linux-kernel - Re: [PATCH 3/5] ipc/sem: optimize perform_atomic

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Date:   Mon, 19 Sep 2016 06:41:47 +0200
From:   Manfred Spraul <manfred@...orfullife.com>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 3/5] ipc/sem: optimize perform_atomic_semop()

Hi Davidlohr,

On 09/13/2016 10:33 AM, Davidlohr Bueso wrote:
>
>>
>>> @@ -1751,12 +1820,17 @@ SYSCALL_DEFINE4(semtimedop, int, semid, 
>>> struct sembuf __user *, tsops,
>>>          if (sop->sem_num >= max)
>>>              max = sop->sem_num;
>>>          if (sop->sem_flg & SEM_UNDO)
>>> -            undos = 1;
>>> +            undos = true;
>>>          if (sop->sem_op != 0)
>>> -            alter = 1;
>>> +            alter = true;
>>> +        if (sop->sem_num < SEMOPM_FAST && !dupsop) {
>>> +            if (dup & (1 << sop->sem_num))
>>> +                dupsop = 1;
>>> +            else
>>> +                dup |= 1 << sop->sem_num;
>>> +        }
>>>      }
>> At least for nsops=2, sops[0].sem_num !=sops[1].sem_num can detect 
>> absense of duplicated ops regardless of the array size.
>> Should we support that?
>
> There are various individual cases like that (ie obviously nsops == 1, 
> alter == 0, etc)
> where the dup detection would be unnecessary, but it seems like a 
> stretch to go
> at it like this. The above will work on the common case (assuming 
> lower sem_num
> of course). So I'm not particularly worried about being too smart at 
> the dup detection.
>
What about the attached dup detection?

--
     Manfred

View attachment "0001-ipc-sem-Update-duplicate-sop-detection.patch" of type "text/x-patch" (2247 bytes)