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Sourcecode: postgresql-8.4 version File versions

sysv_sema.c

/*-------------------------------------------------------------------------
 *
 * sysv_sema.c
 *      Implement PGSemaphores using SysV semaphore facilities
 *
 *
 * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *      $PostgreSQL$
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <signal.h>
#include <unistd.h>
#include <sys/file.h>
#ifdef HAVE_SYS_IPC_H
#include <sys/ipc.h>
#endif
#ifdef HAVE_SYS_SEM_H
#include <sys/sem.h>
#endif
#ifdef HAVE_KERNEL_OS_H
#include <kernel/OS.h>
#endif

#include "miscadmin.h"
#include "storage/ipc.h"
#include "storage/pg_sema.h"


#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 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)
      {
            /*
             * 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 (errno == EEXIST || errno == EACCES
#ifdef EIDRM
                  || 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),
                         (errno == ENOSPC) ?
                         errhint("This error does *not* mean that you have run out of disk space.\n"
              "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 "
                                     "(currently %d).\n"
                    "The PostgreSQL documentation contains more information about "
                                     "configuring your system for PostgreSQL.",
                                     MaxBackends) : 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)
            ereport(FATAL,
                        (errmsg_internal("semctl(%d, %d, SETVAL, %d) failed: %m",
                                                 semId, semNum, value),
                         (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;
}


/*
 * 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.
 *
 * The port number is passed for possible use as a key (for SysV, we use
 * it to generate the starting semaphore key).  In a standalone backend,
 * zero will be passed.
 *
 * In the SysV implementation, we acquire semaphore sets on-demand; the
 * maxSemas parameter is just used to size the array that keeps track of
 * acquired sets for subsequent releasing.
 */
void
PGReserveSemaphores(int maxSemas, int port)
{
      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 = port * 1000;
      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
 *
 * Initialize a PGSemaphore structure to represent a sema with count 1
 */
void
PGSemaphoreCreate(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;
      }
      /* 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);
}

/*
 * 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, bool interruptOK)
{
      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.
       *
       * Each time around the loop, we check for a cancel/die interrupt.  On
       * some platforms, if such an interrupt comes in while we are waiting,
       * it will cause the semop() call to exit with errno == EINTR, allowing
       * us to service the interrupt (if not in a critical section already)
       * during the next loop iteration.
       *
       * Once we acquire the lock, we do NOT check for an interrupt before
       * returning.  The caller needs to be able to record ownership of the lock
       * before any interrupt can be accepted.
       *
       * There is a window of a few instructions between CHECK_FOR_INTERRUPTS
       * and entering the semop() call.  If a cancel/die interrupt occurs in
       * that window, we would fail to notice it until after we acquire the lock
       * (or get another interrupt to escape the semop()).  We can avoid this
       * problem by temporarily setting ImmediateInterruptOK to true before we
       * do CHECK_FOR_INTERRUPTS; then, a die() interrupt in this interval will
       * execute directly.  However, there is a huge pitfall: there is another
       * window of a few instructions after the semop() before we are able to
       * reset ImmediateInterruptOK.      If an interrupt occurs then, we'll lose
       * control, which means that the lock has been acquired but our caller did
       * not get a chance to record the fact. Therefore, we only set
       * ImmediateInterruptOK if the caller tells us it's OK to do so, ie, the
       * caller does not need to record acquiring the lock.  (This is currently
       * true for lockmanager locks, since the process that granted us the lock
       * did all the necessary state updates. It's not true for SysV semaphores
       * used to implement LW locks or emulate spinlocks --- but the wait time
       * for such locks should not be very long, anyway.)
       *
       * On some platforms, signals marked SA_RESTART (which is most, for us)
       * will not interrupt the semop(); it will just keep waiting.  Therefore
       * it's necessary for cancel/die interrupts to be serviced directly by
       * the signal handler.  On these platforms the behavior is really the same
       * whether the signal arrives just before the semop() begins, or while it
       * is waiting.  The loop on EINTR is thus important only for other types
       * of interrupts.
       */
      do
      {
            ImmediateInterruptOK = interruptOK;
            CHECK_FOR_INTERRUPTS();
            errStatus = semop(sema->semId, &sops, 1);
            ImmediateInterruptOK = false;
      } 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;
}

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