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

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Date:	Fri, 15 Jul 2011 18:06:36 +0800
From:	Shan Hai <haishan.bai@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
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 07/15/2011 05:50 PM, Peter Zijlstra wrote:
> On Fri, 2011-07-15 at 17:08 +0800, Shan Hai wrote:
>> The whole scenario should be,
>> - the child process triggers a page fault at the first time access to
>>       the lock, and it got its own writable page, but its *clean* for
>>       the reason just for checking the status of the lock.
>>       I am sorry for above "unbreakable COW".
>> - the futex_lock_pi() is invoked because of the lock contention,
>>       and the futex_atomic_cmpxchg_inatomic() tries to get the lock,
>>       it found out the lock is free so tries to write to the lock for
>>       reservation, a page fault occurs, because the page is read only
>>       for kernel(e500 specific), and returns -EFAULT to the caller
>> - the fault_in_user_writeable() tries to fix the fault,
>>       but from the get_user_pages() view everything is ok, because
>>       the COW was already broken, retry futex_lock_pi_atomic()
> but that's a bug right there, gup(.write=1) _should_ be a complete write
> fault, and as such toggle your sw dirty/young tracking.
>

The fault causing futex_atomic_cmpxchg_inatomic() is
protected by pagefault_disable(), so the page fault handler has
no chance to toggle the SW dirty/young tracking.

Thanks
Shan Hai

>> - futex_lock_pi_atomic() -->  futex_atomic_cmpxchg_inatomic(),
>>       another write protection page fault
>> - infinite loop

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