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

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Date:	Fri, 15 Jul 2011 11:50:41 +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 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.

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

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