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Tue, 6 Jul 2021 22:43:40 +0000 (GMT) Subject: Re: [PATCH] s390/vfio-ap: do not use open locks during VFIO_GROUP_NOTIFY_SET_KVM notification To: Jason Gunthorpe Cc: Halil Pasic , linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org, pasic@linux.vnet.ibm.com, jjherne@linux.ibm.com References: <20210625220758.80365-1-akrowiak@linux.ibm.com> <20210628222923.21a257c8.pasic@linux.ibm.com> <25edecce-0795-3b00-a155-bfcc8499f1be@linux.ibm.com> <20210701003941.685c524c.pasic@linux.ibm.com> <8b742188-d8a2-cf4e-e9de-0ca6f3d829b3@linux.ibm.com> <20210705141348.GN4459@nvidia.com> <11025fe5-0751-b7ab-9250-b21c10382edd@linux.ibm.com> <20210706134939.GX4459@nvidia.com> From: Tony Krowiak Message-ID: Date: Tue, 6 Jul 2021 18:43:39 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: <20210706134939.GX4459@nvidia.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: 3Vm9dF-9DIevD65to1lZ8T0qNlDddDuV X-Proofpoint-GUID: 3Vm9dF-9DIevD65to1lZ8T0qNlDddDuV X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.391,18.0.790 definitions=2021-07-06_13:2021-07-06,2021-07-06 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 suspectscore=0 lowpriorityscore=0 spamscore=0 bulkscore=0 mlxlogscore=999 malwarescore=0 phishscore=0 priorityscore=1501 clxscore=1015 adultscore=0 impostorscore=0 mlxscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2104190000 definitions=main-2107060106 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 7/6/21 9:49 AM, Jason Gunthorpe wrote: > On Tue, Jul 06, 2021 at 09:39:29AM -0400, Tony Krowiak wrote: >> >> On 7/5/21 10:13 AM, Jason Gunthorpe wrote: >>> On Thu, Jul 01, 2021 at 10:28:52AM -0400, Tony Krowiak wrote: >>> >>>>> I think Jason was talking about open coding locks in general. >>>> That may be so, but his comments were in support of his >>>> statement that the  mutex + wait_queue did not resolve >>>> the issue reported vai the lockdep splat because it turned >>>> off lockdep. >>> Rgiht, if this used to be proper locks and lockdep complained then >>> whatever potential deadlock it found is not magically removed by going >>> to a wait_queue. It just removes the lockdep annotations that would >>> identify the issue early. >>> >>> This is why people should not open code locks, it completely defeats >>> lockdep. That alone is merit enough for this patch. >> When you use the phrase "open code locks", to what are you >> specifically referring? I am confused by the use of the phrase >> "open code" in this context because open coding, at least as >> I understand it, has to do with data analysis. > "open code" here means you write the algorithm of a standard lock in > your own functions instead of calling the standard library. > > Testing/setting the busy and sleeping on a wait_event is exactly a > standard lock. > > Ie if I write > > for (len = 0; str[len] != 0; len++) > ; > > Then I have open coded strlen() > > Jason Thanks for the explanation.