linux-kernel - Re: [patch 6/6] futex: Provide FUTEX_LOCK

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Date:   Sat, 24 Apr 2021 00:36:48 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     André Almeida <andrealmeid@...eup.net>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Adhemerval Zanella <adhemerval.zanella@...aro.org>,
        Lukasz Majewski <lukma@...x.de>,
        Florian Weimer <fweimer@...hat.com>,
        Carlos O'Donell <carlos@...hat.com>,
        "Michael Kerrisk \(man-pages\)" <mtk.manpages@...il.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Ingo Molnar <mingo@...nel.org>,
        Kurt Kanzenbach <kurt@...utronix.de>,
        Darren Hart <dvhart@...radead.org>,
        Andrei Vagin <avagin@...il.com>, kernel@...labora.com
Subject: Re: [patch 6/6] futex: Provide FUTEX_LOCK_PI2 to support clock selection

André!

On Fri, Apr 23 2021 at 19:20, André Almeida wrote:
>> @@ -21,6 +21,7 @@
>>   #define FUTEX_WAKE_BITSET	10
>>   #define FUTEX_WAIT_REQUEUE_PI	11
>>   #define FUTEX_CMP_REQUEUE_PI	12
>> +#define FUTEX_LOCK_PI2		13
>>   
>>   #define FUTEX_PRIVATE_FLAG	128
>>   #define FUTEX_CLOCK_REALTIME	256
>
> To keep consistency with other operations, maybe add a 
> FUTEX_LOCK_PI2_PRIVATE?

Good point! Missed that.

>> --- a/kernel/futex.c
>> +++ b/kernel/futex.c
>> @@ -3711,7 +3711,8 @@ long do_futex(u32 __user *uaddr, int op,
>>   
>>   	if (op & FUTEX_CLOCK_REALTIME) {
>>   		flags |= FLAGS_CLOCKRT;
>> -		if (cmd != FUTEX_WAIT_BITSET && cmd != FUTEX_WAIT_REQUEUE_PI)
>> +		if (cmd != FUTEX_WAIT_BITSET && cmd != FUTEX_WAIT_REQUEUE_PI &&
>> +		    cmd != FUTEX_LOCK_PI2)
>>   			return -ENOSYS;
>>   	}
>
> As FUTEX_LOCK_PI, FUTEX_LOCK_PI2 also requires FUTEX_CMPXCHG right? 
> Then, add it here:
>
> 	switch (cmd) {
> 	case FUTEX_LOCK_PI:
> +	case FUTEX_LOCK_PI2:
> 	case FUTEX_UNLOCK_PI:
> 	case FUTEX_TRYLOCK_PI:
> 	case FUTEX_WAIT_REQUEUE_PI:
> 	case FUTEX_CMP_REQUEUE_PI:
>   		if (!futex_cmpxchg_enabled)
>   			return -ENOSYS;
>   	}

Indeed. Forgot about that completely.

Thanks for spotting that!

       tglx