linux-kernel - Re: [PATCH] Mark thread stack correctly in proc/<pid>/maps

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Message-ID: <CAAHN_R2Awa5X3B09541grAPLkm9RzL9DnixUKFJXpz=1ZkPTFg@mail.gmail.com>
Date:	Fri, 24 Feb 2012 10:59:03 +0530
From:	Siddhesh Poyarekar <siddhesh.poyarekar@...il.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, Jamie Lokier <jamie@...reable.org>,
	vapier@...too.org, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] Mark thread stack correctly in proc/<pid>/maps

On Fri, Feb 24, 2012 at 4:47 AM, KOSAKI Motohiro
<kosaki.motohiro@...il.com> wrote:
> How protect this loop from task exiting? AFAIK, while_each_thread
> require rcu_read_lock or task_list_lock.

I missed this, thanks. I'll send a patch for this on top of my earlier
patch since Andrew has already included the earlier patch.

> Sigh. No, I missed one thing. If application use
> makecontext()/swapcontext() pair,
> ESP is not reliable way to detect pthread stack. At that time the
> stack is still marked
> as anonymous memory.

This is not wrong, because it essentially gives the correct picture of
the state of that task -- the task is using another vma as a stack
during that point and not the one it was allotted by pthreads during
thread creation.

I don't think we can successfully stick to the idea of trying to mark
stack space allocated by pthreads but not used by any task *currently*
as stack as long as the allocation happens outside the kernel space.
The only way to mark this is either by marking the stack as
VM_GROWSDOWN (which will make the stack grow and break some pthreads
functions) or create a new flag, which a simple display such as this
does not deserve. So it's best that this sticks to what the kernel
*knows* is being used as stack.

-- 
Siddhesh Poyarekar
http://siddhesh.in
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