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

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Date:	Tue, 6 Jan 2015 17:25:42 +0800
From:	Baoquan He <bhe@...hat.com>
To:	Waiman Long <waiman.long@...com>,
	Peter Zijlstra <peterz@...radead.org>, mingo@...hat.com
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] locking/rwlocks: clean up of qrwlock

Hi all,

Thanks for reviewing. 

Do you have any other concerns about this small change?

Thanks
Baoquan


On 12/17/14 at 04:16pm, Waiman Long wrote:
> 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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