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McKenney" To: "H. Peter Anvin" Cc: Alexander Potapenko , Peter Zijlstra , Ingo Molnar , LKML , Dmitriy Vyukov , James Y Knight Subject: Re: Potentially missing "memory" clobbers in bitops.h for x86 Reply-To: paulmck@linux.ibm.com References: <20190328162222.GO4102@linux.ibm.com> <8e32ab34-c14c-1ccb-76f9-0dcd729a0ef6@zytor.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <8e32ab34-c14c-1ccb-76f9-0dcd729a0ef6@zytor.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19032921-0040-0000-0000-000004D9423C X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010836; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000283; SDB=6.01181524; UDB=6.00606765; IPR=6.00962244; MB=3.00026214; MTD=3.00000008; XFM=3.00000015; UTC=2019-03-29 21:09:19 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19032921-0041-0000-0000-000008E465D2 Message-Id: <20190329210918.GZ4102@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-03-29_12:,, 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-1903290145 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Mar 29, 2019 at 01:52:33PM -0700, H. Peter Anvin wrote: > On 3/29/19 8:54 AM, Alexander Potapenko wrote: > > > >> Of course, this would force the compiler to actually compute the > >> offset, which would slow things down. I have no idea whether this > >> would be better or worse than just using the "memory" clobber. > > Just adding the "memory" clobber to clear_bit() changes sizes of 5 > > kernel functions (the three mentioned above, plus hub_activate() and > > native_send_call_func_ipi()) by a small margin. > > This probably means the performance impact of this clobber is > > negligible in this case. > > I would agree with that. > > Could you perhaps verify whether or not any of the above functions > contains a currently manifest bug? > > Note: the atomic versions of these functions obviously need to have > "volatile" and the clobber anyway, as they are by definition barriers > and moving memory operations around them would be a very serious error. The atomic functions that return void don't need to order anything except the input and output arguments. The oddness with clear_bit() is that the memory changed isn't necessarily the quantity referenced by the argument, if the number of bits specified is large. So (for example) atomic_inc() does not need a "memory" clobber, right? Thanx, Paul