linux-kernel - Re: [PATCH] locking/rwlocks: clean up of qrwlock

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Date:	Wed, 17 Dec 2014 16:16:28 -0500
From:	Waiman Long <waiman.long@...com>
To:	Baoquan He <bhe@...hat.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, mingo@...hat.com
Subject: Re: [PATCH] locking/rwlocks: clean up of qrwlock

On 12/16/2014 10:36 AM, Baoquan He wrote:
> On 12/16/14 at 10:01am, Peter Zijlstra wrote:
>> On Tue, Dec 16, 2014 at 02:00:40PM +0800, Baoquan He wrote:
>>> In queue_read_lock_slowpath, when writer count becomes 0, we need
>>> increment the read count and get the lock. Then need call
>>> rspin_until_writer_unlock to check again if an incoming writer
>>> steals the lock in the gap. But in rspin_until_writer_unlock
>>> it only checks the writer count, namely low 8 bit of lock->cnts,
>>> no need to subtract the reader count unit specifically. So remove
>>> that subtraction to make it clearer, rspin_until_writer_unlock
>>> just takes the actual lock->cnts as the 2nd argument.
>>>
>>> And also change the code comment in queue_write_lock_slowpath to
>>> make it more exact and explicit.
>>>
>>> Signed-off-by: Baoquan He<bhe@...hat.com>
>>> ---
>>>   kernel/locking/qrwlock.c | 8 ++++----
>>>   1 file changed, 4 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
>>> index f956ede..ae66c10 100644
>>> --- a/kernel/locking/qrwlock.c
>>> +++ b/kernel/locking/qrwlock.c
>>> @@ -76,7 +76,7 @@ void queue_read_lock_slowpath(struct qrwlock *lock)
>>>   	while (atomic_read(&lock->cnts)&  _QW_WMASK)
>>>   		cpu_relax_lowlatency();
>>>
>>> -	cnts = atomic_add_return(_QR_BIAS,&lock->cnts) - _QR_BIAS;
>>> +	cnts = atomic_add_return(_QR_BIAS,&lock->cnts);
>>>   	rspin_until_writer_unlock(lock, cnts);
>> Did you actually look at the ASM generated? I suspect your change makes
>> it bigger.
>
> It does make it bigger. But it doesn't matter. Because in
> rspin_until_writer_unlock it only compqre (cnts&  _QW_WMASK)
> with _QW_LOCKED. So using incremented reader count doesn't impact
> the result. Anyway it will get the actual lock->cnts in
> rspin_until_writer_unlock in next loop. I can't see why we need
> subtract that reader count increment specifically.
>
> When I read this code, thought there's some special usage. Finally I
> realized it doesn't have special usage, and doesn't have to do that.

The "- _QR_BIAS" expression was added to simulate xadd() which is 
present in x86, but not in some other architectures. There is no 
equivalent functionality in the set of atomic helper functions. Anyway, 
I have no objection to the change as it is in the slowpath.

Acked-by: Waiman Long <Waiman.Long@...com>

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