Refactor shared memory allocation for semaphores

Before commit e25626677f, spinlocks were implemented using semaphores
on some platforms (--disable-spinlocks). That made it necessary to
initialize semaphores early, before any spinlocks could be used. Now
that we don't support --disable-spinlocks anymore, we can allocate the
shared memory needed for semaphores the same way as other shared
memory structures. Since the semaphores are used only in the PGPROC
array, move the semaphore shmem size estimation and initialization
calls to ProcGlobalShmemSize() and InitProcGlobal().

Author: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Discussion: https://www.postgresql.org/message-id/CAExHW5seSZpPx-znjidVZNzdagGHOk06F+Ds88MpPUbxd1kTaA@mail.gmail.com

src/backend/port/posix_sema.c

@@ -215,12 +215,8 @@ PGReserveSemaphores(int maxSemas)
 		elog(PANIC, "out of memory");
 #else
 
-	/*
-	 * We must use ShmemAllocUnlocked(), since the spinlock protecting
-	 * ShmemAlloc() won't be ready yet.
-	 */
 	sharedSemas = (PGSemaphore)
-		ShmemAllocUnlocked(PGSemaphoreShmemSize(maxSemas));
+		ShmemAlloc(PGSemaphoreShmemSize(maxSemas));
 #endif
 
 	numSems = 0;

src/backend/port/sysv_sema.c

@@ -343,12 +343,8 @@ PGReserveSemaphores(int maxSemas)
 				 errmsg("could not stat data directory \"%s\": %m",
 						DataDir)));
 
-	/*
-	 * We must use ShmemAllocUnlocked(), since the spinlock protecting
-	 * ShmemAlloc() won't be ready yet.
-	 */
 	sharedSemas = (PGSemaphore)
-		ShmemAllocUnlocked(PGSemaphoreShmemSize(maxSemas));
+		ShmemAlloc(PGSemaphoreShmemSize(maxSemas));
 	numSharedSemas = 0;
 	maxSharedSemas = maxSemas;
 

src/backend/storage/ipc/ipci.c

@@ -81,23 +81,12 @@ RequestAddinShmemSpace(Size size)
 
 /*
  * CalculateShmemSize
- *		Calculates the amount of shared memory and number of semaphores needed.
- *
- * If num_semaphores is not NULL, it will be set to the number of semaphores
- * required.
+ *		Calculates the amount of shared memory needed.
  */
 Size
-CalculateShmemSize(int *num_semaphores)
+CalculateShmemSize(void)
 {
 	Size		size;
-	int			numSemas;
-
-	/* Compute number of semaphores we'll need */
-	numSemas = ProcGlobalSemas();
-
-	/* Return the number of semaphores if requested by the caller */
-	if (num_semaphores)
-		*num_semaphores = numSemas;
 
 	/*
 	 * Size of the Postgres shared-memory block is estimated via moderately-
@@ -109,7 +98,6 @@ CalculateShmemSize(int *num_semaphores)
 	 * during the actual allocation phase.
 	 */
 	size = 100000;
-	size = add_size(size, PGSemaphoreShmemSize(numSemas));
 	size = add_size(size, hash_estimate_size(SHMEM_INDEX_SIZE,
 											 sizeof(ShmemIndexEnt)));
 	size = add_size(size, dsm_estimate_size());
@@ -204,12 +192,11 @@ CreateSharedMemoryAndSemaphores(void)
 	PGShmemHeader *shim;
 	PGShmemHeader *seghdr;
 	Size		size;
-	int			numSemas;
 
 	Assert(!IsUnderPostmaster);
 
 	/* Compute the size of the shared-memory block */
-	size = CalculateShmemSize(&numSemas);
+	size = CalculateShmemSize();
 	elog(DEBUG3, "invoking IpcMemoryCreate(size=%zu)", size);
 
 	/*
@@ -226,13 +213,6 @@ CreateSharedMemoryAndSemaphores(void)
 
 	InitShmemAccess(seghdr);
 
-	/*
-	 * Create semaphores.  (This is done here for historical reasons.  We used
-	 * to support emulating spinlocks with semaphores, which required
-	 * initializing semaphores early.)
-	 */
-	PGReserveSemaphores(numSemas);
-
 	/*
 	 * Set up shared memory allocation mechanism
 	 */
@@ -363,12 +343,11 @@ InitializeShmemGUCs(void)
 	Size		size_b;
 	Size		size_mb;
 	Size		hp_size;
-	int			num_semas;
 
 	/*
 	 * Calculate the shared memory size and round up to the nearest megabyte.
 	 */
-	size_b = CalculateShmemSize(&num_semas);
+	size_b = CalculateShmemSize();
 	size_mb = add_size(size_b, (1024 * 1024) - 1) / (1024 * 1024);
 	sprintf(buf, "%zu", size_mb);
 	SetConfigOption("shared_memory_size", buf,
@@ -388,6 +367,6 @@ InitializeShmemGUCs(void)
 						PGC_INTERNAL, PGC_S_DYNAMIC_DEFAULT);
 	}
 
-	sprintf(buf, "%d", num_semas);
+	sprintf(buf, "%d", ProcGlobalSemas());
 	SetConfigOption("num_os_semaphores", buf, PGC_INTERNAL, PGC_S_DYNAMIC_DEFAULT);
 }

src/backend/storage/ipc/shmem.c

@@ -76,6 +76,7 @@
 #include "utils/builtins.h"
 
 static void *ShmemAllocRaw(Size size, Size *allocated_size);
+static void *ShmemAllocUnlocked(Size size);
 
 /* shared memory global variables */
 
@@ -234,7 +235,7 @@ ShmemAllocRaw(Size size, Size *allocated_size)
  *
  * We consider maxalign, rather than cachealign, sufficient here.
  */
-void *
+static void *
 ShmemAllocUnlocked(Size size)
 {
 	Size		newStart;

src/backend/storage/lmgr/proc.c

@@ -145,6 +145,7 @@ ProcGlobalShmemSize(void)
 	size = add_size(size, sizeof(PROC_HDR));
 	size = add_size(size, sizeof(slock_t));
 
+	size = add_size(size, PGSemaphoreShmemSize(ProcGlobalSemas()));
 	size = add_size(size, PGProcShmemSize());
 	size = add_size(size, FastPathLockShmemSize());
 
@@ -287,6 +288,9 @@ InitProcGlobal(void)
 	/* For asserts checking we did not overflow. */
 	fpEndPtr = fpPtr + requestSize;
 
+	/* Reserve space for semaphores. */
+	PGReserveSemaphores(ProcGlobalSemas());
+
 	for (i = 0; i < TotalProcs; i++)
 	{
 		PGPROC	   *proc = &procs[i];

src/include/storage/ipc.h

@@ -77,7 +77,7 @@ extern void check_on_shmem_exit_lists_are_empty(void);
 /* ipci.c */
 extern PGDLLIMPORT shmem_startup_hook_type shmem_startup_hook;
 
-extern Size CalculateShmemSize(int *num_semaphores);
+extern Size CalculateShmemSize(void);
 extern void CreateSharedMemoryAndSemaphores(void);
 #ifdef EXEC_BACKEND
 extern void AttachSharedMemoryStructs(void);

src/include/storage/shmem.h

@@ -33,7 +33,6 @@ extern void InitShmemAccess(PGShmemHeader *seghdr);
 extern void InitShmemAllocation(void);
 extern void *ShmemAlloc(Size size);
 extern void *ShmemAllocNoError(Size size);
-extern void *ShmemAllocUnlocked(Size size);
 extern bool ShmemAddrIsValid(const void *addr);
 extern void InitShmemIndex(void);
 extern HTAB *ShmemInitHash(const char *name, int64 init_size, int64 max_size,