linux-kernel - Re: Thread flags modified without set_thread

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Message-ID: <20070305144033.GG22829@bingen.suse.de>
Date:	Mon, 5 Mar 2007 15:40:33 +0100
From:	Andi Kleen <ak@...e.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Mathieu Desnoyers <compudj@...gle.com>,
	"Martin J. Bligh" <mbligh@...gle.com>,
	linux-kernel@...r.kernel.org, Andi Kleen <ak@...e.de>,
	"David S. Miller" <davem@...emloft.net>,
	Paul Mackerras <paulus@...ba.org>,
	"Luck, Tony" <tony.luck@...el.com>,
	Haavard Skinnemoen <hskinnemoen@...el.com>
Subject: Re: Thread flags modified without set_thread_flag() (non atomically)

> It does seem risky.  Perhaps it is a micro-optimisation which utilises
> knowledge that this thread_struct cannot be looked up via any path in this
> context.
> 
> Or perhaps it is a bug.  Andi, can you please comment?

On flush_thread nobody else can mess with the thread, so yes it's a micro
optimization.

> 
> > And about this specific flush_thread, I am puzzled about the t->flags ^= 
> > (_TIF_ABI_PENDING | _TIF_IA32); line. The XOR will clearly flip the 
> > _TIF_ABI_PENDING bit to 0, and very likely set _TIF_IA32 to the opposite 
> > of its current value. Why does this change need to be written atomically 
> > (can other threads play with these flags ?) ?
> > 
> 
> Don't know.

iirc it came from DaveM originally. He just likes to write things in 
comp^wclever ways :0) It's just a little shorter.

> No, I don't immediately see anything in the flush_old_exec() code path
> which tells us that nobody else can look up this thread_info (or be holding
> a ref to it) in this context.

Normally the process flags atomicity should only matter with signals;
i don't think you can send a signal to a process being in exec this way.

-Andi
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