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Herne" Organization: IBM Message-ID: Date: Tue, 25 May 2021 10:07:46 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.0 MIME-Version: 1.0 In-Reply-To: <20210525132628.GX1002214@nvidia.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: qW0Pzx8Nds36aUmGn6v21o3V3TzH1g97 X-Proofpoint-GUID: WDnYRfk3GmayeNPctu6VH5AWgotXd-Z6 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.391,18.0.761 definitions=2021-05-25_07:2021-05-25,2021-05-25 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 bulkscore=0 suspectscore=0 adultscore=0 spamscore=0 malwarescore=0 phishscore=0 impostorscore=0 mlxlogscore=999 clxscore=1015 mlxscore=0 lowpriorityscore=0 priorityscore=1501 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2104190000 definitions=main-2105250088 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 5/25/21 9:26 AM, Jason Gunthorpe wrote: > On Tue, May 25, 2021 at 09:24:59AM -0400, Jason J. Herne wrote: >> change the function pointer to point to vfio_ap_ops:handle_pqap(). When we >> unload the module we change the function pointer back to the stub. The >> updates should be atomic operations so no lock needed, right? > > No > > Jason > Okay... Would you be willing to elaborate, please? A counter argument, or a simple explanation would be appreciated. A simple "no" does not really do much to advance the discussion :). I'm fairly sure that a 64-bit pointer would be updated atomically. A reader of this value is either going to see value A or value B, not the high half of A and the low half of B. Maybe we also need a memory barrier to prevent stale values from being seen on another core? -- -- Jason J. Herne (jjherne@linux.ibm.com)