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Mon, 17 Sep 2018 19:06:55 +0000 Received: from userv0122.oracle.com (userv0122.oracle.com [156.151.31.75]) by aserv0021.oracle.com (8.14.4/8.14.4) with ESMTP id w8HJ6smA004126 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 17 Sep 2018 19:06:54 GMT Received: from abhmp0004.oracle.com (abhmp0004.oracle.com [141.146.116.10]) by userv0122.oracle.com (8.14.4/8.14.4) with ESMTP id w8HJ6pcT008186; Mon, 17 Sep 2018 19:06:51 GMT Received: from dhcp-burlington7-2nd-B-east-10-152-55-162.usdhcp.oraclecorp.com (/10.152.32.65) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Mon, 17 Sep 2018 12:06:51 -0700 Subject: Re: [PATCH 1/6] xenbus: prepare data structures and parameter for xenwatch multithreading To: Dongli Zhang , xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org Cc: jgross@suse.com, paul.durrant@citrix.com, wei.liu2@citrix.com, konrad.wilk@oracle.com, roger.pau@citrix.com, srinivas.eeda@oracle.com References: <1536910456-13337-1-git-send-email-dongli.zhang@oracle.com> <1536910456-13337-2-git-send-email-dongli.zhang@oracle.com> <04d7dee9-3011-512a-09b0-0e8dcbdd99d6@oracle.com> <797673a8-fa7e-0bfc-332e-4e0190c8d1ed@oracle.com> From: Boris Ostrovsky Openpgp: preference=signencrypt Autocrypt: addr=boris.ostrovsky@oracle.com; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.0 MIME-Version: 1.0 In-Reply-To: <797673a8-fa7e-0bfc-332e-4e0190c8d1ed@oracle.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Language: en-US X-Proofpoint-Virus-Version: vendor=nai engine=5900 definitions=9019 signatures=668708 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=0 malwarescore=0 phishscore=0 bulkscore=0 spamscore=0 mlxscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1807170000 definitions=main-1809170188 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 9/16/18 9:20 PM, Dongli Zhang wrote: > Hi Boris, > > On 09/17/2018 04:17 AM, Boris Ostrovsky wrote: >> >> On 9/14/18 3:34 AM, Dongli Zhang wrote: >> >>> + >>> +struct mtwatch_info { >>> + /* >>> + * The mtwatch_domain is put on both a hash table and a list. >>> + * domain_list is used to optimize xenbus_watch un-registration. >>> + * >>> + * The mtwatch_domain is removed from domain_hash (with state set >>> + * to MTWATCH_DOMAIN_DOWN) when its refcnt is zero. However, it is >>> + * left on domain_list until all events belong to such >>> + * mtwatch_domain are processed in mtwatch_thread(). >> >> Do we really need to keep mwatch_domain on both lists? Why is keeping it on, >> say, only the hash not sufficient? > In the state of the art xenbus, when a watch is unregistered (e.g., > unregister_xenbus_watch()), we need to traverse the list 'watch_events' to > remove all inflight/pending events (for such watch) from 'watch_events'. > > About this patch set, as each domain would have its own event list, we need to > traverse the list of each domain to remove the pending events for the watch to > be unregistered. > > E.g., > unregister_xenbus_watch()-->unregister_mtwatch()-->unregister_all_mtwatch() in > [PATCH 2/6] xenbus: implement the xenwatch multithreading framework. > > To traverse a hash table is not as efficient as traversing a list. That's why a > domain is kept on both the hash table and list. Keeping the same object on two lists also has costs. More importantly IMO, it increases chances on introducing a bug  when people update one instance but not the other. > >>> + * >>> + * While there may exist two mtwatch_domain with the same domid on >>> + * domain_list simultaneously, >> >> How is it possible to have two domids on the list at the same time? Don't you >> want to remove it (which IIUIC means moving it to the purge list) when domain is >> destroyed? > Here is one case (suppose the domid/frontend-id is 9): > > 1. Suppose the last pv driver device is removed from domid=9, and therefore the > reference count of per-domU xenwatch thread for domid=9 (which we call as old > thread below) should be removed. We remove it from hash table (it is left in the > list). > > Here we cannot remove the domain from the list immediately because there might > be pending events being processed by the corresponding per-domU xenwatch thread. > If we remove it from the list while there is related watch being unregistered as > mentioned for last question, we may hit page fault when processing watch event. Don't you need to grab domain->domain_mutex to remove the driver? Meaning that events for that mtwatch thread cannot be processed? In any case, I think that having two mtwatch_domains for the same domain should be avoided. (and if you keep the hash list only then this issue gets resolved automatically ;-)) -boris > > 2. Now the administrator attaches new pv device to domid=9 immediately and > therefore reference count is initially set to 1. The per-domU xenwatch thread > for domid=9 (called new thread) is created again. It is inserted to both the > hash table and list. > > 3. As the old thread for domid=9 might still be on the list, we would have two > threads for domid=9 (one old one to be removed and one newly inserted one to be > used by new pv devices). > > Dongli Zhang > >> >> -boris >> >> >>> + * all mtwatch_domain on hash_hash >>> + * should have unique domid. >>> + */ >>> + spinlock_t domain_lock; >>> + struct hlist_head domain_hash[MTWATCH_HASH_SIZE]; >>> + struct list_head domain_list; >>> + >>> + /* >>> + * When a per-domU kthread is going to be destroyed, it is put >>> + * on the purge_list, and will be flushed by purge_work later. >>> + */ >>> + struct work_struct purge_work; >>> + spinlock_t purge_lock; >>> + struct list_head purge_list; >>> +}; >>