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Documentation improvement and minor code cleanups for the latch facility.

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Improve the documentation around weak-memory-ordering risks, and do a pass
of general editorialization on the comments in the latch code.  Make the
Windows latch code more like the Unix latch code where feasible; in
particular provide the same Assert checks in both implementations.
Fix poorly-placed WaitLatch call in syncrep.c.

This patch resolves, for the moment, concerns around weak-memory-ordering
bugs in latch-related code: we have documented the restrictions and checked
that existing calls meet them.  In 9.2 I hope that we will install suitable
memory barrier instructions in SetLatch/ResetLatch, so that their callers
don't need to be quite so careful.
This commit is contained in:
Tom Lane
2011-08-09 15:30:51 -04:00
parent 028a0c5a29
commit 6760a4d402
5 changed files with 163 additions and 102 deletions
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149
src/backend/port/unix_latch.c
View File

@@ -3,60 +3,6 @@
* unix_latch.c
* Routines for inter-process latches
*
* A latch is a boolean variable, with operations that let you to sleep
* until it is set. A latch can be set from another process, or a signal
* handler within the same process.
*
* The latch interface is a reliable replacement for the common pattern of
* using pg_usleep() or select() to wait until a signal arrives, where the
* signal handler sets a global variable. Because on some platforms, an
* incoming signal doesn't interrupt sleep, and even on platforms where it
* does there is a race condition if the signal arrives just before
* entering the sleep, the common pattern must periodically wake up and
* poll the global variable. pselect() system call was invented to solve
* the problem, but it is not portable enough. Latches are designed to
* overcome these limitations, allowing you to sleep without polling and
* ensuring a quick response to signals from other processes.
*
* There are two kinds of latches: local and shared. A local latch is
* initialized by InitLatch, and can only be set from the same process.
* A local latch can be used to wait for a signal to arrive, by calling
* SetLatch in the signal handler. A shared latch resides in shared memory,
* and must be initialized at postmaster startup by InitSharedLatch. Before
* a shared latch can be waited on, it must be associated with a process
* with OwnLatch. Only the process owning the latch can wait on it, but any
* process can set it.
*
* There are three basic operations on a latch:
*
* SetLatch - Sets the latch
* ResetLatch - Clears the latch, allowing it to be set again
* WaitLatch - Waits for the latch to become set
*
* The correct pattern to wait for an event is:
*
* for (;;)
* {
* ResetLatch();
* if (work to do)
* Do Stuff();
*
* WaitLatch();
* }
*
* It's important to reset the latch *before* checking if there's work to
* do. Otherwise, if someone sets the latch between the check and the
* ResetLatch call, you will miss it and Wait will block.
*
* To wake up the waiter, you must first set a global flag or something
* else that the main loop tests in the "if (work to do)" part, and call
* SetLatch *after* that. SetLatch is designed to return quickly if the
* latch is already set.
*
*
* Implementation
* --------------
*
* The Unix implementation uses the so-called self-pipe trick to overcome
* the race condition involved with select() and setting a global flag
* in the signal handler. When a latch is set and the current process
@@ -65,8 +11,8 @@
* interrupt select() on all platforms, and even on platforms where it
* does, a signal that arrives just before the select() call does not
* prevent the select() from entering sleep. An incoming byte on a pipe
* however reliably interrupts the sleep, and makes select() to return
* immediately if the signal arrives just before select() begins.
* however reliably interrupts the sleep, and causes select() to return
* immediately even if the signal arrives before select() begins.
*
* When SetLatch is called from the same process that owns the latch,
* SetLatch writes the byte directly to the pipe. If it's owned by another
@@ -99,7 +45,7 @@
/* Are we currently in WaitLatch? The signal handler would like to know. */
static volatile sig_atomic_t waiting = false;
/* Read and write end of the self-pipe */
/* Read and write ends of the self-pipe */
static int selfpipe_readfd = -1;
static int selfpipe_writefd = -1;
@@ -115,7 +61,7 @@ static void sendSelfPipeByte(void);
void
InitLatch(volatile Latch *latch)
{
/* Initialize the self pipe if this is our first latch in the process */
/* Initialize the self-pipe if this is our first latch in the process */
if (selfpipe_readfd == -1)
initSelfPipe();
@@ -126,13 +72,14 @@ InitLatch(volatile Latch *latch)
/*
* Initialize a shared latch that can be set from other processes. The latch
* is initially owned by no-one, use OwnLatch to associate it with the
* is initially owned by no-one; use OwnLatch to associate it with the
* current process.
*
* InitSharedLatch needs to be called in postmaster before forking child
* processes, usually right after allocating the shared memory block
* containing the latch with ShmemInitStruct. The Unix implementation
* doesn't actually require that, but the Windows one does.
* containing the latch with ShmemInitStruct. (The Unix implementation
* doesn't actually require that, but the Windows one does.) Because of
* this restriction, we have no concurrency issues to worry about here.
*/
void
InitSharedLatch(volatile Latch *latch)
@@ -144,23 +91,30 @@ InitSharedLatch(volatile Latch *latch)
/*
* Associate a shared latch with the current process, allowing it to
* wait on it.
* wait on the latch.
*
* Make sure that latch_sigusr1_handler() is called from the SIGUSR1 signal
* handler, as shared latches use SIGUSR1 to for inter-process communication.
* Although there is a sanity check for latch-already-owned, we don't do
* any sort of locking here, meaning that we could fail to detect the error
* if two processes try to own the same latch at about the same time. If
* there is any risk of that, caller must provide an interlock to prevent it.
*
* In any process that calls OwnLatch(), make sure that
* latch_sigusr1_handler() is called from the SIGUSR1 signal handler,
* as shared latches use SIGUSR1 for inter-process communication.
*/
void
OwnLatch(volatile Latch *latch)
{
Assert(latch->is_shared);
/* Initialize the self pipe if this is our first latch in the process */
/* Initialize the self-pipe if this is our first latch in this process */
if (selfpipe_readfd == -1)
initSelfPipe();
/* sanity check */
if (latch->owner_pid != 0)
elog(ERROR, "latch already owned");
latch->owner_pid = MyProcPid;
}
@@ -172,6 +126,7 @@ DisownLatch(volatile Latch *latch)
{
Assert(latch->is_shared);
Assert(latch->owner_pid == MyProcPid);
latch->owner_pid = 0;
}
@@ -229,21 +184,31 @@ WaitLatchOrSocket(volatile Latch *latch, pgsocket sock, bool forRead,
int hifd;
/*
* Clear the pipe, and check if the latch is set already. If someone
* Clear the pipe, then check if the latch is set already. If someone
* sets the latch between this and the select() below, the setter will
* write a byte to the pipe (or signal us and the signal handler will
* do that), and the select() will return immediately.
*
* Note: we assume that the kernel calls involved in drainSelfPipe()
* and SetLatch() will provide adequate synchronization on machines
* with weak memory ordering, so that we cannot miss seeing is_set
* if the signal byte is already in the pipe when we drain it.
*/
drainSelfPipe();
if (latch->is_set)
{
result = 1;
break;
}
/* Must wait ... set up the event masks for select() */
FD_ZERO(&input_mask);
FD_ZERO(&output_mask);
FD_SET(selfpipe_readfd, &input_mask);
hifd = selfpipe_readfd;
if (sock != PGINVALID_SOCKET && forRead)
{
FD_SET(sock, &input_mask);
@@ -251,7 +216,6 @@ WaitLatchOrSocket(volatile Latch *latch, pgsocket sock, bool forRead,
hifd = sock;
}
FD_ZERO(&output_mask);
if (sock != PGINVALID_SOCKET && forWrite)
{
FD_SET(sock, &output_mask);
@@ -288,14 +252,23 @@ WaitLatchOrSocket(volatile Latch *latch, pgsocket sock, bool forRead,
}
/*
* Sets a latch and wakes up anyone waiting on it. Returns quickly if the
* latch is already set.
* Sets a latch and wakes up anyone waiting on it.
*
* This is cheap if the latch is already set, otherwise not so much.
*/
void
SetLatch(volatile Latch *latch)
{
pid_t owner_pid;
/*
* XXX there really ought to be a memory barrier operation right here,
* to ensure that any flag variables we might have changed get flushed
* to main memory before we check/set is_set. Without that, we have to
* require that callers provide their own synchronization for machines
* with weak memory ordering (see latch.h).
*/
/* Quick exit if already set */
if (latch->is_set)
return;
@@ -307,13 +280,21 @@ SetLatch(volatile Latch *latch)
* we're in a signal handler. We use the self-pipe to wake up the select()
* in that case. If it's another process, send a signal.
*
* Fetch owner_pid only once, in case the owner simultaneously disowns the
* latch and clears owner_pid. XXX: This assumes that pid_t is atomic,
* which isn't guaranteed to be true! In practice, the effective range of
* pid_t fits in a 32 bit integer, and so should be atomic. In the worst
* case, we might end up signaling wrong process if the right one disowns
* the latch just as we fetch owner_pid. Even then, you're very unlucky if
* a process with that bogus pid exists.
* Fetch owner_pid only once, in case the latch is concurrently getting
* owned or disowned. XXX: This assumes that pid_t is atomic, which isn't
* guaranteed to be true! In practice, the effective range of pid_t fits
* in a 32 bit integer, and so should be atomic. In the worst case, we
* might end up signaling the wrong process. Even then, you're very
* unlucky if a process with that bogus pid exists and belongs to
* Postgres; and PG database processes should handle excess SIGUSR1
* interrupts without a problem anyhow.
*
* Another sort of race condition that's possible here is for a new process
* to own the latch immediately after we look, so we don't signal it.
* This is okay so long as all callers of ResetLatch/WaitLatch follow the
* standard coding convention of waiting at the bottom of their loops,
* not the top, so that they'll correctly process latch-setting events that
* happen before they enter the loop.
*/
owner_pid = latch->owner_pid;
if (owner_pid == 0)
@@ -335,11 +316,23 @@ ResetLatch(volatile Latch *latch)
Assert(latch->owner_pid == MyProcPid);
latch->is_set = false;
/*
* XXX there really ought to be a memory barrier operation right here, to
* ensure that the write to is_set gets flushed to main memory before we
* examine any flag variables. Otherwise a concurrent SetLatch might
* falsely conclude that it needn't signal us, even though we have missed
* seeing some flag updates that SetLatch was supposed to inform us of.
* For the moment, callers must supply their own synchronization of flag
* variables (see latch.h).
*/
}
/*
* SetLatch uses SIGUSR1 to wake up the process waiting on the latch. Wake
* up WaitLatch.
* SetLatch uses SIGUSR1 to wake up the process waiting on the latch.
*
* Wake up WaitLatch, if we're waiting. (We might not be, since SIGUSR1 is
* overloaded for multiple purposes.)
*/
void
latch_sigusr1_handler(void)

25
src/backend/port/win32_latch.c
View File

@@ -1,9 +1,10 @@
/*-------------------------------------------------------------------------
*
* win32_latch.c
* Windows implementation of latches.
* Routines for inter-process latches
*
* See unix_latch.c for information on usage.
* See unix_latch.c for header comments for the exported functions;
* the API presented here is supposed to be the same as there.
*
* The Windows implementation uses Windows events that are inherited by
* all postmaster child processes.
@@ -23,7 +24,6 @@
#include <unistd.h>
#include "miscadmin.h"
#include "replication/walsender.h"
#include "storage/latch.h"
#include "storage/shmem.h"
@@ -88,7 +88,7 @@ WaitLatch(volatile Latch *latch, long timeout)
}
int
WaitLatchOrSocket(volatile Latch *latch, SOCKET sock, bool forRead,
WaitLatchOrSocket(volatile Latch *latch, pgsocket sock, bool forRead,
bool forWrite, long timeout)
{
DWORD rc;
@@ -98,6 +98,9 @@ WaitLatchOrSocket(volatile Latch *latch, SOCKET sock, bool forRead,
int numevents;
int result = 0;
if (latch->owner_pid != MyProcPid)
elog(ERROR, "cannot wait on a latch owned by another process");
latchevent = latch->event;
events[0] = latchevent;
@@ -187,15 +190,10 @@ SetLatch(volatile Latch *latch)
/*
* See if anyone's waiting for the latch. It can be the current process if
* we're in a signal handler. Use a local variable here in case the latch
* is just disowned between the test and the SetEvent call, and event
* field set to NULL.
* we're in a signal handler.
*
* Fetch handle field only once, in case the owner simultaneously disowns
* the latch and clears handle. This assumes that HANDLE is atomic, which
* isn't guaranteed to be true! In practice, it should be, and in the
* worst case we end up calling SetEvent with a bogus handle, and SetEvent
* will return an error with no harm done.
* Use a local variable here just in case somebody changes the event field
* concurrently (which really should not happen).
*/
handle = latch->event;
if (handle)
@@ -212,5 +210,8 @@ SetLatch(volatile Latch *latch)
void
ResetLatch(volatile Latch *latch)
{
/* Only the owner should reset the latch */
Assert(latch->owner_pid == MyProcPid);
latch->is_set = false;
}

20
src/backend/replication/syncrep.c
View File

@@ -166,13 +166,6 @@ SyncRepWaitForLSN(XLogRecPtr XactCommitLSN)
{
int syncRepState;
/*
* Wait on latch for up to 60 seconds. This allows us to check for
* postmaster death regularly while waiting. Note that timeout here
* does not necessarily release from loop.
*/
WaitLatch(&MyProc->waitLatch, 60000000L);
/* Must reset the latch before testing state. */
ResetLatch(&MyProc->waitLatch);
@@ -184,6 +177,12 @@ SyncRepWaitForLSN(XLogRecPtr XactCommitLSN)
* walsender changes the state to SYNC_REP_WAIT_COMPLETE, it will
* never update it again, so we can't be seeing a stale value in that
* case.
*
* Note: on machines with weak memory ordering, the acquisition of
* the lock is essential to avoid race conditions: we cannot be sure
* the sender's state update has reached main memory until we acquire
* the lock. We could get rid of this dance if SetLatch/ResetLatch
* contained memory barriers.
*/
syncRepState = MyProc->syncRepState;
if (syncRepState == SYNC_REP_WAITING)
@@ -246,6 +245,13 @@ SyncRepWaitForLSN(XLogRecPtr XactCommitLSN)
SyncRepCancelWait();
break;
}
/*
* Wait on latch for up to 60 seconds. This allows us to check for
* cancel/die signal or postmaster death regularly while waiting. Note
* that timeout here does not necessarily release from loop.
*/
WaitLatch(&MyProc->waitLatch, 60000000L);
}
/*

2
src/backend/storage/lmgr/proc.c
View File

@@ -338,7 +338,7 @@ InitProcess(void)
MyProc->waitLSN.xrecoff = 0;
MyProc->syncRepState = SYNC_REP_NOT_WAITING;
SHMQueueElemInit(&(MyProc->syncRepLinks));
OwnLatch((Latch *) &MyProc->waitLatch);
OwnLatch(&MyProc->waitLatch);
/*
* We might be reusing a semaphore that belonged to a failed process. So

69
src/include/storage/latch.h
View File

@@ -3,6 +3,66 @@
* latch.h
* Routines for interprocess latches
*
* A latch is a boolean variable, with operations that let processes sleep
* until it is set. A latch can be set from another process, or a signal
* handler within the same process.
*
* The latch interface is a reliable replacement for the common pattern of
* using pg_usleep() or select() to wait until a signal arrives, where the
* signal handler sets a flag variable. Because on some platforms an
* incoming signal doesn't interrupt sleep, and even on platforms where it
* does there is a race condition if the signal arrives just before
* entering the sleep, the common pattern must periodically wake up and
* poll the flag variable. The pselect() system call was invented to solve
* this problem, but it is not portable enough. Latches are designed to
* overcome these limitations, allowing you to sleep without polling and
* ensuring quick response to signals from other processes.
*
* There are two kinds of latches: local and shared. A local latch is
* initialized by InitLatch, and can only be set from the same process.
* A local latch can be used to wait for a signal to arrive, by calling
* SetLatch in the signal handler. A shared latch resides in shared memory,
* and must be initialized at postmaster startup by InitSharedLatch. Before
* a shared latch can be waited on, it must be associated with a process
* with OwnLatch. Only the process owning the latch can wait on it, but any
* process can set it.
*
* There are three basic operations on a latch:
*
* SetLatch - Sets the latch
* ResetLatch - Clears the latch, allowing it to be set again
* WaitLatch - Waits for the latch to become set
*
* WaitLatch includes a provision for timeouts (which should hopefully not
* be necessary once the code is fully latch-ified).
* See unix_latch.c for detailed specifications for the exported functions.
*
* The correct pattern to wait for event(s) is:
*
* for (;;)
* {
* ResetLatch();
* if (work to do)
* Do Stuff();
* WaitLatch();
* }
*
* It's important to reset the latch *before* checking if there's work to
* do. Otherwise, if someone sets the latch between the check and the
* ResetLatch call, you will miss it and Wait will incorrectly block.
*
* To wake up the waiter, you must first set a global flag or something
* else that the wait loop tests in the "if (work to do)" part, and call
* SetLatch *after* that. SetLatch is designed to return quickly if the
* latch is already set.
*
* Presently, when using a shared latch for interprocess signalling, the
* flag variable(s) set by senders and inspected by the wait loop must
* be protected by spinlocks or LWLocks, else it is possible to miss events
* on machines with weak memory ordering (such as PPC). This restriction
* will be lifted in future by inserting suitable memory barriers into
* SetLatch and ResetLatch.
*
*
* Portions Copyright (c) 1996-2011, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
@@ -44,16 +104,17 @@ extern int WaitLatchOrSocket(volatile Latch *latch, pgsocket sock,
extern void SetLatch(volatile Latch *latch);
extern void ResetLatch(volatile Latch *latch);
#define TestLatch(latch) (((volatile Latch *) latch)->is_set)
/* beware of memory ordering issues if you use this macro! */
#define TestLatch(latch) (((volatile Latch *) (latch))->is_set)
/*
* Unix implementation uses SIGUSR1 for inter-process signaling, Win32 doesn't
* need this.
* Unix implementation uses SIGUSR1 for inter-process signaling.
* Win32 doesn't need this.
*/
#ifndef WIN32
extern void latch_sigusr1_handler(void);
#else
#define latch_sigusr1_handler()
#define latch_sigusr1_handler() ((void) 0)
#endif
#endif /* LATCH_H */