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/*-------------------------------------------------------------------------
*
* sysv_sema.c
* Implement PGSemaphores using SysV semaphore facilities
*
*
* Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
* src/backend/port/sysv_sema.c
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include <signal.h>
#include <unistd.h>
#include <sys/file.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/stat.h>
#include "miscadmin.h"
#include "storage/ipc.h"
#include "storage/pg_sema.h"
#include "storage/shmem.h"
typedef struct PGSemaphoreData
{
int semId; /* semaphore set identifier */
int semNum; /* semaphore number within set */
} PGSemaphoreData;
#ifndef HAVE_UNION_SEMUN
union semun
{
int val;
struct semid_ds *buf;
unsigned short *array;
};
#endif
typedef key_t IpcSemaphoreKey; /* semaphore key passed to semget(2) */
typedef int IpcSemaphoreId; /* semaphore ID returned by semget(2) */
/*
* SEMAS_PER_SET is the number of useful semaphores in each semaphore set
* we allocate. It must be *less than* your kernel's SEMMSL (max semaphores
* per set) parameter, which is often around 25. (Less than, because we
* allocate one extra sema in each set for identification purposes.)
*/
#define SEMAS_PER_SET 16
#define IPCProtection (0600) /* access/modify by user only */
#define PGSemaMagic 537 /* must be less than SEMVMX */
static PGSemaphore sharedSemas; /* array of PGSemaphoreData in shared memory */
static int numSharedSemas; /* number of PGSemaphoreDatas used so far */
static int maxSharedSemas; /* allocated size of PGSemaphoreData array */
static IpcSemaphoreId *mySemaSets; /* IDs of sema sets acquired so far */
static int numSemaSets; /* number of sema sets acquired so far */
static int maxSemaSets; /* allocated size of mySemaSets array */
static IpcSemaphoreKey nextSemaKey; /* next key to try using */
static int nextSemaNumber; /* next free sem num in last sema set */
static IpcSemaphoreId InternalIpcSemaphoreCreate(IpcSemaphoreKey semKey,
int numSems);
static void IpcSemaphoreInitialize(IpcSemaphoreId semId, int semNum,
int value);
static void IpcSemaphoreKill(IpcSemaphoreId semId);
static int IpcSemaphoreGetValue(IpcSemaphoreId semId, int semNum);
static pid_t IpcSemaphoreGetLastPID(IpcSemaphoreId semId, int semNum);
static IpcSemaphoreId IpcSemaphoreCreate(int numSems);
static void ReleaseSemaphores(int status, Datum arg);
/*
* InternalIpcSemaphoreCreate
*
* Attempt to create a new semaphore set with the specified key.
* Will fail (return -1) if such a set already exists.
*
* If we fail with a failure code other than collision-with-existing-set,
* print out an error and abort. Other types of errors suggest nonrecoverable
* problems.
*/
static IpcSemaphoreId
InternalIpcSemaphoreCreate(IpcSemaphoreKey semKey, int numSems)
{
int semId;
semId = semget(semKey, numSems, IPC_CREAT | IPC_EXCL | IPCProtection);
if (semId < 0)
{
int saved_errno = errno;
/*
* Fail quietly if error indicates a collision with existing set. One
* would expect EEXIST, given that we said IPC_EXCL, but perhaps we
* could get a permission violation instead? Also, EIDRM might occur
* if an old set is slated for destruction but not gone yet.
*/
if (saved_errno == EEXIST || saved_errno == EACCES
#ifdef EIDRM
|| saved_errno == EIDRM
#endif
)
return -1;
/*
* Else complain and abort
*/
ereport(FATAL,
(errmsg("could not create semaphores: %m"),
errdetail("Failed system call was semget(%lu, %d, 0%o).",
(unsigned long) semKey, numSems,
IPC_CREAT | IPC_EXCL | IPCProtection),
(saved_errno == ENOSPC) ?
errhint("This error does *not* mean that you have run out of disk space. "
"It occurs when either the system limit for the maximum number of "
"semaphore sets (SEMMNI), or the system wide maximum number of "
"semaphores (SEMMNS), would be exceeded. You need to raise the "
"respective kernel parameter. Alternatively, reduce PostgreSQL's "
"consumption of semaphores by reducing its max_connections parameter.\n"
"The PostgreSQL documentation contains more information about "
"configuring your system for PostgreSQL.") : 0));
}
return semId;
}
/*
* Initialize a semaphore to the specified value.
*/
static void
IpcSemaphoreInitialize(IpcSemaphoreId semId, int semNum, int value)
{
union semun semun;
semun.val = value;
if (semctl(semId, semNum, SETVAL, semun) < 0)
{
int saved_errno = errno;
ereport(FATAL,
(errmsg_internal("semctl(%d, %d, SETVAL, %d) failed: %m",
semId, semNum, value),
(saved_errno == ERANGE) ?
errhint("You possibly need to raise your kernel's SEMVMX value to be at least "
"%d. Look into the PostgreSQL documentation for details.",
value) : 0));
}
}
/*
* IpcSemaphoreKill(semId) - removes a semaphore set
*/
static void
IpcSemaphoreKill(IpcSemaphoreId semId)
{
union semun semun;
semun.val = 0; /* unused, but keep compiler quiet */
if (semctl(semId, 0, IPC_RMID, semun) < 0)
elog(LOG, "semctl(%d, 0, IPC_RMID, ...) failed: %m", semId);
}
/* Get the current value (semval) of the semaphore */
static int
IpcSemaphoreGetValue(IpcSemaphoreId semId, int semNum)
{
union semun dummy; /* for Solaris */
dummy.val = 0; /* unused */
return semctl(semId, semNum, GETVAL, dummy);
}
/* Get the PID of the last process to do semop() on the semaphore */
static pid_t
IpcSemaphoreGetLastPID(IpcSemaphoreId semId, int semNum)
{
union semun dummy; /* for Solaris */
dummy.val = 0; /* unused */
return semctl(semId, semNum, GETPID, dummy);
}
/*
* Create a semaphore set with the given number of useful semaphores
* (an additional sema is actually allocated to serve as identifier).
* Dead Postgres sema sets are recycled if found, but we do not fail
* upon collision with non-Postgres sema sets.
*
* The idea here is to detect and re-use keys that may have been assigned
* by a crashed postmaster or backend.
*/
static IpcSemaphoreId
IpcSemaphoreCreate(int numSems)
{
IpcSemaphoreId semId;
union semun semun;
PGSemaphoreData mysema;
/* Loop till we find a free IPC key */
for (nextSemaKey++;; nextSemaKey++)
{
pid_t creatorPID;
/* Try to create new semaphore set */
semId = InternalIpcSemaphoreCreate(nextSemaKey, numSems + 1);
if (semId >= 0)
break; /* successful create */
/* See if it looks to be leftover from a dead Postgres process */
semId = semget(nextSemaKey, numSems + 1, 0);
if (semId < 0)
continue; /* failed: must be some other app's */
if (IpcSemaphoreGetValue(semId, numSems) != PGSemaMagic)
continue; /* sema belongs to a non-Postgres app */
/*
* If the creator PID is my own PID or does not belong to any extant
* process, it's safe to zap it.
*/
creatorPID = IpcSemaphoreGetLastPID(semId, numSems);
if (creatorPID <= 0)
continue; /* oops, GETPID failed */
if (creatorPID != getpid())
{
if (kill(creatorPID, 0) == 0 || errno != ESRCH)
continue; /* sema belongs to a live process */
}
/*
* The sema set appears to be from a dead Postgres process, or from a
* previous cycle of life in this same process. Zap it, if possible.
* This probably shouldn't fail, but if it does, assume the sema set
* belongs to someone else after all, and continue quietly.
*/
semun.val = 0; /* unused, but keep compiler quiet */
if (semctl(semId, 0, IPC_RMID, semun) < 0)
continue;
/*
* Now try again to create the sema set.
*/
semId = InternalIpcSemaphoreCreate(nextSemaKey, numSems + 1);
if (semId >= 0)
break; /* successful create */
/*
* Can only get here if some other process managed to create the same
* sema key before we did. Let him have that one, loop around to try
* next key.
*/
}
/*
* OK, we created a new sema set. Mark it as created by this process. We
* do this by setting the spare semaphore to PGSemaMagic-1 and then
* incrementing it with semop(). That leaves it with value PGSemaMagic
* and sempid referencing this process.
*/
IpcSemaphoreInitialize(semId, numSems, PGSemaMagic - 1);
mysema.semId = semId;
mysema.semNum = numSems;
PGSemaphoreUnlock(&mysema);
return semId;
}
/*
* Report amount of shared memory needed for semaphores
*/
Size
PGSemaphoreShmemSize(int maxSemas)
{
return mul_size(maxSemas, sizeof(PGSemaphoreData));
}
/*
* PGReserveSemaphores --- initialize semaphore support
*
* This is called during postmaster start or shared memory reinitialization.
* It should do whatever is needed to be able to support up to maxSemas
* subsequent PGSemaphoreCreate calls. Also, if any system resources
* are acquired here or in PGSemaphoreCreate, register an on_shmem_exit
* callback to release them.
*
* In the SysV implementation, we acquire semaphore sets on-demand; the
* maxSemas parameter is just used to size the arrays. There is an array
* of PGSemaphoreData structs in shared memory, and a postmaster-local array
* with one entry per SysV semaphore set, which we use for releasing the
* semaphore sets when done. (This design ensures that postmaster shutdown
* doesn't rely on the contents of shared memory, which a failed backend might
* have clobbered.)
*/
void
PGReserveSemaphores(int maxSemas)
{
struct stat statbuf;
/*
* We use the data directory's inode number to seed the search for free
* semaphore keys. This minimizes the odds of collision with other
* postmasters, while maximizing the odds that we will detect and clean up
* semaphores left over from a crashed postmaster in our own directory.
*/
if (stat(DataDir, &statbuf) < 0)
ereport(FATAL,
(errcode_for_file_access(),
errmsg("could not stat data directory \"%s\": %m",
DataDir)));
/*
* We must use ShmemAllocUnlocked(), since the spinlock protecting
* ShmemAlloc() won't be ready yet. (This ordering is necessary when we
* are emulating spinlocks with semaphores.)
*/
sharedSemas = (PGSemaphore)
ShmemAllocUnlocked(PGSemaphoreShmemSize(maxSemas));
numSharedSemas = 0;
maxSharedSemas = maxSemas;
maxSemaSets = (maxSemas + SEMAS_PER_SET - 1) / SEMAS_PER_SET;
mySemaSets = (IpcSemaphoreId *)
malloc(maxSemaSets * sizeof(IpcSemaphoreId));
if (mySemaSets == NULL)
elog(PANIC, "out of memory");
numSemaSets = 0;
nextSemaKey = statbuf.st_ino;
nextSemaNumber = SEMAS_PER_SET; /* force sema set alloc on 1st call */
on_shmem_exit(ReleaseSemaphores, 0);
}
/*
* Release semaphores at shutdown or shmem reinitialization
*
* (called as an on_shmem_exit callback, hence funny argument list)
*/
static void
ReleaseSemaphores(int status, Datum arg)
{
int i;
for (i = 0; i < numSemaSets; i++)
IpcSemaphoreKill(mySemaSets[i]);
free(mySemaSets);
}
/*
* PGSemaphoreCreate
*
* Allocate a PGSemaphore structure with initial count 1
*/
PGSemaphore
PGSemaphoreCreate(void)
{
PGSemaphore sema;
/* Can't do this in a backend, because static state is postmaster's */
Assert(!IsUnderPostmaster);
if (nextSemaNumber >= SEMAS_PER_SET)
{
/* Time to allocate another semaphore set */
if (numSemaSets >= maxSemaSets)
elog(PANIC, "too many semaphores created");
mySemaSets[numSemaSets] = IpcSemaphoreCreate(SEMAS_PER_SET);
numSemaSets++;
nextSemaNumber = 0;
}
/* Use the next shared PGSemaphoreData */
if (numSharedSemas >= maxSharedSemas)
elog(PANIC, "too many semaphores created");
sema = &sharedSemas[numSharedSemas++];
/* Assign the next free semaphore in the current set */
sema->semId = mySemaSets[numSemaSets - 1];
sema->semNum = nextSemaNumber++;
/* Initialize it to count 1 */
IpcSemaphoreInitialize(sema->semId, sema->semNum, 1);
return sema;
}
/*
* PGSemaphoreReset
*
* Reset a previously-initialized PGSemaphore to have count 0
*/
void
PGSemaphoreReset(PGSemaphore sema)
{
IpcSemaphoreInitialize(sema->semId, sema->semNum, 0);
}
/*
* PGSemaphoreLock
*
* Lock a semaphore (decrement count), blocking if count would be < 0
*/
void
PGSemaphoreLock(PGSemaphore sema)
{
int errStatus;
struct sembuf sops;
sops.sem_op = -1; /* decrement */
sops.sem_flg = 0;
sops.sem_num = sema->semNum;
/*
* Note: if errStatus is -1 and errno == EINTR then it means we returned
* from the operation prematurely because we were sent a signal. So we
* try and lock the semaphore again.
*
* We used to check interrupts here, but that required servicing
* interrupts directly from signal handlers. Which is hard to do safely
* and portably.
*/
do
{
errStatus = semop(sema->semId, &sops, 1);
} while (errStatus < 0 && errno == EINTR);
if (errStatus < 0)
elog(FATAL, "semop(id=%d) failed: %m", sema->semId);
}
/*
* PGSemaphoreUnlock
*
* Unlock a semaphore (increment count)
*/
void
PGSemaphoreUnlock(PGSemaphore sema)
{
int errStatus;
struct sembuf sops;
sops.sem_op = 1; /* increment */
sops.sem_flg = 0;
sops.sem_num = sema->semNum;
/*
* Note: if errStatus is -1 and errno == EINTR then it means we returned
* from the operation prematurely because we were sent a signal. So we
* try and unlock the semaphore again. Not clear this can really happen,
* but might as well cope.
*/
do
{
errStatus = semop(sema->semId, &sops, 1);
} while (errStatus < 0 && errno == EINTR);
if (errStatus < 0)
elog(FATAL, "semop(id=%d) failed: %m", sema->semId);
}
/*
* PGSemaphoreTryLock
*
* Lock a semaphore only if able to do so without blocking
*/
bool
PGSemaphoreTryLock(PGSemaphore sema)
{
int errStatus;
struct sembuf sops;
sops.sem_op = -1; /* decrement */
sops.sem_flg = IPC_NOWAIT; /* but don't block */
sops.sem_num = sema->semNum;
/*
* Note: if errStatus is -1 and errno == EINTR then it means we returned
* from the operation prematurely because we were sent a signal. So we
* try and lock the semaphore again.
*/
do
{
errStatus = semop(sema->semId, &sops, 1);
} while (errStatus < 0 && errno == EINTR);
if (errStatus < 0)
{
/* Expect EAGAIN or EWOULDBLOCK (platform-dependent) */
#ifdef EAGAIN
if (errno == EAGAIN)
return false; /* failed to lock it */
#endif
#if defined(EWOULDBLOCK) && (!defined(EAGAIN) || (EWOULDBLOCK != EAGAIN))
if (errno == EWOULDBLOCK)
return false; /* failed to lock it */
#endif
/* Otherwise we got trouble */
elog(FATAL, "semop(id=%d) failed: %m", sema->semId);
}
return true;
}
|