From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f198.google.com (mail-wr0-f198.google.com [209.85.128.198]) by kanga.kvack.org (Postfix) with ESMTP id A69176B0038 for ; Wed, 13 Dec 2017 19:36:43 -0500 (EST) Received: by mail-wr0-f198.google.com with SMTP id 96so2225492wrk.7 for ; Wed, 13 Dec 2017 16:36:43 -0800 (PST) Received: from mail.linuxfoundation.org (mail.linuxfoundation.org. [140.211.169.12]) by mx.google.com with ESMTPS id p108si2366332wrb.221.2017.12.13.16.36.42 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 13 Dec 2017 16:36:42 -0800 (PST) Date: Wed, 13 Dec 2017 16:36:39 -0800 From: Andrew Morton Subject: Re: [PATCHv4 09/12] x86/mm: Provide pmdp_establish() helper Message-Id: <20171213163639.7e1fb5c4082888d2e399b310@linux-foundation.org> In-Reply-To: <20171214003318.xli42qgybplln754@node.shutemov.name> References: <20171213105756.69879-1-kirill.shutemov@linux.intel.com> <20171213105756.69879-10-kirill.shutemov@linux.intel.com> <20171213160951.249071f2aecdccb38b6bb646@linux-foundation.org> <20171214003318.xli42qgybplln754@node.shutemov.name> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" Cc: "Kirill A. Shutemov" , Vlastimil Babka , Andrea Arcangeli , Michal Hocko , linux-arch@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Ingo Molnar , "H . Peter Anvin" , Thomas Gleixner On Thu, 14 Dec 2017 03:33:18 +0300 "Kirill A. Shutemov" wrote: > On Wed, Dec 13, 2017 at 04:09:51PM -0800, Andrew Morton wrote: > > > @@ -181,6 +182,40 @@ static inline pmd_t native_pmdp_get_and_clear(pmd_t *pmdp) > > > #define native_pmdp_get_and_clear(xp) native_local_pmdp_get_and_clear(xp) > > > #endif > > > > > > +#ifndef pmdp_establish > > > +#define pmdp_establish pmdp_establish > > > +static inline pmd_t pmdp_establish(struct vm_area_struct *vma, > > > + unsigned long address, pmd_t *pmdp, pmd_t pmd) > > > +{ > > > + pmd_t old; > > > + > > > + /* > > > + * If pmd has present bit cleared we can get away without expensive > > > + * cmpxchg64: we can update pmdp half-by-half without racing with > > > + * anybody. > > > + */ > > > + if (!(pmd_val(pmd) & _PAGE_PRESENT)) { > > > + union split_pmd old, new, *ptr; > > > + > > > + ptr = (union split_pmd *)pmdp; > > > + > > > + new.pmd = pmd; > > > + > > > + /* xchg acts as a barrier before setting of the high bits */ > > > + old.pmd_low = xchg(&ptr->pmd_low, new.pmd_low); > > > + old.pmd_high = ptr->pmd_high; > > > + ptr->pmd_high = new.pmd_high; > > > + return old.pmd; > > > + } > > > + > > > + { > > > + old = *pmdp; > > > + } while (cmpxchg64(&pmdp->pmd, old.pmd, pmd.pmd) != old.pmd); > > > > um, what happened here? > > Ouch.. Yeah, we need 'do' here. :-/ > > Apparently, it's a valid C code that would run the body once and it worked for > me because I didn't hit the race condition. So how the heck do we test this? Add an artificial delay on the other side to open the race window? -- 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