From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from merlin.infradead.org (merlin.infradead.org [IPv6:2001:4978:20e::2]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client did not present a certificate) by ozlabs.org (Postfix) with ESMTPS id 9E063B6F18 for ; Fri, 15 Jul 2011 19:50:59 +1000 (EST) Subject: Re: [PATCH 0/1] Fixup write permission of TLB on powerpc e500 core From: Peter Zijlstra To: Shan Hai In-Reply-To: <4E20037C.5070506@gmail.com> References: <1310717238-13857-1-git-send-email-haishan.bai@gmail.com> <1310718056.2586.275.camel@twins> <4E1FFC7B.4000209@gmail.com> <1310719445.2586.288.camel@twins> <4E20037C.5070506@gmail.com> Content-Type: text/plain; charset="UTF-8" Date: Fri, 15 Jul 2011 11:50:41 +0200 Message-ID: <1310723441.2586.291.camel@twins> Mime-Version: 1.0 Cc: tony.luck@intel.com, linux-kernel@vger.kernel.org, cmetcalf@tilera.com, dhowells@redhat.com, paulus@samba.org, tglx@linutronix.de, walken@google.com, linuxppc-dev@lists.ozlabs.org, akpm@linux-foundation.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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=3D1) _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