From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933374AbcDKONb (ORCPT ); Mon, 11 Apr 2016 10:13:31 -0400 Received: from e06smtp09.uk.ibm.com ([195.75.94.105]:39465 "EHLO e06smtp09.uk.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932178AbcDKON2 (ORCPT ); Mon, 11 Apr 2016 10:13:28 -0400 X-IBM-Helo: d06dlp01.portsmouth.uk.ibm.com X-IBM-MailFrom: borntraeger@de.ibm.com X-IBM-RcptTo: linux-kernel@vger.kernel.org;stable@vger.kernel.org Subject: Re: [PATCH 3.12 05/98] kernel: Provide READ_ONCE and ASSIGN_ONCE To: Jiri Slaby , stable@vger.kernel.org References: Cc: linux-kernel@vger.kernel.org From: Christian Borntraeger Message-ID: <570BB102.1010001@de.ibm.com> Date: Mon, 11 Apr 2016 16:13:22 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=iso-8859-15 Content-Transfer-Encoding: 7bit X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 16041114-0037-0000-0000-00000A8F0F9A Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/11/2016 03:22 PM, Jiri Slaby wrote: > From: Christian Borntraeger > > 3.12-stable review patch. If anyone has any objections, please let me know. > As I wrote you 2 weeks ago, there are several patches on top, which are necessary to not break compile on some architectures or to get the final names. Please do not apply this stand-alone. > =============== > > commit 230fa253df6352af12ad0a16128760b5cb3f92df upstream. > > ACCESS_ONCE does not work reliably on non-scalar types. For > example gcc 4.6 and 4.7 might remove the volatile tag for such > accesses during the SRA (scalar replacement of aggregates) step > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145) > > Let's provide READ_ONCE/ASSIGN_ONCE that will do all accesses via > scalar types as suggested by Linus Torvalds. Accesses larger than > the machines word size cannot be guaranteed to be atomic. These > macros will use memcpy and emit a build warning. > > Signed-off-by: Christian Borntraeger > Signed-off-by: Jiri Slaby > --- > include/linux/compiler.h | 74 ++++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 74 insertions(+) > > diff --git a/include/linux/compiler.h b/include/linux/compiler.h > index 19a199414bd0..237063adbe1b 100644 > --- a/include/linux/compiler.h > +++ b/include/linux/compiler.h > @@ -179,6 +179,80 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect); > # define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __LINE__) > #endif > > +#include > + > +static __always_inline void data_access_exceeds_word_size(void) > +#ifdef __compiletime_warning > +__compiletime_warning("data access exceeds word size and won't be atomic") > +#endif > +; > + > +static __always_inline void data_access_exceeds_word_size(void) > +{ > +} > + > +static __always_inline void __read_once_size(volatile void *p, void *res, int size) > +{ > + switch (size) { > + case 1: *(__u8 *)res = *(volatile __u8 *)p; break; > + case 2: *(__u16 *)res = *(volatile __u16 *)p; break; > + case 4: *(__u32 *)res = *(volatile __u32 *)p; break; > +#ifdef CONFIG_64BIT > + case 8: *(__u64 *)res = *(volatile __u64 *)p; break; > +#endif > + default: > + barrier(); > + __builtin_memcpy((void *)res, (const void *)p, size); > + data_access_exceeds_word_size(); > + barrier(); > + } > +} > + > +static __always_inline void __assign_once_size(volatile void *p, void *res, int size) > +{ > + switch (size) { > + case 1: *(volatile __u8 *)p = *(__u8 *)res; break; > + case 2: *(volatile __u16 *)p = *(__u16 *)res; break; > + case 4: *(volatile __u32 *)p = *(__u32 *)res; break; > +#ifdef CONFIG_64BIT > + case 8: *(volatile __u64 *)p = *(__u64 *)res; break; > +#endif > + default: > + barrier(); > + __builtin_memcpy((void *)p, (const void *)res, size); > + data_access_exceeds_word_size(); > + barrier(); > + } > +} > + > +/* > + * Prevent the compiler from merging or refetching reads or writes. The > + * compiler is also forbidden from reordering successive instances of > + * READ_ONCE, ASSIGN_ONCE and ACCESS_ONCE (see below), but only when the > + * compiler is aware of some particular ordering. One way to make the > + * compiler aware of ordering is to put the two invocations of READ_ONCE, > + * ASSIGN_ONCE or ACCESS_ONCE() in different C statements. > + * > + * In contrast to ACCESS_ONCE these two macros will also work on aggregate > + * data types like structs or unions. If the size of the accessed data > + * type exceeds the word size of the machine (e.g., 32 bits or 64 bits) > + * READ_ONCE() and ASSIGN_ONCE() will fall back to memcpy and print a > + * compile-time warning. > + * > + * Their two major use cases are: (1) Mediating communication between > + * process-level code and irq/NMI handlers, all running on the same CPU, > + * and (2) Ensuring that the compiler does not fold, spindle, or otherwise > + * mutilate accesses that either do not require ordering or that interact > + * with an explicit memory barrier or atomic instruction that provides the > + * required ordering. > + */ > + > +#define READ_ONCE(x) \ > + ({ typeof(x) __val; __read_once_size(&x, &__val, sizeof(__val)); __val; }) > + > +#define ASSIGN_ONCE(val, x) \ > + ({ typeof(x) __val; __val = val; __assign_once_size(&x, &__val, sizeof(__val)); __val; }) > + > #endif /* __KERNEL__ */ > > #endif /* __ASSEMBLY__ */ >