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Fix issues around the "variable" support in the lwlock infrastructure.

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The lwlock scalability work introduced two race conditions into the
lwlock variable support provided for xlog.c. First, and harmlessly on
most platforms, it set/read the variable without the spinlock in some
places. Secondly, due to the removal of the spinlock, it was possible
that a backend missed changes to the variable's state if it changed in
the wrong moment because checking the lock's state, the variable's state
and the queuing are not protected by a single spinlock acquisition
anymore.

To fix first move resetting the variable's from LWLockAcquireWithVar to
WALInsertLockRelease, via a new function LWLockReleaseClearVar. That
prevents issues around waiting for a variable's value to change when a
new locker has acquired the lock, but not yet set the value. Secondly
re-check that the variable hasn't changed after enqueing, that prevents
the issue that the lock has been released and already re-acquired by the
time the woken up backend checks for the lock's state.

Reported-By: Jeff Janes
Analyzed-By: Heikki Linnakangas
Reviewed-By: Heikki Linnakangas
Discussion: 5592DB35.2060401@iki.fi
Backpatch: 9.5, where the lwlock scalability went in
pull/7/head
Andres Freund 10 years ago
parent f69b4b9495
commit 7039760114
3 changed files with 129 additions and 100 deletions
Show all changes Ignore whitespace when comparing lines Ignore changes in amount of whitespace Ignore changes in whitespace at EOL
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  1. 34
      src/backend/access/transam/xlog.c
  2. 193
      src/backend/storage/lmgr/lwlock.c
  3. 2
      src/include/storage/lwlock.h

34
src/backend/access/transam/xlog.c
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@ -1408,9 +1408,7 @@ WALInsertLockAcquire(void)
* The insertingAt value is initially set to 0, as we don't know our
* insert location yet.
*/
immed = LWLockAcquireWithVar(&WALInsertLocks[MyLockNo].l.lock,
&WALInsertLocks[MyLockNo].l.insertingAt,
0);
immed = LWLockAcquire(&WALInsertLocks[MyLockNo].l.lock, LW_EXCLUSIVE);
if (!immed)
{
/*
@ -1435,26 +1433,28 @@ WALInsertLockAcquireExclusive(void)
int i;
/*
* When holding all the locks, we only update the last lock's insertingAt
* indicator. The others are set to 0xFFFFFFFFFFFFFFFF, which is higher
* than any real XLogRecPtr value, to make sure that no-one blocks waiting
* on those.
* When holding all the locks, all but the last lock's insertingAt
* indicator is set to 0xFFFFFFFFFFFFFFFF, which is higher than any real
* XLogRecPtr value, to make sure that no-one blocks waiting on those.
*/
for (i = 0; i < NUM_XLOGINSERT_LOCKS - 1; i++)
{
LWLockAcquireWithVar(&WALInsertLocks[i].l.lock,
&WALInsertLocks[i].l.insertingAt,
PG_UINT64_MAX);
LWLockAcquire(&WALInsertLocks[i].l.lock, LW_EXCLUSIVE);
LWLockUpdateVar(&WALInsertLocks[i].l.lock,
&WALInsertLocks[i].l.insertingAt,
PG_UINT64_MAX);
}
LWLockAcquireWithVar(&WALInsertLocks[i].l.lock,
&WALInsertLocks[i].l.insertingAt,
0);
/* Variable value reset to 0 at release */
LWLockAcquire(&WALInsertLocks[i].l.lock, LW_EXCLUSIVE);
holdingAllLocks = true;
}
/*
* Release our insertion lock (or locks, if we're holding them all).
*
* NB: Reset all variables to 0, so they cause LWLockWaitForVar to block the
* next time the lock is acquired.
*/
static void
WALInsertLockRelease(void)
@ -1464,13 +1464,17 @@ WALInsertLockRelease(void)
int i;
for (i = 0; i < NUM_XLOGINSERT_LOCKS; i++)
LWLockRelease(&WALInsertLocks[i].l.lock);
LWLockReleaseClearVar(&WALInsertLocks[i].l.lock,
&WALInsertLocks[i].l.insertingAt,
0);
holdingAllLocks = false;
}
else
{
LWLockRelease(&WALInsertLocks[MyLockNo].l.lock);
LWLockReleaseClearVar(&WALInsertLocks[MyLockNo].l.lock,
&WALInsertLocks[MyLockNo].l.insertingAt,
0);
}
}

193
src/backend/storage/lmgr/lwlock.c
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@ -10,13 +10,15 @@
* locking should be done with the full lock manager --- which depends on
* LWLocks to protect its shared state.
*
* In addition to exclusive and shared modes, lightweight locks can be used
* to wait until a variable changes value. The variable is initially set
* when the lock is acquired with LWLockAcquireWithVar, and can be updated
* In addition to exclusive and shared modes, lightweight locks can be used to
* wait until a variable changes value. The variable is initially not set
* when the lock is acquired with LWLockAcquire, i.e. it remains set to the
* value it was set to when the lock was released last, and can be updated
* without releasing the lock by calling LWLockUpdateVar. LWLockWaitForVar
* waits for the variable to be updated, or until the lock is free. The
* meaning of the variable is up to the caller, the lightweight lock code
* just assigns and compares it.
* waits for the variable to be updated, or until the lock is free. When
* releasing the lock with LWLockReleaseClearVar() the value can be set to an
* appropriate value for a free lock. The meaning of the variable is up to
* the caller, the lightweight lock code just assigns and compares it.
*
* Portions Copyright (c) 1996-2015, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
@ -150,9 +152,6 @@ static LWLockHandle held_lwlocks[MAX_SIMUL_LWLOCKS];
static int lock_addin_request = 0;
static bool lock_addin_request_allowed = true;
static inline bool LWLockAcquireCommon(LWLock *l, LWLockMode mode,
uint64 *valptr, uint64 val);
#ifdef LWLOCK_STATS
typedef struct lwlock_stats_key
{
@ -899,25 +898,7 @@ LWLockDequeueSelf(LWLock *lock)
* Side effect: cancel/die interrupts are held off until lock release.
*/
bool
LWLockAcquire(LWLock *l, LWLockMode mode)
{
return LWLockAcquireCommon(l, mode, NULL, 0);
}
/*
* LWLockAcquireWithVar - like LWLockAcquire, but also sets *valptr = val
*
* The lock is always acquired in exclusive mode with this function.
*/
bool
LWLockAcquireWithVar(LWLock *l, uint64 *valptr, uint64 val)
{
return LWLockAcquireCommon(l, LW_EXCLUSIVE, valptr, val);
}
/* internal function to implement LWLockAcquire and LWLockAcquireWithVar */
static inline bool
LWLockAcquireCommon(LWLock *lock, LWLockMode mode, uint64 *valptr, uint64 val)
LWLockAcquire(LWLock *lock, LWLockMode mode)
{
PGPROC *proc = MyProc;
bool result = true;
@ -1064,10 +1045,6 @@ LWLockAcquireCommon(LWLock *lock, LWLockMode mode, uint64 *valptr, uint64 val)
result = false;
}
/* If there's a variable associated with this lock, initialize it */
if (valptr)
*valptr = val;
TRACE_POSTGRESQL_LWLOCK_ACQUIRE(T_NAME(lock), T_ID(lock), mode);
/* Add lock to list of locks held by this backend */
@ -1258,6 +1235,71 @@ LWLockAcquireOrWait(LWLock *lock, LWLockMode mode)
return !mustwait;
}
/*
* Does the lwlock in its current state need to wait for the variable value to
* change?
*
* If we don't need to wait, and it's because the value of the variable has
* changed, store the current value in newval.
*
* *result is set to true if the lock was free, and false otherwise.
*/
static bool
LWLockConflictsWithVar(LWLock *lock,
uint64 *valptr, uint64 oldval, uint64 *newval,
bool *result)
{
bool mustwait;
uint64 value;
#ifdef LWLOCK_STATS
lwlock_stats *lwstats;
lwstats = get_lwlock_stats_entry(lock);
#endif
/*
* Test first to see if it the slot is free right now.
*
* XXX: the caller uses a spinlock before this, so we don't need a memory
* barrier here as far as the current usage is concerned. But that might
* not be safe in general.
*/
mustwait = (pg_atomic_read_u32(&lock->state) & LW_VAL_EXCLUSIVE) != 0;
if (!mustwait)
{
*result = true;
return false;
}
*result = false;
/*
* Read value using spinlock as we can't rely on atomic 64 bit
* reads/stores. TODO: On platforms with a way to do atomic 64 bit
* reads/writes the spinlock could be optimized away.
*/
#ifdef LWLOCK_STATS
lwstats->spin_delay_count += SpinLockAcquire(&lock->mutex);
#else
SpinLockAcquire(&lock->mutex);
#endif
value = *valptr;
SpinLockRelease(&lock->mutex);
if (value != oldval)
{
mustwait = false;
*newval = value;
}
else
{
mustwait = true;
}
return mustwait;
}
/*
* LWLockWaitForVar - Wait until lock is free, or a variable is updated.
*
@ -1268,11 +1310,6 @@ LWLockAcquireOrWait(LWLock *lock, LWLockMode mode)
* matches oldval, returns false and sets *newval to the current value in
* *valptr.
*
* It's possible that the lock holder releases the lock, but another backend
* acquires it again before we get a chance to observe that the lock was
* momentarily released. We wouldn't need to wait for the new lock holder,
* but we cannot distinguish that case, so we will have to wait.
*
* Note: this function ignores shared lock holders; if the lock is held
* in shared mode, returns 'true'.
*/
@ -1290,16 +1327,6 @@ LWLockWaitForVar(LWLock *lock, uint64 *valptr, uint64 oldval, uint64 *newval)
PRINT_LWDEBUG("LWLockWaitForVar", lock, LW_WAIT_UNTIL_FREE);
/*
* Quick test first to see if it the slot is free right now.
*
* XXX: the caller uses a spinlock before this, so we don't need a memory
* barrier here as far as the current usage is concerned. But that might
* not be safe in general.
*/
if ((pg_atomic_read_u32(&lock->state) & LW_VAL_EXCLUSIVE) == 0)
return true;
/*
* Lock out cancel/die interrupts while we sleep on the lock. There is no
* cleanup mechanism to remove us from the wait queue if we got
@ -1313,39 +1340,9 @@ LWLockWaitForVar(LWLock *lock, uint64 *valptr, uint64 oldval, uint64 *newval)
for (;;)
{
bool mustwait;
uint64 value;
mustwait = (pg_atomic_read_u32(&lock->state) & LW_VAL_EXCLUSIVE) != 0;
if (mustwait)
{
/*
* Perform comparison using spinlock as we can't rely on atomic 64
* bit reads/stores.
*/
#ifdef LWLOCK_STATS
lwstats->spin_delay_count += SpinLockAcquire(&lock->mutex);
#else
SpinLockAcquire(&lock->mutex);
#endif
/*
* XXX: We can significantly optimize this on platforms with 64bit
* atomics.
*/
value = *valptr;
if (value != oldval)
{
result = false;
mustwait = false;
*newval = value;
}
else
mustwait = true;
SpinLockRelease(&lock->mutex);
}
else
mustwait = false;
mustwait = LWLockConflictsWithVar(lock, valptr, oldval, newval,
&result);
if (!mustwait)
break; /* the lock was free or value didn't match */
@ -1354,7 +1351,9 @@ LWLockWaitForVar(LWLock *lock, uint64 *valptr, uint64 oldval, uint64 *newval)
* Add myself to wait queue. Note that this is racy, somebody else
* could wakeup before we're finished queuing. NB: We're using nearly
* the same twice-in-a-row lock acquisition protocol as
* LWLockAcquire(). Check its comments for details.
* LWLockAcquire(). Check its comments for details. The only
* difference is that we also have to check the variable's values when
* checking the state of the lock.
*/
LWLockQueueSelf(lock, LW_WAIT_UNTIL_FREE);
@ -1365,12 +1364,13 @@ LWLockWaitForVar(LWLock *lock, uint64 *valptr, uint64 oldval, uint64 *newval)
pg_atomic_fetch_or_u32(&lock->state, LW_FLAG_RELEASE_OK);
/*
* We're now guaranteed to be woken up if necessary. Recheck the
* lock's state.
* We're now guaranteed to be woken up if necessary. Recheck the lock
* and variables state.
*/
mustwait = (pg_atomic_read_u32(&lock->state) & LW_VAL_EXCLUSIVE) != 0;
mustwait = LWLockConflictsWithVar(lock, valptr, oldval, newval,
&result);
/* Ok, lock is free after we queued ourselves. Undo queueing. */
/* Ok, no conflict after we queued ourselves. Undo queueing. */
if (!mustwait)
{
LOG_LWDEBUG("LWLockWaitForVar", lock, "free, undoing queue");
@ -1587,6 +1587,31 @@ LWLockRelease(LWLock *lock)
RESUME_INTERRUPTS();
}
/*
* LWLockReleaseClearVar - release a previously acquired lock, reset variable
*/
void
LWLockReleaseClearVar(LWLock *lock, uint64 *valptr, uint64 val)
{
#ifdef LWLOCK_STATS
lwlock_stats *lwstats;
lwstats = get_lwlock_stats_entry(lock);
lwstats->spin_delay_count += SpinLockAcquire(&lock->mutex);
#else
SpinLockAcquire(&lock->mutex);
#endif
/*
* Set the variable's value before releasing the lock, that prevents race
* a race condition wherein a new locker acquires the lock, but hasn't yet
* set the variables value.
*/
*valptr = val;
SpinLockRelease(&lock->mutex);
LWLockRelease(lock);
}
/*
* LWLockReleaseAll - release all currently-held locks

2
src/include/storage/lwlock.h
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@ -182,10 +182,10 @@ extern bool LWLockAcquire(LWLock *lock, LWLockMode mode);
extern bool LWLockConditionalAcquire(LWLock *lock, LWLockMode mode);
extern bool LWLockAcquireOrWait(LWLock *lock, LWLockMode mode);
extern void LWLockRelease(LWLock *lock);
extern void LWLockReleaseClearVar(LWLock *lock, uint64 *valptr, uint64 val);
extern void LWLockReleaseAll(void);
extern bool LWLockHeldByMe(LWLock *lock);
extern bool LWLockAcquireWithVar(LWLock *lock, uint64 *valptr, uint64 val);
extern bool LWLockWaitForVar(LWLock *lock, uint64 *valptr, uint64 oldval, uint64 *newval);
extern void LWLockUpdateVar(LWLock *lock, uint64 *valptr, uint64 value);

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