$PostgreSQL: pgsql/src/backend/storage/lmgr/README,v 1.21 2006/09/18 22:40:36 tgl Exp $
LOCKING OVERVIEW
Postgres uses three types of interprocess locks:
* Spinlocks. These are intended for *very* short-term locks. If a lock
is to be held more than a few dozen instructions, or across any sort of
kernel call (or even a call to a nontrivial subroutine), don't use a
spinlock. Spinlocks are primarily used as infrastructure for lightweight
locks. They are implemented using a hardware atomic-test-and-set
instruction, if available. Waiting processes busy-loop until they can
get the lock. There is no provision for deadlock detection, automatic
release on error, or any other nicety. There is a timeout if the lock
cannot be gotten after a minute or so (which is approximately forever in
comparison to the intended lock hold time, so this is certainly an error
condition).
* Lightweight locks (LWLocks). These locks are typically used to
interlock access to datastructures in shared memory. LWLocks support
both exclusive and shared lock modes (for read/write and read-only
access to a shared object). There is no provision for deadlock
detection, but the LWLock manager will automatically release held
LWLocks during elog() recovery, so it is safe to raise an error while
holding LWLocks. Obtaining or releasing an LWLock is quite fast (a few
dozen instructions) when there is no contention for the lock. When a
process has to wait for an LWLock, it blocks on a SysV semaphore so as
to not consume CPU time. Waiting processes will be granted the lock in
arrival order. There is no timeout.
* Regular locks (a/k/a heavyweight locks). The regular lock manager
supports a variety of lock modes with table-driven semantics, and it has
full deadlock detection and automatic release at transaction end.
Regular locks should be used for all user-driven lock requests.
Acquisition of either a spinlock or a lightweight lock causes query
cancel and die() interrupts to be held off until all such locks are
released. No such restriction exists for regular locks, however. Also
note that we can accept query cancel and die() interrupts while waiting
for a regular lock, but we will not accept them while waiting for
spinlocks or LW locks. It is therefore not a good idea to use LW locks
when the wait time might exceed a few seconds.
The rest of this README file discusses the regular lock manager in detail.
LOCK DATA STRUCTURES
Lock methods describe the overall locking behavior. Currently there are
two lock methods: DEFAULT and USER.
Lock modes describe the type of the lock (read/write or shared/exclusive).
In principle, each lock method can have its own set of lock modes with
different conflict rules, but currently DEFAULT and USER methods use
identical lock mode sets. See src/tools/backend/index.html and
src/include/storage/lock.h for more details. (Lock modes are also called
lock types in some places in the code and documentation.)
There are two fundamental lock structures in shared memory: the
per-lockable-object LOCK struct, and the per-lock-and-requestor PROCLOCK
struct. A LOCK object exists for each lockable object that currently has
locks held or requested on it. A PROCLOCK struct exists for each backend
that is holding or requesting lock(s) on each LOCK object.
In addition to these, each backend maintains an unshared LOCALLOCK structure
for each lockable object and lock mode that it is currently holding or
requesting. The shared lock structures only allow a single lock grant to
be made per lockable object/lock mode/backend. Internally to a backend,
however, the same lock may be requested and perhaps released multiple times
in a transaction, and it can also be held both transactionally and session-
wide. The internal request counts are held in LOCALLOCK so that the shared
data structures need not be accessed to alter them.
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The lock manager's LOCK objects contain:
tag -
The key fields that are used for hashing locks in the shared memory
lock hash table. The contents of the tag essentially define an
individual lockable object. See include/storage/lock.h for details
about the supported types of lockable objects. This is declared as
a separate struct to ensure that we always zero out the correct number
of bytes. It is critical that any alignment-padding bytes the compiler
might insert in the struct be zeroed out, else the hash computation
will be random. (Currently, we are careful to define struct LOCKTAG
so that there are no padding bytes.)
grantMask -
This bitmask indicates what types of locks are currently held on the
given lockable object. It is used (against the lock table's conflict
table) to determine if a new lock request will conflict with existing
lock types held. Conflicts are determined by bitwise AND operations
between the grantMask and the conflict table entry for the requested
lock type. Bit i of grantMask is 1 if and only if granted[i] > 0.
waitMask -
This bitmask shows the types of locks being waited for. Bit i of waitMask
is 1 if and only if requested[i] > granted[i].
procLocks -
This is a shared memory queue of all the PROCLOCK structs associated with
the lock object. Note that both granted and waiting PROCLOCKs are in this
list (indeed, the same PROCLOCK might have some already-granted locks and
be waiting for more!).
waitProcs -
This is a shared memory queue of all PGPROC structures corresponding to
backends that are waiting (sleeping) until another backend releases this
lock. The process structure holds the information needed to determine
if it should be woken up when the lock is released.
nRequested -
Keeps a count of how many times this lock has been attempted to be
acquired. The count includes attempts by processes which were put
to sleep due to conflicts. It also counts the same backend twice
if, for example, a backend process first acquires a read and then
acquires a write. (But multiple acquisitions of the same lock/lock mode
within a backend are not multiply counted here; they are recorded
only in the backend's LOCALLOCK structure.)
requested -
Keeps a count of how many locks of each type have been attempted. Only
elements 1 through MAX_LOCKMODES-1 are used as they correspond to the lock
type defined constants. Summing the values of requested[] should come out
equal to nRequested.
nGranted -
Keeps count of how many times this lock has been successfully acquired.
This count does not include attempts that are waiting due to conflicts.
Otherwise the counting rules are the same as for nRequested.
granted -
Keeps count of how many locks of each type are currently held. Once again
only elements 1 through MAX_LOCKMODES-1 are used (0 is not). Also, like
requested[], summing the values of granted[] should total to the value
of nGranted.
We should always have 0 <= nGranted <= nRequested, and
0 <= granted[i] <= requested[i] for each i. When all the request counts
go to zero, the LOCK object is no longer needed and can be freed.
---------------------------------------------------------------------------
The lock manager's PROCLOCK objects contain:
tag -
The key fields that are used for hashing entries in the shared memory
PROCLOCK hash table. This is declared as a separate struct to ensure that
we always zero out the correct number of bytes. It is critical that any
alignment-padding bytes the compiler might insert in the struct be zeroed
out, else the hash computation will be random. (Currently, we are careful
to define struct PROCLOCKTAG so that there are no padding bytes.)
tag.myLock
Pointer to the shared LOCK object this PROCLOCK is for.
tag.myProc
Pointer to the PGPROC of backend process that owns this PROCLOCK.
Note: it's OK to use pointers here because a PROCLOCK never outlives
either its lock or its proc. The tag is therefore unique for as long
as it needs to be, even though the same tag values might mean something
else at other times.
holdMask -
A bitmask for the lock modes successfully acquired by this PROCLOCK.
This should be a subset of the LOCK object's grantMask, and also a
subset of the PGPROC object's heldLocks mask (if the PGPROC is
currently waiting for another lock mode on this lock).
releaseMask -
A bitmask for the lock modes due to be released during LockReleaseAll.
This must be a subset of the holdMask. Note that it is modified without
taking the partition LWLock, and therefore it is unsafe for any
backend except the one owning the PROCLOCK to examine/change it.
lockLink -
List link for shared memory queue of all the PROCLOCK objects for the
same LOCK.
procLink -
List link for shared memory queue of all the PROCLOCK objects for the
same backend.
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LOCK MANAGER INTERNAL LOCKING
Before PostgreSQL 8.2, all of the shared-memory data structures used by
the lock manager were protected by a single LWLock, the LockMgrLock;
any operation involving these data structures had to exclusively lock
LockMgrLock. Not too surprisingly, this became a contention bottleneck.
To reduce contention, the lock manager's data structures have been split
into multiple "partitions", each protected by an independent LWLock.
Most operations only need to lock the single partition they are working in.
Here are the details:
* Each possible lock is assigned to one partition according to a hash of
its LOCKTAG value. The partition's LWLock is considered to protect all the
LOCK objects of that partition as well as their subsidiary PROCLOCKs.
* The shared-memory hash tables for LOCKs and PROCLOCKs are organized
so that different partitions use different hash chains, and thus there
is no conflict in working with objects in different partitions. This
is supported directly by dynahash.c's "partitioned table" mechanism
for the LOCK table: we need only ensure that the partition number is
taken from the low-order bits of the dynahash hash value for the LOCKTAG.
To make it work for PROCLOCKs, we have to ensure that a PROCLOCK's hash
value has the same low-order bits as its associated LOCK. This requires
a specialized hash function (see proclock_hash).
* Formerly, each PGPROC had a single list of PROCLOCKs belonging to it.
This has now been split into per-partition lists, so that access to a
particular PROCLOCK list can be protected by the associated partition's
LWLock. (This is not strictly necessary at the moment, because at this
writing a PGPROC's PROCLOCK list is only accessed by the owning backend
anyway. But it seems forward-looking to maintain a convention for how
other backends could access it. In any case LockReleaseAll needs to be
able to quickly determine which partition each LOCK belongs to, and
for the currently contemplated number of partitions, this way takes less
shared memory than explicitly storing a partition number in LOCK structs
would require.)
* The other lock-related fields of a PGPROC are only interesting when
the PGPROC is waiting for a lock, so we consider that they are protected
by the partition LWLock of the awaited lock.
For normal lock acquisition and release, it is sufficient to lock the
partition containing the desired lock. Deadlock checking needs to touch
multiple partitions in general; for simplicity, we just make it lock all
the partitions in partition-number order. (To prevent LWLock deadlock,
we establish the rule that any backend needing to lock more than one
partition at once must lock them in partition-number order.) It's
possible that deadlock checking could be done without touching every
partition in typical cases, but since in a properly functioning system
deadlock checking should not occur often enough to be performance-critical,
trying to make this work does not seem a productive use of effort.
A backend's internal LOCALLOCK hash table is not partitioned. We do store
a copy of the locktag hash code in LOCALLOCK table entries, from which the
partition number can be computed, but this is a straight speed-for-space
tradeoff: we could instead recalculate the partition number from the LOCKTAG
when needed.
THE DEADLOCK DETECTION ALGORITHM
Since we allow user transactions to request locks in any order, deadlock
is possible. We use a deadlock detection/breaking algorithm that is
fairly standard in essence, but there are many special considerations
needed to deal with Postgres' generalized locking model.
A key design consideration is that we want to make routine operations
(lock grant and release) run quickly when there is no deadlock, and
avoid the overhead of deadlock handling as much as possible. We do this
using an "optimistic waiting" approach: if a process cannot acquire the
lock it wants immediately, it goes to sleep without any deadlock check.
But it also sets a delay timer, with a delay of DeadlockTimeout
milliseconds (typically set to one second). If the delay expires before
the process is granted the lock it wants, it runs the deadlock
detection/breaking code. Normally this code will determine that there is
no deadlock condition, and then the process will go back to sleep and
wait quietly until it is granted the lock. But if a deadlock condition
does exist, it will be resolved, usually by aborting the detecting
process' transaction. In this way, we avoid deadlock handling overhead
whenever the wait time for a lock is less than DeadlockTimeout, while
not imposing an unreasonable delay of detection when there is an error.
Lock acquisition (routines LockAcquire and ProcSleep) follows these rules:
1. A lock request is granted immediately if it does not conflict with
any existing or waiting lock request, or if the process already holds an
instance of the same lock type (eg, there's no penalty to acquire a read
lock twice). Note that a process never conflicts with itself, eg one
can obtain read lock when one already holds exclusive lock.
2. Otherwise the process joins the lock's wait queue. Normally it will
be added to the end of the queue, but there is an exception: if the
process already holds locks on this same lockable object that conflict
with the request of any pending waiter, then the process will be
inserted in the wait queue just ahead of the first such waiter. (If we
did not make this check, the deadlock detection code would adjust the
queue order to resolve the conflict, but it's relatively cheap to make
the check in ProcSleep and avoid a deadlock timeout delay in this case.)
Note special case when inserting before the end of the queue: if the
process's request does not conflict with any existing lock nor any
waiting request before its insertion point, then go ahead and grant the
lock without waiting.
When a lock is released, the lock release routine (ProcLockWakeup) scans
the lock object's wait queue. Each waiter is awoken if (a) its request
does not conflict with already-granted locks, and (b) its request does
not conflict with the requests of prior un-wakable waiters. Rule (b)
ensures that conflicting requests are granted in order of arrival. There
are cases where a later waiter must be allowed to go in front of
conflicting earlier waiters to avoid deadlock, but it is not
ProcLockWakeup's responsibility to recognize these cases; instead, the
deadlock detection code will re-order the wait queue when necessary.
To perform deadlock checking, we use the standard method of viewing the
various processes as nodes in a directed graph (the waits-for graph or
WFG). There is a graph edge leading from process A to process B if A
waits for B, ie, A is waiting for some lock and B holds a conflicting
lock. There is a deadlock condition if and only if the WFG contains a
cycle. We detect cycles by searching outward along waits-for edges to
see if we return to our starting point. There are three possible
outcomes:
1. All outgoing paths terminate at a running process (which has no
outgoing edge).
2. A deadlock is detected by looping back to the start point. We
resolve such a deadlock by canceling the start point's lock request and
reporting an error in that transaction, which normally leads to
transaction abort and release of that transaction's held locks. Note
that it's sufficient to cancel one request to remove the cycle; we don't
need to kill all the transactions involved.
3. Some path(s) loop back to a node other than the start point. This
indicates a deadlock, but one that does not involve our starting
process. We ignore this condition on the grounds that resolving such a
deadlock is the responsibility of the processes involved --- killing our
start- point process would not resolve the deadlock. So, cases 1 and 3
both report "no deadlock".
Postgres' situation is a little more complex than the standard discussion
of deadlock detection, for two reasons:
1. A process can be waiting for more than one other process, since there
might be multiple PROCLOCKs of (non-conflicting) lock types that all
conflict with the waiter's request. This creates no real difficulty
however; we simply need to be prepared to trace more than one outgoing
edge.
2. If a process A is behind a process B in some lock's wait queue, and
their requested locks conflict, then we must say that A waits for B, since
ProcLockWakeup will never awaken A before B. This creates additional
edges in the WFG. We call these "soft" edges, as opposed to the "hard"
edges induced by locks already held. Note that if B already holds any
locks conflicting with A's request, then their relationship is a hard edge
not a soft edge.
A "soft" block, or wait-priority block, has the same potential for
inducing deadlock as a hard block. However, we may be able to resolve
a soft block without aborting the transactions involved: we can instead
rearrange the order of the wait queue. This rearrangement reverses the
direction of the soft edge between two processes with conflicting requests
whose queue order is reversed. If we can find a rearrangement that
eliminates a cycle without creating new ones, then we can avoid an abort.
Checking for such possible rearrangements is the trickiest part of the
algorithm.
The workhorse of the deadlock detector is a routine FindLockCycle() which
is given a starting point process (which must be a waiting process).
It recursively scans outward across waits-for edges as discussed above.
If it finds no cycle involving the start point, it returns "false".
(As discussed above, we can ignore cycles not involving the start point.)
When such a cycle is found, FindLockCycle() returns "true", and as it
unwinds it also builds a list of any "soft" edges involved in the cycle.
If the resulting list is empty then there is a hard deadlock and the
configuration cannot succeed. However, if the list is not empty, then
reversing any one of the listed edges through wait-queue rearrangement
will eliminate that cycle. Since such a reversal might create cycles
elsewhere, we may need to try every possibility. Therefore, we need to
be able to invoke FindLockCycle() on hypothetical configurations (wait
orders) as well as the current real order.
The easiest way to handle this seems to be to have a lookaside table that
shows the proposed new queue order for each wait queue that we are
considering rearranging. This table is checked by FindLockCycle, and it
believes the proposed queue order rather than the real order for each lock
that has an entry in the lookaside table.
We build a proposed new queue order by doing a "topological sort" of the
existing entries. Each soft edge that we are currently considering
reversing creates a property of the partial order that the topological sort
has to enforce. We must use a sort method that preserves the input
ordering as much as possible, so as not to gratuitously break arrival
order for processes not involved in a deadlock. (This is not true of the
tsort method shown in Knuth, for example, but it's easily done by a simple
doubly-nested-loop method that emits the first legal candidate at each
step. Fortunately, we don't need a highly efficient sort algorithm, since
the number of partial order constraints is not likely to be large.) Note
that failure of the topological sort tells us we have conflicting ordering
constraints, and therefore that the last-added soft edge reversal
conflicts with a prior edge reversal. We need to detect this case to
avoid an infinite loop in the case where no possible rearrangement will
work: otherwise, we might try a reversal, find that it still leads to
a cycle, then try to un-reverse the reversal while trying to get rid of
that cycle, etc etc. Topological sort failure tells us the un-reversal
is not a legitimate move in this context.
So, the basic step in our rearrangement method is to take a list of
soft edges in a cycle (as returned by FindLockCycle()) and successively
try the reversal of each one as a topological-sort constraint added to
whatever constraints we are already considering. We recursively search
through all such sets of constraints to see if any one eliminates all
the deadlock cycles at once. Although this might seem impossibly
inefficient, it shouldn't be a big problem in practice, because there
will normally be very few, and not very large, deadlock cycles --- if
any at all. So the combinatorial inefficiency isn't going to hurt us.
Besides, it's better to spend some time to guarantee that we've checked
all possible escape routes than to abort a transaction when we didn't
really have to.
Each edge reversal constraint can be viewed as requesting that the waiting
process A be moved to before the blocking process B in the wait queue they
are both in. This action will reverse the desired soft edge, as well as
any other soft edges between A and other processes it is advanced over.
No other edges will be affected (note this is actually a constraint on our
topological sort method to not re-order the queue more than necessary.)
Therefore, we can be sure we have not created any new deadlock cycles if
neither FindLockCycle(A) nor FindLockCycle(B) discovers any cycle. Given
the above-defined behavior of FindLockCycle, each of these searches is
necessary as well as sufficient, since FindLockCycle starting at the
original start point will not complain about cycles that include A or B
but not the original start point.
In short then, a proposed rearrangement of the wait queue(s) is determined
by one or more broken soft edges A->B, fully specified by the output of
topological sorts of each wait queue involved, and then tested by invoking
FindLockCycle() starting at the original start point as well as each of
the mentioned processes (A's and B's). If none of the tests detect a
cycle, then we have a valid configuration and can implement it by
reordering the wait queues per the sort outputs (and then applying
ProcLockWakeup on each reordered queue, in case a waiter has become wakable).
If any test detects a soft cycle, we can try to resolve it by adding each
soft link in that cycle, in turn, to the proposed rearrangement list.
This is repeated recursively until we either find a workable rearrangement
or determine that none exists. In the latter case, the outer level
resolves the deadlock by aborting the original start-point transaction.
The particular order in which rearrangements are tried depends on the
order FindLockCycle() happens to scan in, so if there are multiple
workable rearrangements of the wait queues, then it is unspecified which
one will be chosen. What's more important is that we guarantee to try
every queue rearrangement that could lead to success. (For example,
if we have A before B before C and the needed order constraints are
C before A and B before C, we would first discover that A before C
doesn't work and try the rearrangement C before A before B. This would
eventually lead to the discovery of the additional constraint B before C.)
Got that?
Miscellaneous notes:
1. It is easily proven that no deadlock will be missed due to our
asynchronous invocation of deadlock checking. A deadlock cycle in the WFG
is formed when the last edge in the cycle is added; therefore the last
process in the cycle to wait (the one from which that edge is outgoing) is
certain to detect and resolve the cycle when it later runs CheckDeadLock.
This holds even if that edge addition created multiple cycles; the process
may indeed abort without ever noticing those additional cycles, but we
don't particularly care. The only other possible creation of deadlocks is
during deadlock resolution's rearrangement of wait queues, and we already
saw that that algorithm will prove that it creates no new deadlocks before
it attempts to actually execute any rearrangement.
2. It is not certain that a deadlock will be resolved by aborting the
last-to-wait process. If earlier waiters in the cycle have not yet run
CheckDeadLock, then the first one to do so will be the victim.
3. No live (wakable) process can be missed by ProcLockWakeup, since it
examines every member of the wait queue (this was not true in the 7.0
implementation, BTW). Therefore, if ProcLockWakeup is always invoked
after a lock is released or a wait queue is rearranged, there can be no
failure to wake a wakable process. One should also note that
LockWaitCancel (abort a waiter due to outside factors) must run
ProcLockWakeup, in case the canceled waiter was soft-blocking other
waiters.
4. We can minimize excess rearrangement-trial work by being careful to
scan the wait queue from the front when looking for soft edges. For
example, if we have queue order A,B,C and C has deadlock conflicts with
both A and B, we want to generate the "C before A" constraint first,
rather than wasting time with "C before B", which won't move C far
enough up. So we look for soft edges outgoing from C starting at the
front of the wait queue.
5. The working data structures needed by the deadlock detection code can
be limited to numbers of entries computed from MaxBackends. Therefore,
we can allocate the worst-case space needed during backend startup. This
seems a safer approach than trying to allocate workspace on the fly; we
don't want to risk having the deadlock detector run out of memory, else
we really have no guarantees at all that deadlock will be detected.
USER LOCKS
User locks are handled totally on the application side as long term
cooperative locks which extend beyond the normal transaction boundaries.
Their purpose is to indicate to an application that someone is `working'
on an item. So it is possible to put an user lock on a tuple's oid,
retrieve the tuple, work on it for an hour and then update it and remove
the lock. While the lock is active other clients can still read and write
the tuple but they can be aware that it has been locked at the application
level by someone.
User locks and normal locks are completely orthogonal and they don't
interfere with each other.
User locks are always held as session locks, so that they are not released at
transaction end. They must be released explicitly by the application --- but
they are released automatically when a backend terminates.
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