-8-longman@redhat.com>
 <20190416154937.GL12232@hirez.programming.kicks-ass.net>
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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 !

> 
> 	rwsem_set_owner(sem);
> 	return true;
> 
> it almost looks like sensible code.