linux-kernel - Re: [RFC PATCH v6 1/5] Thread-local ABI system call: cache CPU number of running thread

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Message-ID: <20160407111938.GR3430@twins.programming.kicks-ass.net>
Date:	Thu, 7 Apr 2016 13:19:38 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Florian Weimer <fweimer@...hat.com>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Russell King <linux@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	linux-api <linux-api@...r.kernel.org>,
	Paul Turner <pjt@...gle.com>, Andrew Hunter <ahh@...gle.com>,
	Andy Lutomirski <luto@...capital.net>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Watson <davejwatson@...com>, Chris Lameter <cl@...ux.com>,
	Ben Maurer <bmaurer@...com>, rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Boqun Feng <boqun.feng@...il.com>
Subject: Re: [RFC PATCH v6 1/5] Thread-local ABI system call: cache CPU
 number of running thread

On Thu, Apr 07, 2016 at 12:39:21PM +0200, Florian Weimer wrote:
> On 04/07/2016 12:31 PM, Peter Zijlstra wrote:
> > On Thu, Apr 07, 2016 at 11:01:25AM +0200, Florian Weimer wrote:
> >>> Because ideally this structure would be part of the initial (glibc) TCB
> >>> with fixed offset etc.
> >>
> >> This is not possible because we have layering violations and code
> >> assumes it knows the precise of the glibc TCB.  I think Address
> >> Sanitizer is in this category.  This means we cannot adjust the TCB size
> >> based on the kernel headers used to compile glibc, and there will have
> >> to be some indirection.
> > 
> > So with the proposed fixed sized object it would work, right?
> 
> I didn't see a proposal for a fixed size buffer, in the sense that the
> size of struct sockaddr_in is fixed.

This thing proposed a single 64byte structure (with the possibility of
eventually adding more 64byte structures). Basically:

struct tlabi {
	union {
		__u8[64] __foo;
		struct {
			/* fields go here */
		};
	};
} __aligned__(64);

People objected against the fixed size scheme, but it being possible to
get a fixed TCB offset and reduce indirections is a big win IMO.