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From: Roland McGrath <roland@redhat.com>
To: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>,
       Andrew Morton <akpm@linux-foundation.org>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       linux-kernel@vger.kernel.org,
       Tomasz Grobelny <tomasz@grobelny.oswiecenia.net>
Subject: Re: [PATCH] x86 tls prevent_tail_call
In-Reply-To: Ingo Molnar's message of  Wednesday, 27 February 2008 08:26:17 +0100 <20080227072617.GB4638@elte.hu>
References: <20080226210018.A031B26F9A4@magilla.localdomain>
	<20080227072617.GB4638@elte.hu>
Message-Id: <20080227073821.0C34C2700FD@magilla.localdomain>
Date: Tue, 26 Feb 2008 23:38:21 -0800 (PST)
Sender: linux-kernel-owner@vger.kernel.org
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> i'm wondering, have you seen this happen in practice? We use 
> sys_set_thread_area() for every new task started up. I guess we havent 
> seen problems in the field yet because this early during startup tasks 
> do not normally receive signals? (or if they do they are fatal and no 
> user signal context is used.)

Tomasz saw it.  I don't know what compiler or exact options to it he used.

> btw., this whole thing of us having to notice such tail-optimization 
> incidents is totally fragile and unreliable. Shouldnt there be a "dont 
> tail-optimize me" attribute which we could stick into asmlinkage? 

I agree.  It's come up before.  I'll talk to compiler folks about it again.

> Perhaps sparse could detect asmlinkage functions that do not do 
> prevent_tail_call()s?

That sounds like a good idea to me.


Thanks,
Roland

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