linux-kernel - Re: [patch V2 3/7] futex: Add op for hash preallocation

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Thu, 19 May 2016 14:24:57 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Darren Hart <darren@...art.com>,
	Ingo Molnar <mingo@...nel.org>,
	Michael Kerrisk <mtk.manpages@...glemail.com>,
	Davidlohr Bueso <dave@...olabs.net>, Chris Mason <clm@...com>,
	Carlos O'Donell <carlos@...hat.com>,
	Torvald Riegel <triegel@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [patch V2 3/7] futex: Add op for hash preallocation

On Thu, May 05, 2016 at 08:44:05PM -0000, Thomas Gleixner wrote:
> From: Sebastian Siewior <bigeasy@...utronix.de>
> 
> The per process hash is allocated on the fly at the first futex operation of a
> process. The size of the hash is determined by a system wide default setting
> controlled by the sys admin, This is suboptimal for RT applications and
> applications with pathological futex abuse,
> 
>  - For RT applications its important to allocate the per process hash before the
>    first futex operation to avoid the allocation on the first futex operation.
> 
>  - For pathological applications which use gazillions of futexes its useful to
>    allocate a hash greater than the default hash size.
> 
> Add a futex op which allows to preallocate the hash with the requested
> size. The size is limited by the systemwide maximum hash size, which can be
> set by the admin. The requested size is rounded up to the next order of 2.
> 
> The function can be called several times, but ony the first call results in a
> hash allocation of the requested size as there is no non-intrusive way to
> reallocate/rehash in a multithreaded application.
> 
> Note, that this call must be issued before the first futex operation in the
> process because that would automatically allocate the default sized hash.
> 
> The function returns the actual hash size or 0 if the global hash is used. The
> latter is the case on UP and in the rare case that the allocation failed and
> the global hash is used as a fallback.

OK, so no on-line rehashing possible?