sysv_sema.c

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/*-------------------------------------------------------------------------
 *
 * sysv_sema.c
 *    Implement PGSemaphores using SysV semaphore facilities
 *
 *
 * Portions Copyright (c) 1996-2025, 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.)
 *
 * The present value of 19 is chosen with one eye on NetBSD/OpenBSD's default
 * SEMMNS setting of 60.  Remembering the extra sema per set, this lets us
 * allocate three sets with 57 useful semaphores before exceeding that, which
 * is enough to run our core regression tests.  Users of those systems will
 * still want to raise SEMMNS for any sort of production work, though.
 */
#define SEMAS_PER_SET   19
 
#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.
     */
    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;
}