/*
* Sets a latch and wakes up anyone waiting on it. Returns quickly if the
* latch is already set.
*/
void
SetLatch(volatile Latch *latch)
{
pid_t owner_pid;
/* Quick exit if already set */
if (latch->is_set)
return;
latch->is_set = true;
/*
* See if anyone's waiting for the latch. It can be the current process
* if 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.
*/
owner_pid = latch->owner_pid;
if (owner_pid == 0)
return;
else if (owner_pid == MyProcPid)
sendSelfPipeByte();
else
kill(owner_pid, SIGUSR1);
}
/*
* Sets a latch and wakes up anyone waiting on it.
*
* This is cheap if the latch is already set, otherwise not so much.
*
* NB: when calling this in a signal handler, be sure to save and restore
* errno around it. (That's standard practice in most signal handlers, of
* course, but we used to omit it in handlers that only set a flag.)
*
* NB: this function is called from critical sections and signal handlers so
* throwing an error is not a good idea.
*/
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;
latch->is_set = true;
/*
* See if anyone's waiting for the latch. It can be the current process if
* 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 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)
return;
else if (owner_pid == MyProcPid)
{
if (waiting)
sendSelfPipeByte();
}
else
kill(owner_pid, SIGUSR1);
}
/*
* 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; or we might not have reached WaitLatch
* yet, in which case we don't need to fill the pipe either.)
*
* NB: when calling this in a signal handler, be sure to save and restore
* errno around it.
*/
void
latch_sigusr1_handler(void)
{
if (waiting)
sendSelfPipeByte();
}
/*
* Sets a latch and wakes up anyone waiting on it.
*
* This is cheap if the latch is already set, otherwise not so much.
*
* NB: when calling this in a signal handler, be sure to save and restore
* errno around it. (That's standard practice in most signal handlers, of
* course, but we used to omit it in handlers that only set a flag.)
*
* NB: this function is called from critical sections and signal handlers so
* throwing an error is not a good idea.
*/
void
SetLatch(volatile Latch *latch)
{
#ifndef WIN32
pid_t owner_pid;
#else
HANDLE handle;
#endif
/*
* The memory barrier has to be placed here to ensure that any flag
* variables possibly changed by this process have been flushed to main
* memory, before we check/set is_set.
*/
pg_memory_barrier();
/* Quick exit if already set */
if (latch->is_set)
return;
latch->is_set = true;
#ifndef WIN32
/*
* See if anyone's waiting for the latch. It can be the current process if
* 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 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)
return;
else if (owner_pid == MyProcPid)
{
if (waiting)
sendSelfPipeByte();
}
else
kill(owner_pid, SIGUSR1);
#else
/*
* 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 just in case somebody changes the event field
* concurrently (which really should not happen).
*/
handle = latch->event;
if (handle)
{
SetEvent(handle);
/*
* Note that we silently ignore any errors. We might be in a signal
* handler or other critical path where it's not safe to call elog().
*/
}
#endif
}
