linux-kernel - Re: Futex Question (64 bits?)

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Date:	Mon, 21 Apr 2014 10:50:23 -0700
From:	Randy Dunlap <rdunlap@...radead.org>
To:	Ryan Nicholl <r.p.nicholl@...il.com>, linux-kernel@...r.kernel.org
CC:	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: Futex Question (64 bits?)

[adding Rusty]

On 03/24/2014 02:19 PM, Ryan Nicholl wrote:
> I would like to know if it would be possible to use a 64 bit futex
> call? If not, I would like to request it be added. (It is acknowledged
> in the manpages that it must be added eventually anyway.)
> 
> I am programming a C++11+ library. One of the constructed features is
> a "locking pointer". Unfortunately, pointers are usually 64 bits,
> while the futex call supports only 32 bits. I could not discover any
> documented way to invoke a 64 bit futex call.
> 
> It's unfortunate in that the futex call must have a timeout set
> because of the possible deadlock condition where only the half of the
> pointer not futexed is modified between the time the system call is
> entered and executed.
> 
> This creates a situation that prevents the locking_ptr from being
> realtime safe unless the timeout is set at a very low value.
> Furthermore, this causes excess CPU usage due to spurious wakeups.
> 
> Thank you,
> Ryan Nicholl
> --



-- 
~Randy
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