From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <480C9619.2050201@qumranet.com> Date: Mon, 21 Apr 2008 16:26:49 +0300 From: Avi Kivity MIME-Version: 1.0 Subject: Re: [patch 2/2]: introduce fast_gup References: <20080328025455.GA8083@wotan.suse.de> <20080328030023.GC8083@wotan.suse.de> <1208444605.7115.2.camel@twins> <480C81C4.8030200@qumranet.com> <1208781013.7115.173.camel@twins> In-Reply-To: <1208781013.7115.173.camel@twins> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Peter Zijlstra Cc: Linus Torvalds , Nick Piggin , Andrew Morton , shaggy@austin.ibm.com, axboe@kernel.dk, linux-mm@kvack.org, linux-arch@vger.kernel.org, Clark Williams , Ingo Molnar List-ID: Peter Zijlstra wrote: > On Mon, 2008-04-21 at 15:00 +0300, Avi Kivity wrote: > >> Linus Torvalds wrote: >> >>> Finally, I don't think that comment is correct in the first place. It's >>> not that simple. The thing is, even *with* the memory barrier in place, we >>> may have: >>> >>> CPU#1 CPU#2 >>> ===== ===== >>> >>> fast_gup: >>> - read low word >>> >>> native_set_pte_present: >>> - set low word to 0 >>> - set high word to new value >>> >>> - read high word >>> >>> - set low word to new value >>> >>> and so you read a low word that is associated with a *different* high >>> word! Notice? >>> >>> So trivial memory ordering is _not_ enough. >>> >>> So I think the code literally needs to be something like this >>> >>> #ifdef CONFIG_X86_PAE >>> >>> static inline pte_t native_get_pte(pte_t *ptep) >>> { >>> pte_t pte; >>> >>> retry: >>> pte.pte_low = ptep->pte_low; >>> smp_rmb(); >>> pte.pte_high = ptep->pte_high; >>> smp_rmb(); >>> if (unlikely(pte.pte_low != ptep->pte_low) >>> goto retry; >>> return pte; >>> } >>> >>> >>> >> I think this is still broken. Suppose that after reading pte_high >> native_set_pte() is called again on another cpu, changing pte_low back >> to the original value (but with a different pte_high). You now have >> pte_low from second native_set_pte() but pte_high from the first >> native_set_pte(). >> > > I think the idea was that for user pages we only use set_pte_present() > which does the low=0 thing first. > Doesn't matter. The second native_set_pte() (or set_pte_present()) executes atomically: fast_gup: - read low word (l0) native_set_pte_present: - set low word to 0 - set high word to new value (h1) - set low word to new value (l1) - read high word (h1) native_set_pte_present: - set low word to 0 - set high word to new value (h2) - set low word to new value (l2) - re-read low word (l2) If l2 happens to be equal to l0, then the check succeeds and we have a splintered pte h1:l0. -- error compiling committee.c: too many arguments to function -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org