linux-kernel - Re: [QUESTION] srcu: Remove the SCAN2 state

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Date:   Thu, 22 Feb 2018 14:05:18 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     paulmck@...ux.vnet.ibm.com
Cc:     jiangshanlai@...il.com, josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, linux-kernel@...r.kernel.org,
        kernel-team@....com
Subject: Re: [QUESTION] srcu: Remove the SCAN2 state

On 2/22/2018 11:11 AM, Paul E. McKenney wrote:
> On Thu, Feb 22, 2018 at 08:57:27AM +0900, Byungchul Park wrote:
>> Hello,
>>
>> I'm sorry for bothering you, and I seem to be obviously missing
>> something, but I'm really wondering why we check try_check_zero()
>> again in the state, SCAN1, for the previous srcu_idx.
>>
>> I mean, since we've already checked try_check_zero() in the previous
>> grace period and gotten 'true' as a return value, all readers who see
>> the flipped idx via srcu_flip() won't update the src_{lock,unlock}_count
>> for the previous idx until it gets flipped back again.
>>
>> Is there any reasons we check try_check_zero() again in the state, SCAN1?
>> Is there any problems if the following patch's applied?
> 
> Indeed there are!  Removing the second scan exposes us to a nasty race
> condition where a reader is preempted (or interrupted or whatever) just

Indeed! I missed the cases. It should be as it is.

Thanks a lot for pointing it out.

> after fetching its counter.  A detailed explanation for an essentially

-- 
Thanks,
Byungchul