From 1a917ae8610d44985fd2027da0cfe60ccece9104 Mon Sep 17 00:00:00 2001 From: Alvaro Herrera Date: Thu, 24 Apr 2014 15:41:55 -0300 Subject: [PATCH] Fix race when updating a tuple concurrently locked by another process If a tuple is locked, and this lock is later upgraded either to an update or to a stronger lock, and in the meantime some other process tries to lock, update or delete the same tuple, it (the tuple) could end up being updated twice, or having conflicting locks held. The reason for this is that the second updater checks for a change in Xmax value, or in the HEAP_XMAX_IS_MULTI infomask bit, after noticing the first lock; and if there's a change, it restarts and re-evaluates its ability to update the tuple. But it neglected to check for changes in lock strength or in lock-vs-update status when those two properties stayed the same. This would lead it to take the wrong decision and continue with its own update, when in reality it shouldn't do so but instead restart from the top. This could lead to either an assertion failure much later (when a multixact containing multiple updates is detected), or duplicate copies of tuples. To fix, make sure to compare the other relevant infomask bits alongside the Xmax value and HEAP_XMAX_IS_MULTI bit, and restart from the top if necessary. Also, in the belt-and-suspenders spirit, add a check to MultiXactCreateFromMembers that a multixact being created does not have two or more members that are claimed to be updates. This should protect against other bugs that might cause similar bogus situations. Backpatch to 9.3, where the possibility of multixacts containing updates was introduced. (In prior versions it was possible to have the tuple lock upgraded from shared to exclusive, and an update would not restart from the top; yet we're protected against a bug there because there's always a sleep to wait for the locking transaction to complete before continuing to do anything. Really, the fact that tuple locks always conflicted with concurrent updates is what protected against bugs here.) Per report from Andrew Dunstan and Josh Berkus in thread at http://www.postgresql.org/message-id/534C8B33.9050807@pgexperts.com Bug analysis by Andres Freund. --- src/backend/access/heap/heapam.c | 42 ++++++++++++++++++-------- src/backend/access/transam/multixact.c | 18 ++++++++++- src/include/access/multixact.h | 4 +++ 3 files changed, 51 insertions(+), 13 deletions(-) diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c index 7217e96d6a..a04763207f 100644 --- a/src/backend/access/heap/heapam.c +++ b/src/backend/access/heap/heapam.c @@ -190,9 +190,6 @@ static const int MultiXactStatusLock[MaxMultiXactStatus + 1] = /* Get the LockTupleMode for a given MultiXactStatus */ #define TUPLOCK_from_mxstatus(status) \ (MultiXactStatusLock[(status)]) -/* Get the is_update bit for a given MultiXactStatus */ -#define ISUPDATE_from_mxstatus(status) \ - ((status) > MultiXactStatusForUpdate) /* ---------------------------------------------------------------- * heap support routines @@ -2588,6 +2585,27 @@ compute_infobits(uint16 infomask, uint16 infomask2) XLHL_KEYS_UPDATED : 0); } +/* + * Given two versions of the same t_infomask for a tuple, compare them and + * return whether the relevant status for a tuple Xmax has changed. This is + * used after a buffer lock has been released and reacquired: we want to ensure + * that the tuple state continues to be the same it was when we previously + * examined it. + * + * Note the Xmax field itself must be compared separately. + */ +static inline bool +xmax_infomask_changed(uint16 new_infomask, uint16 old_infomask) +{ + const uint16 interesting = + HEAP_XMAX_IS_MULTI | HEAP_XMAX_LOCK_ONLY | HEAP_LOCK_MASK; + + if ((new_infomask & interesting) != (old_infomask & interesting)) + return true; + + return false; +} + /* * heap_delete - delete a tuple * @@ -2725,7 +2743,7 @@ l1: * update this tuple before we get to this point. Check for xmax * change, and start over if so. */ - if (!(tp.t_data->t_infomask & HEAP_XMAX_IS_MULTI) || + if (xmax_infomask_changed(tp.t_data->t_infomask, infomask) || !TransactionIdEquals(HeapTupleHeaderGetRawXmax(tp.t_data), xwait)) goto l1; @@ -2751,7 +2769,7 @@ l1: * other xact could update this tuple before we get to this point. * Check for xmax change, and start over if so. */ - if ((tp.t_data->t_infomask & HEAP_XMAX_IS_MULTI) || + if (xmax_infomask_changed(tp.t_data->t_infomask, infomask) || !TransactionIdEquals(HeapTupleHeaderGetRawXmax(tp.t_data), xwait)) goto l1; @@ -3278,7 +3296,7 @@ l2: * update this tuple before we get to this point. Check for xmax * change, and start over if so. */ - if (!(oldtup.t_data->t_infomask & HEAP_XMAX_IS_MULTI) || + if (xmax_infomask_changed(oldtup.t_data->t_infomask, infomask) || !TransactionIdEquals(HeapTupleHeaderGetRawXmax(oldtup.t_data), xwait)) goto l2; @@ -3332,7 +3350,7 @@ l2: * recheck the locker; if someone else changed the tuple while * we weren't looking, start over. */ - if ((oldtup.t_data->t_infomask & HEAP_XMAX_IS_MULTI) || + if (xmax_infomask_changed(oldtup.t_data->t_infomask, infomask) || !TransactionIdEquals( HeapTupleHeaderGetRawXmax(oldtup.t_data), xwait)) @@ -3353,7 +3371,7 @@ l2: * some other xact could update this tuple before we get to * this point. Check for xmax change, and start over if so. */ - if ((oldtup.t_data->t_infomask & HEAP_XMAX_IS_MULTI) || + if (xmax_infomask_changed(oldtup.t_data->t_infomask, infomask) || !TransactionIdEquals( HeapTupleHeaderGetRawXmax(oldtup.t_data), xwait)) @@ -4357,7 +4375,7 @@ l3: * over. */ LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE); - if (!(tuple->t_data->t_infomask & HEAP_XMAX_IS_MULTI) || + if (xmax_infomask_changed(tuple->t_data->t_infomask, infomask) || !TransactionIdEquals(HeapTupleHeaderGetRawXmax(tuple->t_data), xwait)) { @@ -4376,7 +4394,7 @@ l3: LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE); /* if the xmax changed in the meantime, start over */ - if ((tuple->t_data->t_infomask & HEAP_XMAX_IS_MULTI) || + if (xmax_infomask_changed(tuple->t_data->t_infomask, infomask) || !TransactionIdEquals( HeapTupleHeaderGetRawXmax(tuple->t_data), xwait)) @@ -4444,7 +4462,7 @@ l3: * could update this tuple before we get to this point. Check * for xmax change, and start over if so. */ - if (!(tuple->t_data->t_infomask & HEAP_XMAX_IS_MULTI) || + if (xmax_infomask_changed(tuple->t_data->t_infomask, infomask) || !TransactionIdEquals( HeapTupleHeaderGetRawXmax(tuple->t_data), xwait)) @@ -4500,7 +4518,7 @@ l3: * some other xact could update this tuple before we get to * this point. Check for xmax change, and start over if so. */ - if ((tuple->t_data->t_infomask & HEAP_XMAX_IS_MULTI) || + if (xmax_infomask_changed(tuple->t_data->t_infomask, infomask) || !TransactionIdEquals( HeapTupleHeaderGetRawXmax(tuple->t_data), xwait)) diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c index d4ad6787a5..459f59cb4e 100644 --- a/src/backend/access/transam/multixact.c +++ b/src/backend/access/transam/multixact.c @@ -457,7 +457,7 @@ MultiXactIdExpand(MultiXactId multi, TransactionId xid, MultiXactStatus status) for (i = 0, j = 0; i < nmembers; i++) { if (TransactionIdIsInProgress(members[i].xid) || - ((members[i].status > MultiXactStatusForUpdate) && + (ISUPDATE_from_mxstatus(members[i].status) && TransactionIdDidCommit(members[i].xid))) { newMembers[j].xid = members[i].xid; @@ -713,6 +713,22 @@ MultiXactIdCreateFromMembers(int nmembers, MultiXactMember *members) return multi; } + /* Verify that there is a single update Xid among the given members. */ + { + int i; + bool has_update = false; + + for (i = 0; i < nmembers; i++) + { + if (ISUPDATE_from_mxstatus(members[i].status)) + { + if (has_update) + elog(ERROR, "new multixact has more than one updating member"); + has_update = true; + } + } + } + /* * Assign the MXID and offsets range to use, and make sure there is space * in the OFFSETs and MEMBERs files. NB: this routine does diff --git a/src/include/access/multixact.h b/src/include/access/multixact.h index 9729f27508..1f048e8ed5 100644 --- a/src/include/access/multixact.h +++ b/src/include/access/multixact.h @@ -48,6 +48,10 @@ typedef enum #define MaxMultiXactStatus MultiXactStatusUpdate +/* does a status value correspond to a tuple update? */ +#define ISUPDATE_from_mxstatus(status) \ + ((status) > MultiXactStatusForUpdate) + typedef struct MultiXactMember {