From: Waiman Long <longman@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: "Hillf Danton" <hdanton@sina.com>, 马振华 <mazhenhua@xiaomi.com>,
mingo <mingo@redhat.com>, will <will@kernel.org>,
"boqun.feng" <boqun.feng@gmail.com>,
linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [BUG]locking/rwsem: only clean RWSEM_FLAG_HANDOFF when already set
Date: Thu, 11 Nov 2021 16:55:12 -0500 [thread overview]
Message-ID: <1158f843-ee83-8dea-fade-e59f529a88fb@redhat.com> (raw)
In-Reply-To: <YY2Q220iEiFdaKlT@hirez.programming.kicks-ass.net>
On 11/11/21 16:53, Peter Zijlstra wrote:
> On Thu, Nov 11, 2021 at 04:25:56PM -0500, Waiman Long wrote:
>> On 11/11/21 16:01, Waiman Long wrote:
>>> On 11/11/21 15:26, Peter Zijlstra wrote:
>>>> On Thu, Nov 11, 2021 at 02:36:52PM -0500, Waiman Long wrote:
>>>>
>>>>> @@ -434,6 +430,7 @@ static void rwsem_mark_wake(struct
>>>>> rw_semaphore *sem,
>>>>> if (!(oldcount & RWSEM_FLAG_HANDOFF) &&
>>>>> time_after(jiffies, waiter->timeout)) {
>>>>> adjustment -= RWSEM_FLAG_HANDOFF;
>>>>> + waiter->handoff_set = true;
>>>>> lockevent_inc(rwsem_rlock_handoff);
>>>>> }
>>>> Do we really need this flag? Wouldn't it be the same as waiter-is-first
>>>> AND sem-has-handoff ?
>>> That is true. The only downside is that we have to read the count first
>>> in rwsem_out_nolock_clear_flags(). Since this is not a fast path, it
>>> should be OK to do that.
>> I just realize that I may still need this flag for writer to determine if it
>> should spin after failing to acquire the lock. Or I will have to do extra
>> read of count value in the loop. I don't need to use it for writer now.
> Maybe it's too late here, but afaict this is right after failing
> try_write_lock(), which will have done at least that load you're
> interested in, no?
>
> Simply have try_write_lock() update &count or something.
You are right. I have actually decided to do an extra read after second
thought.
Cheers,
Longman
next prev parent reply other threads:[~2021-11-11 21:55 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <4fafad133b074f279dbab1aa3642e23f@xiaomi.com>
2021-11-07 3:25 ` [BUG]locking/rwsem: only clean RWSEM_FLAG_HANDOFF when already set Waiman Long
2021-11-07 3:28 ` Waiman Long
[not found] ` <20211107090131.1535-1-hdanton@sina.com>
2021-11-07 15:24 ` Waiman Long
2021-11-07 19:52 ` Waiman Long
2021-11-10 21:38 ` Peter Zijlstra
2021-11-11 2:42 ` Maria Yu
2021-11-11 15:08 ` Peter Zijlstra
2021-11-11 19:14 ` Waiman Long
2021-11-11 19:20 ` Peter Zijlstra
2021-11-11 19:36 ` Waiman Long
2021-11-11 19:52 ` Waiman Long
2021-11-11 20:26 ` Peter Zijlstra
2021-11-11 21:01 ` Waiman Long
2021-11-11 21:25 ` Waiman Long
2021-11-11 21:53 ` Peter Zijlstra
2021-11-11 21:55 ` Waiman Long [this message]
2021-11-11 22:00 ` Waiman Long
2021-11-11 21:38 ` Peter Zijlstra
2021-11-11 21:46 ` Waiman Long
2021-11-11 20:35 ` Peter Zijlstra
2021-11-11 20:39 ` Peter Zijlstra
2021-11-11 20:45 ` Waiman Long
2021-11-11 21:27 ` Peter Zijlstra
2021-11-11 21:54 ` Waiman Long
2021-11-11 20:50 ` Peter Zijlstra
2021-11-11 21:09 ` Waiman Long
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