linux-kernel - Re: [patch 5/6] futex: Prepare futex_lock

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Message-ID: <20210423113424.19e26b77@jawa>
Date:   Fri, 23 Apr 2021 11:34:24 +0200
From:   Lukasz Majewski <lukma@...x.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Adhemerval Zanella <adhemerval.zanella@...aro.org>,
        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>
Subject: Re: [patch 5/6] futex: Prepare futex_lock_pi() for runtime clock
 selection

Hi Thomas,

> futex_lock_pi() is the only futex operation which cannot select the
> clock for timeouts (CLOCK_MONOTONIC/CLOCK_REALTIME). That's
> inconsistent and there is no particular reason why this cannot be
> supported.
> 
> This was overlooked when CLOCK_REALTIME_FLAG was introduced and
> unfortunately not reported when the inconsistency was discovered in
> glibc.
> 
> Prepare the function and enforce the CLOCK_REALTIME_FLAG on
> FUTEX_LOCK_PI so that a new FUTEX_LOCK_PI2 can implement it correctly.
> 
> Reported-by: Kurt Kanzenbach <kurt@...utronix.de>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
>  kernel/futex.c |    5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -2786,7 +2786,7 @@ static int futex_lock_pi(u32 __user *uad
>  	if (refill_pi_state_cache())
>  		return -ENOMEM;
>  
> -	to = futex_setup_timer(time, &timeout, FLAGS_CLOCKRT, 0);
> +	to = futex_setup_timer(time, &timeout, flags, 0);
>  
>  retry:
>  	ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &q.key,
> FUTEX_WRITE); @@ -3711,7 +3711,7 @@ 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) return -ENOSYS;

What is the exact change for those two lines above? Looks like some
different tab/spaces...

>  	}
>  
> @@ -3743,6 +3743,7 @@ long do_futex(u32 __user *uaddr, int op,
>  	case FUTEX_WAKE_OP:
>  		return futex_wake_op(uaddr, flags, uaddr2, val,
> val2, val3); case FUTEX_LOCK_PI:
> +		flags |= FLAGS_CLOCKRT;
>  		return futex_lock_pi(uaddr, flags, timeout, 0);
>  	case FUTEX_UNLOCK_PI:
>  		return futex_unlock_pi(uaddr, flags);
> 



Best regards,

Lukasz Majewski

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