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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Mon, 27 May 2019 17:10:47 +0100 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23033.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x4RGAkLo14549502 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 27 May 2019 16:10:46 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 95E73B205F; Mon, 27 May 2019 16:10:46 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 6086FB2065; Mon, 27 May 2019 16:10:46 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.80.199.73]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Mon, 27 May 2019 16:10:46 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 5BDFA16C37C6; Mon, 27 May 2019 09:10:50 -0700 (PDT) Date: Mon, 27 May 2019 09:10:50 -0700 From: "Paul E. McKenney" To: Andrea Parri Cc: linux-kernel@vger.kernel.org, Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Joel Fernandes , rcu@vger.kernel.org, Peter Zijlstra , Will Deacon , Mark Rutland , Matthew Wilcox , Sasha Levin , apw@canonical.com, joe@perches.com Subject: Re: [PATCH v2] rcu: Don't return a value from rcu_assign_pointer() Reply-To: paulmck@linux.ibm.com References: <1558946997-25559-1-git-send-email-andrea.parri@amarulasolutions.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1558946997-25559-1-git-send-email-andrea.parri@amarulasolutions.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19052716-0040-0000-0000-000004F50320 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00011172; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000286; SDB=6.01209379; UDB=6.00635308; IPR=6.00990405; MB=3.00027075; MTD=3.00000008; XFM=3.00000015; UTC=2019-05-27 16:10:51 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19052716-0041-0000-0000-000009011B24 Message-Id: <20190527161050.GK28207@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-05-27_08:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1905270113 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, May 27, 2019 at 10:49:57AM +0200, Andrea Parri wrote: > Quoting Paul [1]: > > "Given that a quick (and perhaps error-prone) search of the uses > of rcu_assign_pointer() in v5.1 didn't find a single use of the > return value, let's please instead change the documentation and > implementation to eliminate the return value." > > [1] https://lkml.kernel.org/r/20190523135013.GL28207@linux.ibm.com Queued, thank you! Adding the checkpatch maintainers on CC as well. The "do { } while (0)" prevents the return value from being used, by design. Given the checkpatch complaint, is there some better way to achieve this? Thanx, Paul > Signed-off-by: Andrea Parri > Cc: "Paul E. McKenney" > Cc: Josh Triplett > Cc: Steven Rostedt > Cc: Mathieu Desnoyers > Cc: Lai Jiangshan > Cc: Joel Fernandes > Cc: rcu@vger.kernel.org > Cc: Peter Zijlstra > Cc: Will Deacon > Cc: Mark Rutland > Cc: Matthew Wilcox > Cc: Sasha Levin > --- > Changes in v2: > - Fix documentation and style (Paul E. McKenney) > - Improve subject line (Mark Rutland) > --- > Documentation/RCU/whatisRCU.txt | 8 ++++---- > include/linux/rcupdate.h | 5 ++--- > tools/include/linux/rcu.h | 4 ++-- > tools/testing/radix-tree/linux/rcupdate.h | 2 +- > 4 files changed, 9 insertions(+), 10 deletions(-) > > diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt > index 981651a8b65d2..7e1a8721637ab 100644 > --- a/Documentation/RCU/whatisRCU.txt > +++ b/Documentation/RCU/whatisRCU.txt > @@ -212,7 +212,7 @@ synchronize_rcu() > > rcu_assign_pointer() > > - typeof(p) rcu_assign_pointer(p, typeof(p) v); > + void rcu_assign_pointer(p, typeof(p) v); > > Yes, rcu_assign_pointer() -is- implemented as a macro, though it > would be cool to be able to declare a function in this manner. > @@ -220,9 +220,9 @@ rcu_assign_pointer() > > The updater uses this function to assign a new value to an > RCU-protected pointer, in order to safely communicate the change > - in value from the updater to the reader. This function returns > - the new value, and also executes any memory-barrier instructions > - required for a given CPU architecture. > + in value from the updater to the reader. This macro does not > + evaluate to an rvalue, but it does execute any memory-barrier > + instructions required for a given CPU architecture. > > Perhaps just as important, it serves to document (1) which > pointers are protected by RCU and (2) the point at which a > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > index a8ed624da5556..0c9b92799abc7 100644 > --- a/include/linux/rcupdate.h > +++ b/include/linux/rcupdate.h > @@ -367,7 +367,7 @@ static inline void rcu_preempt_sleep_check(void) { } > * other macros that it invokes. > */ > #define rcu_assign_pointer(p, v) \ > -({ \ > +do { \ > uintptr_t _r_a_p__v = (uintptr_t)(v); \ > rcu_check_sparse(p, __rcu); \ > \ > @@ -375,8 +375,7 @@ static inline void rcu_preempt_sleep_check(void) { } > WRITE_ONCE((p), (typeof(p))(_r_a_p__v)); \ > else \ > smp_store_release(&p, RCU_INITIALIZER((typeof(p))_r_a_p__v)); \ > - _r_a_p__v; \ > -}) > +} while (0) > > /** > * rcu_swap_protected() - swap an RCU and a regular pointer > diff --git a/tools/include/linux/rcu.h b/tools/include/linux/rcu.h > index 7d02527e5bcea..9554d3fa54f33 100644 > --- a/tools/include/linux/rcu.h > +++ b/tools/include/linux/rcu.h > @@ -19,7 +19,7 @@ static inline bool rcu_is_watching(void) > return false; > } > > -#define rcu_assign_pointer(p, v) ((p) = (v)) > -#define RCU_INIT_POINTER(p, v) p=(v) > +#define rcu_assign_pointer(p, v) do { (p) = (v); } while (0) > +#define RCU_INIT_POINTER(p, v) do { (p) = (v); } while (0) > > #endif > diff --git a/tools/testing/radix-tree/linux/rcupdate.h b/tools/testing/radix-tree/linux/rcupdate.h > index fd280b070fdb1..fed468fb0c78d 100644 > --- a/tools/testing/radix-tree/linux/rcupdate.h > +++ b/tools/testing/radix-tree/linux/rcupdate.h > @@ -7,6 +7,6 @@ > #define rcu_dereference_raw(p) rcu_dereference(p) > #define rcu_dereference_protected(p, cond) rcu_dereference(p) > #define rcu_dereference_check(p, cond) rcu_dereference(p) > -#define RCU_INIT_POINTER(p, v) (p) = (v) > +#define RCU_INIT_POINTER(p, v) do { (p) = (v); } while (0) > > #endif > -- > 2.7.4 >