linux-kernel - Re: [PATCH 1/2] x86: Cleanup TIF value gaps in shift range

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Message-ID: <alpine.DEB.2.00.1010181340080.10965@chino.kir.corp.google.com>
Date:	Mon, 18 Oct 2010 13:47:56 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/2] x86: Cleanup TIF value gaps in shift range

On Mon, 18 Oct 2010, Frederic Weisbecker wrote:

> 9, 19 and 26 values are missing from the TIF shift range, probably
> due to flags that were removed by the past. Now repack the range
> so that we can quickly retrieve the remaining free shift slots.
> 
> But take care of keeping the seperation between high and low bits
> as some masks are created on top of this boundary.
> 

What's the benefit of doing this?

These flags are exported to userspace through SysRq-T, SysRq+W, the hung 
task detector, and the rcu stall detector, so there may be external 
dependencies testing for these bits.

We use this to look for TIF_MEMDIE to determine whether an oom killed task 
has failed to exit and becomes hung after having access to memory 
reserves, and that's one of the bits you've changed here.
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