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Avoid spurious deadlocks when upgrading a tuple lock

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When two (or more) transactions are waiting for transaction T1 to release a
tuple-level lock, and transaction T1 upgrades its lock to a higher level, a
spurious deadlock can be reported among the waiting transactions when T1
finishes.  The simplest example case seems to be:

T1: select id from job where name = 'a' for key share;
Y: select id from job where name = 'a' for update; -- starts waiting for X
Z: select id from job where name = 'a' for key share;
T1: update job set name = 'b' where id = 1;
Z: update job set name = 'c' where id = 1; -- starts waiting for X
T1: rollback;

At this point, transaction Y is rolled back on account of a deadlock: Y
holds the heavyweight tuple lock and is waiting for the Xmax to be released,
while Z holds part of the multixact and tries to acquire the heavyweight
lock (per protocol) and goes to sleep; once X releases its part of the
multixact, Z is awakened only to be put back to sleep on the heavyweight
lock that Y is holding while sleeping.  Kaboom.

This can be avoided by having Z skip the heavyweight lock acquisition.  As
far as I can see, the biggest downside is that if there are multiple Z
transactions, the order in which they resume after X finishes is not
guaranteed.

Backpatch to 9.6.  The patch applies cleanly on 9.5, but the new tests don't
work there (because isolationtester is not smart enough), so I'm not going
to risk it.

Author: Oleksii Kliukin
Discussion: https://postgr.es/m/B9C9D7CD-EB94-4635-91B6-E558ACEC0EC3@hintbits.com
This commit is contained in:
Alvaro Herrera
2019-06-13 17:28:24 -04:00
parent 07accce500
commit 85600b7b5d
5 changed files with 281 additions and 21 deletions
Download Patch File Download Diff File Expand all files Collapse all files

10
src/backend/access/heap/README.tuplock
View File

@ -36,6 +36,16 @@ do LockTuple as well, if there is any conflict, to ensure that they don't
starve out waiting exclusive-lockers. However, if there is not any active
conflict for a tuple, we don't incur any extra overhead.
We make an exception to the above rule for those lockers that already hold
some lock on a tuple and attempt to acquire a stronger one on it. In that
case, we skip the LockTuple() call even when there are conflicts, provided
that the target tuple is being locked, updated or deleted by multiple sessions
concurrently. Failing to skip the lock would risk a deadlock, e.g., between a
session that was first to record its weaker lock in the tuple header and would
be waiting on the LockTuple() call to upgrade to the stronger lock level, and
another session that has already done LockTuple() and is waiting for the first
session transaction to release its tuple header-level lock.
We provide four levels of tuple locking strength: SELECT FOR UPDATE obtains an
exclusive lock which prevents any kind of modification of the tuple. This is
the lock level that is implicitly taken by DELETE operations, and also by

84
src/backend/access/heap/heapam.c
View File

@ -120,7 +120,7 @@ static void GetMultiXactIdHintBits(MultiXactId multi, uint16 *new_infomask,
static TransactionId MultiXactIdGetUpdateXid(TransactionId xmax,
uint16 t_infomask);
static bool DoesMultiXactIdConflict(MultiXactId multi, uint16 infomask,
LockTupleMode lockmode);
LockTupleMode lockmode, bool *current_is_member);
static void MultiXactIdWait(MultiXactId multi, MultiXactStatus status, uint16 infomask,
Relation rel, ItemPointer ctid, XLTW_Oper oper,
int *remaining);
@ -3161,15 +3161,20 @@ l1:
*/
if (infomask & HEAP_XMAX_IS_MULTI)
{
/* wait for multixact */
bool current_is_member = false;
if (DoesMultiXactIdConflict((MultiXactId) xwait, infomask,
LockTupleExclusive))
LockTupleExclusive, &current_is_member))
{
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
/* acquire tuple lock, if necessary */
heap_acquire_tuplock(relation, &(tp.t_self), LockTupleExclusive,
LockWaitBlock, &have_tuple_lock);
/*
* Acquire the lock, if necessary (but skip it when we're
* requesting a lock and already have one; avoids deadlock).
*/
if (!current_is_member)
heap_acquire_tuplock(relation, &(tp.t_self), LockTupleExclusive,
LockWaitBlock, &have_tuple_lock);
/* wait for multixact */
MultiXactIdWait((MultiXactId) xwait, MultiXactStatusUpdate, infomask,
@ -3768,15 +3773,20 @@ l2:
{
TransactionId update_xact;
int remain;
bool current_is_member = false;
if (DoesMultiXactIdConflict((MultiXactId) xwait, infomask,
*lockmode))
*lockmode, &current_is_member))
{
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
/* acquire tuple lock, if necessary */
heap_acquire_tuplock(relation, &(oldtup.t_self), *lockmode,
LockWaitBlock, &have_tuple_lock);
/*
* Acquire the lock, if necessary (but skip it when we're
* requesting a lock and already have one; avoids deadlock).
*/
if (!current_is_member)
heap_acquire_tuplock(relation, &(oldtup.t_self), *lockmode,
LockWaitBlock, &have_tuple_lock);
/* wait for multixact */
MultiXactIdWait((MultiXactId) xwait, mxact_status, infomask,
@ -4746,6 +4756,7 @@ l3:
uint16 infomask;
uint16 infomask2;
bool require_sleep;
bool skip_tuple_lock;
ItemPointerData t_ctid;
/* must copy state data before unlocking buffer */
@ -4771,6 +4782,7 @@ l3:
if (first_time)
{
first_time = false;
skip_tuple_lock = false;
if (infomask & HEAP_XMAX_IS_MULTI)
{
@ -4799,6 +4811,21 @@ l3:
result = HeapTupleMayBeUpdated;
goto out_unlocked;
}
else
{
/*
* Disable acquisition of the heavyweight tuple lock.
* Otherwise, when promoting a weaker lock, we might
* deadlock with another locker that has acquired the
* heavyweight tuple lock and is waiting for our
* transaction to finish.
*
* Note that in this case we still need to wait for
* the multixact if required, to avoid acquiring
* conflicting locks.
*/
skip_tuple_lock = true;
}
}
if (members)
@ -4953,7 +4980,7 @@ l3:
if (infomask & HEAP_XMAX_IS_MULTI)
{
if (!DoesMultiXactIdConflict((MultiXactId) xwait, infomask,
mode))
mode, NULL))
{
/*
* No conflict, but if the xmax changed under us in the
@ -5030,13 +5057,15 @@ l3:
/*
* Acquire tuple lock to establish our priority for the tuple, or
* die trying. LockTuple will release us when we are next-in-line
* for the tuple. We must do this even if we are share-locking.
* for the tuple. We must do this even if we are share-locking,
* but not if we already have a weaker lock on the tuple.
*
* If we are forced to "start over" below, we keep the tuple lock;
* this arranges that we stay at the head of the line while
* rechecking tuple state.
*/
if (!heap_acquire_tuplock(relation, tid, mode, wait_policy,
if (!skip_tuple_lock &&
!heap_acquire_tuplock(relation, tid, mode, wait_policy,
&have_tuple_lock))
{
/*
@ -7214,10 +7243,13 @@ HeapTupleGetUpdateXid(HeapTupleHeader tuple)
* tuple lock of the given strength?
*
* The passed infomask pairs up with the given multixact in the tuple header.
*
* If current_is_member is not NULL, it is set to 'true' if the current
* transaction is a member of the given multixact.
*/
static bool
DoesMultiXactIdConflict(MultiXactId multi, uint16 infomask,
LockTupleMode lockmode)
LockTupleMode lockmode, bool *current_is_member)
{
int nmembers;
MultiXactMember *members;
@ -7238,17 +7270,26 @@ DoesMultiXactIdConflict(MultiXactId multi, uint16 infomask,
TransactionId memxid;
LOCKMODE memlockmode;
if (result && (current_is_member == NULL || *current_is_member))
break;
memlockmode = LOCKMODE_from_mxstatus(members[i].status);
/* ignore members from current xact (but track their presence) */
memxid = members[i].xid;
if (TransactionIdIsCurrentTransactionId(memxid))
{
if (current_is_member != NULL)
*current_is_member = true;
continue;
}
else if (result)
continue;
/* ignore members that don't conflict with the lock we want */
if (!DoLockModesConflict(memlockmode, wanted))
continue;
/* ignore members from current xact */
memxid = members[i].xid;
if (TransactionIdIsCurrentTransactionId(memxid))
continue;
if (ISUPDATE_from_mxstatus(members[i].status))
{
/* ignore aborted updaters */
@ -7265,10 +7306,11 @@ DoesMultiXactIdConflict(MultiXactId multi, uint16 infomask,
/*
* Whatever remains are either live lockers that conflict with our
* wanted lock, and updaters that are not aborted. Those conflict
* with what we want, so return true.
* with what we want. Set up to return true, but keep going to
* look for the current transaction among the multixact members,
* if needed.
*/
result = true;
break;
}
pfree(members);
}