{"id":3958,"date":"2014-10-26T19:20:45","date_gmt":"2014-10-26T18:20:45","guid":{"rendered":"https:\/\/www.dbi-services.com\/blog\/oracle-an-unexpected-lock-behaviour-with-rollback\/"},"modified":"2024-03-13T09:59:28","modified_gmt":"2024-03-13T08:59:28","slug":"oracle-an-unexpected-lock-behaviour-with-rollback","status":"publish","type":"post","link":"https:\/\/www.dbi-services.com\/blog\/oracle-an-unexpected-lock-behaviour-with-rollback\/","title":{"rendered":"Oracle: an unexpected lock behavior with rollback"},"content":{"rendered":"

By Franck Pachot<\/h2>\n

.
\nHere is an odd Oracle behavior I observed when a transaction that acquired a lock is rolled back. Note that this is related to a specific combination of locks that should not be encountered in production. So it’s not a bug. Just something unexpected.<\/p>\n

 <\/p>\n

First session<\/h3>\n

In my first session I lock the DEPT table in share mode (RS)
\n20:56:56 SQL1> lock table dept in row share mode;
\nTable(s) Locked.
\n<\/code>
\nMy first session (SID=53) has acquired a TM lock in mode 2 (row share):
\n20:56:56 SQL1> select object_name,session_id,LOCKED_MODE from v$locked_object join dba_objects using(object_id);
\n 
\nOBJECT_NAME SESSION_ID LOCKED_MODE
\n----------- ---------- -----------
\nDEPT 53 2
\n<\/code>
\n <\/p>\n

Second session<\/h3>\n

In my second session I lock the DEPT table in share + row exclusive mode (SRX). This is compatible with the RS.
\n20:56:59 SQL2> lock table dept in share row exclusive mode;
\nTable(s) Locked.
\n<\/code>
\nMy second session (SID=59) has acquired a TM lock in mode 5 (share + row exclusive):
\n20:56:59 SQL2> select object_name,session_id,LOCKED_MODE from v$locked_object join dba_objects using(object_id);
\n 
\nOBJECT_NAME SESSION_ID LOCKED_MODE
\n----------- ---------- -----------
\nDEPT 59 5
\nDEPT 53 2
\n<\/code>
\nThen I rollback my transaction
\n20:56:59 SQL2> rollback;
\n<\/code>
\nMy lock was released and I’ve only the one from Session 1 (SID=53):
\nOBJECT_NAME SESSION_ID LOCKED_MODE
\n----------- ---------- -----------
\nDEPT 53 2
\n<\/code>
\nNow comes the funny part. I run exactly the same SSX lock, which was immediately acquired before:
\n21:14:30 SQL2> lock table dept in share row exclusive mode wait 5 ;
\n<\/code>
\nBut now it hangs. Let’s see the blocking tree with utllockt.sql:
\nWAITING_SESSION LOCK_TYPE MODE_REQUESTED MODE_HELD LOCK_ID1 LOCK_ID2
\n----------------- ----------------- -------------- -------------- ----------------- -----------------
\n53 None
\n 59 DML Exclusive Row-S (SS) 94228 0
\n<\/code>
\nWhat? My session wants to acquire an Exclusive lock? I’ve never requested that.
\nAnd finally it fails because my Session 1 has a RS lock which prevents exclusive locks.
\nlock table dept in share row exclusive mode wait 5
\n*
\nERROR at line 1:
\nORA-00054: resource busy and acquire with NOWAIT specified or timeout expired
\n<\/code>
\n <\/p>\n

Third session<\/h3>\n

But there is worse. I disconnect my second session and try to do the same from a third one. And I even try only a RS lock:
\n21:15:20 SQL3> lock table dept in row share mode wait 5 ;
\n<\/code>
\nAnd I’m blocked again. It seems that because a session had acquired a SRX lock and has rolled back, while another session held a RS one, then any new transaction that wants to acquire any lock must acquire temporarily an exclusive one before.<\/p>\n

 <\/p>\n

Event 10704 “Print out information about what enqueues are being obtained”<\/h3>\n

In order to get further I traced the locks with event 10704 (see how in a previous post<\/a>).<\/p>\n

Here is the trace when it failed, filtering on ‘00017014’ which is the object_id of DEPT in hexadecimal:
\nksqgtl *** TM-00017014-00000000-00000000-00000000 mode=2 flags=0x400 timeout=5 ***
\nksqrcl: TM-00017014-00000000-00000000-00000000
\nksqgtl *** TM-00017014-00000000-00000000-00000000 mode=6<\/strong> flags=0x400 timeout=5 ***
\nksqcmi: TM-00017014-00000000-00000000-00000000 mode=6 timeout=5
\nksqcmi: returns 51
\nksqgtl: RETURNS 51<\/strong>
\nksqrcl: returns 0
\n<\/code>
\nMy ‘lock table dept in row share mode’ acquires a mode 2 (which is the ‘row share’ mode) but then released it and tried to acquire a mode 6 (which is exclusive)<\/p>\n

And then here is a trace when it was successful, after the transaction in Session 1 has been committed:
\nksqgtl *** TM-00017014-00000000-00000000-00000000 mode=2 flags=0x400 timeout=5 ***
\nksqrcl: TM-00017014-00000000-00000000-00000000
\nksqgtl *** TM-00017014-00000000-00000000-00000000 mode=6<\/strong> flags=0x400 timeout=5 ***
\nksqcmi: TM-00017014-00000000-00000000-00000000 mode=6 timeout=5
\nksqcmi: returns 0
\nksqgtl: RETURNS 0<\/strong>
\nksqgtl *** TM-00017014-00000001-00000000-00000000 mode=3 flags=0x400 timeout=5 ***
\nksqgtl: RETURNS 0
\nksqcnv: TM-00017014-00000000-00000000-00000000 mode=2<\/strong> timeout=5
\nksqcmi: TM-00017014-00000000-00000000-00000000 mode=2 timeout=5
\nksqcmi: returns 0
\nksqrcl: TM-00017014-00000001-00000000-00000000
\nksqrcl: returns 0
\n<\/code>
\nSo it it did the same, but that time the mode 6 can be acquired. Then we see a conversion to mode 2 which is the RS we wanted.<\/p>\n

Finally I tried with all other combinations of locks, but it seems that only that one (RS then rolled back SRX) show that behavior. I tried also with DML instead of ‘lock table’ statement but the Share lock acquired by DML (the non-indexed foreign key case) is released immediately so I cannot rollback it. And a failure in the statement do not trigger the same behaviour.<\/p>\n

Final note: the only reference I’ve find for that behavior is this<\/a> post on oracle-l<\/p>\n","protected":false},"excerpt":{"rendered":"

By Franck Pachot . Here is an odd Oracle behavior I observed when a transaction that acquired a lock is rolled back. Note that this is related to a specific combination of locks that should not be encountered in production. So it’s not a bug. Just something unexpected.   First session In my first session […]<\/p>\n","protected":false},"author":27,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[229,59],"tags":[481,96],"type_dbi":[],"class_list":["post-3958","post","type-post","status-publish","format-standard","hentry","category-database-administration-monitoring","category-oracle","tag-lock","tag-oracle"],"acf":[],"yoast_head":"\nOracle: an unexpected lock behavior with rollback - dbi Blog<\/title>\n<meta name=\"description\" content=\"A weird Oracle behavior when acquired lock is rolled back.\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.dbi-services.com\/blog\/oracle-an-unexpected-lock-behaviour-with-rollback\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Oracle: an unexpected lock behavior with rollback\" \/>\n<meta property=\"og:description\" content=\"A weird Oracle behavior when acquired lock is rolled back.\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.dbi-services.com\/blog\/oracle-an-unexpected-lock-behaviour-with-rollback\/\" \/>\n<meta property=\"og:site_name\" content=\"dbi Blog\" \/>\n<meta property=\"article:published_time\" content=\"2014-10-26T18:20:45+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2024-03-13T08:59:28+00:00\" \/>\n<meta name=\"author\" content=\"Oracle Team\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Oracle Team\" \/>\n\t<meta name=\"twitter:label2\" content=\"Est. reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"3 minutes\" \/>\n<script type=\"application\/ld+json\" class=\"yoast-schema-graph\">{\"@context\":\"https:\/\/schema.org\",\"@graph\":[{\"@type\":\"Article\",\"@id\":\"https:\/\/www.dbi-services.com\/blog\/oracle-an-unexpected-lock-behaviour-with-rollback\/#article\",\"isPartOf\":{\"@id\":\"https:\/\/www.dbi-services.com\/blog\/oracle-an-unexpected-lock-behaviour-with-rollback\/\"},\"author\":{\"name\":\"Oracle Team\",\"@id\":\"https:\/\/www.dbi-services.com\/blog\/#\/schema\/person\/66ab87129f2d357f09971bc7936a77ee\"},\"headline\":\"Oracle: an unexpected lock behavior with rollback\",\"datePublished\":\"2014-10-26T18:20:45+00:00\",\"dateModified\":\"2024-03-13T08:59:28+00:00\",\"mainEntityOfPage\":{\"@id\":\"https:\/\/www.dbi-services.com\/blog\/oracle-an-unexpected-lock-behaviour-with-rollback\/\"},\"wordCount\":476,\"commentCount\":2,\"keywords\":[\"lock\",\"Oracle\"],\"articleSection\":[\"Database Administration & Monitoring\",\"Oracle\"],\"inLanguage\":\"en-US\",\"potentialAction\":[{\"@type\":\"CommentAction\",\"name\":\"Comment\",\"target\":[\"https:\/\/www.dbi-services.com\/blog\/oracle-an-unexpected-lock-behaviour-with-rollback\/#respond\"]}]},{\"@type\":\"WebPage\",\"@id\":\"https:\/\/www.dbi-services.com\/blog\/oracle-an-unexpected-lock-behaviour-with-rollback\/\",\"url\":\"https:\/\/www.dbi-services.com\/blog\/oracle-an-unexpected-lock-behaviour-with-rollback\/\",\"name\":\"Oracle: an unexpected lock behavior with rollback - 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