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Add a default local latch for use in signal handlers.

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To do so, move InitializeLatchSupport() into the new common process
initialization functions, and add a new global variable MyLatch.

MyLatch is usable as soon InitPostmasterChild() has been called
(i.e. very early during startup). Initially it points to a process
local latch that exists in all processes. InitProcess/InitAuxiliaryProcess
then replaces that local latch with PGPROC->procLatch. During shutdown
the reverse happens.

This is primarily advantageous for two reasons: For one it simplifies
dealing with the shared process latch, especially in signal handlers,
because instead of having to check for MyProc, MyLatch can be used
unconditionally. For another, a later patch that makes FEs/BE
communication use latches, now can rely on the existence of a latch,
even before having gone through InitProcess.

Discussion: 20140927191243.GD5423@alap3.anarazel.de
This commit is contained in:
Andres Freund
2015-01-14 18:45:22 +01:00
parent 85a2a8903f
commit 59f71a0d0b
23 changed files with 136 additions and 134 deletions
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42
src/backend/storage/lmgr/proc.c
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@@ -290,13 +290,6 @@ InitProcess(void)
if (MyProc != NULL)
elog(ERROR, "you already exist");
/*
* Initialize process-local latch support. This could fail if the kernel
* is low on resources, and if so we want to exit cleanly before acquiring
* any shared-memory resources.
*/
InitializeLatchSupport();
/*
* Try to get a proc struct from the free list. If this fails, we must be
* out of PGPROC structures (not to mention semaphores).
@@ -391,10 +384,12 @@ InitProcess(void)
SHMQueueElemInit(&(MyProc->syncRepLinks));
/*
* Acquire ownership of the PGPROC's latch, so that we can use WaitLatch.
* Note that there's no particular need to do ResetLatch here.
* Acquire ownership of the PGPROC's latch, so that we can use WaitLatch
* on it. That allows us to repoint the process latch, which so far
* points to process local one, to the shared one.
*/
OwnLatch(&MyProc->procLatch);
SwitchToSharedLatch();
/*
* We might be reusing a semaphore that belonged to a failed process. So
@@ -474,13 +469,6 @@ InitAuxiliaryProcess(void)
if (MyProc != NULL)
elog(ERROR, "you already exist");
/*
* Initialize process-local latch support. This could fail if the kernel
* is low on resources, and if so we want to exit cleanly before acquiring
* any shared-memory resources.
*/
InitializeLatchSupport();
/*
* We use the ProcStructLock to protect assignment and releasing of
* AuxiliaryProcs entries.
@@ -547,10 +535,12 @@ InitAuxiliaryProcess(void)
#endif
/*
* Acquire ownership of the PGPROC's latch, so that we can use WaitLatch.
* Note that there's no particular need to do ResetLatch here.
* Acquire ownership of the PGPROC's latch, so that we can use WaitLatch
* on it. That allows us to repoint the process latch, which so far
* points to process local one, to the shared one.
*/
OwnLatch(&MyProc->procLatch);
SwitchToSharedLatch();
/*
* We might be reusing a semaphore that belonged to a failed process. So
@@ -800,10 +790,12 @@ ProcKill(int code, Datum arg)
ReplicationSlotRelease();
/*
* Clear MyProc first; then disown the process latch. This is so that
* signal handlers won't try to clear the process latch after it's no
* longer ours.
* Reset MyLatch to the process local one. This is so that signal
* handlers et al can continue using the latch after the shared latch
* isn't ours anymore. After that clear MyProc and disown the shared
* latch.
*/
SwitchBackToLocalLatch();
proc = MyProc;
MyProc = NULL;
DisownLatch(&proc->procLatch);
@@ -867,10 +859,12 @@ AuxiliaryProcKill(int code, Datum arg)
LWLockReleaseAll();
/*
* Clear MyProc first; then disown the process latch. This is so that
* signal handlers won't try to clear the process latch after it's no
* longer ours.
* Reset MyLatch to the process local one. This is so that signal
* handlers et al can continue using the latch after the shared latch
* isn't ours anymore. After that clear MyProc and disown the shared
* latch.
*/
SwitchBackToLocalLatch();
proc = MyProc;
MyProc = NULL;
DisownLatch(&proc->procLatch);