linux-kernel - Re: [RFC PATCH] thread_local_abi system call: caching current CPU number (x86)

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Message-ID: <20150717170300.GB1424@x>
Date:	Fri, 17 Jul 2015 10:03:00 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	Ben Maurer <bmaurer@...com>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Paul Turner <pjt@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Hunter <ahh@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-api@...r.kernel.org" <linux-api@...r.kernel.org>
Subject: Re: [RFC PATCH] thread_local_abi system call: caching current CPU
 number (x86)

On Fri, Jul 17, 2015 at 10:49:19AM +0000, Ben Maurer wrote:
> Mathieu Desnoyers wrote:
> > Expose a new system call allowing threads to register a userspace memory
> > area where to store the current CPU number. Scheduler migration sets the
> 
> I really like that this approach makes it easier to add a per-thread interaction between userspace and the kernel in the future.
> 
> >+       if (!tlap || t->thread_local_abi_len <
> >+                       offsetof(struct thread_local_abi, cpu)
> >+                       + sizeof(tlap->cpu))
> 
> Could you save a branch here by enforcing that thread_local_abi_len = 0 if thread_local_abi = null?

"saving a branch" doesn't seem like a good reason to do that; however,
it *is* the convention across other calls: if you pass 0, the pointer
is ignored, but if you pass non-zero, the pointer must be valid or you
get -EFAULT (or an actual segfault).

- Josh Triplett
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