linux-kernel - Re: [PATCH 0/1] Fixup write permission of TLB on powerpc e500 core

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Date:	Fri, 15 Jul 2011 10:20:56 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Shan Hai <haishan.bai@...il.com>
Cc:	benh@...nel.crashing.org, paulus@...ba.org, tglx@...utronix.de,
	walken@...gle.com, dhowells@...hat.com, cmetcalf@...era.com,
	tony.luck@...el.com, akpm@...ux-foundation.org,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/1] Fixup write permission of TLB on powerpc e500 core

On Fri, 2011-07-15 at 16:07 +0800, Shan Hai wrote:
> 
> The following test case could reveal a bug in the futex_lock_pi()
> 
> BUG: On FUTEX_LOCK_PI, there is a infinite loop in the futex_lock_pi() 
>         on Powerpc e500 core.
> Cause: The linux kernel on the e500 core has no write permission on
>         the COW page, refer the head comment of the following test code.
>  
> ftrace on test case:
> [000]   353.990181: futex_lock_pi_atomic <-futex_lock_pi
> [000]   353.990185: cmpxchg_futex_value_locked <-futex_lock_pi_atomic
> [snip]
> [000]   353.990191: do_page_fault <-handle_page_fault
> [000]   353.990192: bad_page_fault <-handle_page_fault
> [000]   353.990193: search_exception_tables <-bad_page_fault
> [snip]
> [000]   353.990199: get_user_pages <-fault_in_user_writeable
> [snip]
> [000]   353.990208: mark_page_accessed <-follow_page
> [000]   353.990222: futex_lock_pi_atomic <-futex_lock_pi
> [snip]
> [000]   353.990230: cmpxchg_futex_value_locked <-futex_lock_pi_atomic
> [ a loop occures here ]
> 


But but but but, that get_user_pages(.write=1, .force=0) should result
in a COW break, getting our own writable page.

What is this e500 thing smoking that this doesn't work?
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