source: branches/minix3-book/servers/fs/lock.c@ 25

/* This file handles advisory file locking as required by POSIX.
 *
 * The entry points into this file are
 *   lock_op:   perform locking operations for FCNTL system call
 *   lock_revive: revive processes when a lock is released
 */

#include "fs.h"
#include <minix/com.h>
#include <fcntl.h>
#include <unistd.h>
#include "file.h"
#include "fproc.h"
#include "inode.h"
#include "lock.h"
#include "param.h"

/*===========================================================================*
 *                              lock_op                                      *
 *===========================================================================*/
PUBLIC int lock_op(f, req)
struct filp *f;
int req;                        /* either F_SETLK or F_SETLKW */
{
/* Perform the advisory locking required by POSIX. */

  int r, ltype, i, conflict = 0, unlocking = 0;
  mode_t mo;
  off_t first, last;
  struct flock flock;
  vir_bytes user_flock;
  struct file_lock *flp, *flp2, *empty;

  /* Fetch the flock structure from user space. */
  user_flock = (vir_bytes) m_in.name1;
  r = sys_datacopy(who, (vir_bytes) user_flock,
        FS_PROC_NR, (vir_bytes) &flock, (phys_bytes) sizeof(flock));
  if (r != OK) return(EINVAL);

  /* Make some error checks. */
  ltype = flock.l_type;
  mo = f->filp_mode;
  if (ltype != F_UNLCK && ltype != F_RDLCK && ltype != F_WRLCK) return(EINVAL);
  if (req == F_GETLK && ltype == F_UNLCK) return(EINVAL);
  if ( (f->filp_ino->i_mode & I_TYPE) != I_REGULAR) return(EINVAL);
  if (req != F_GETLK && ltype == F_RDLCK && (mo & R_BIT) == 0) return(EBADF);
  if (req != F_GETLK && ltype == F_WRLCK && (mo & W_BIT) == 0) return(EBADF);

  /* Compute the first and last bytes in the lock region. */
  switch (flock.l_whence) {
        case SEEK_SET:  first = 0; break;
        case SEEK_CUR:  first = f->filp_pos; break;
        case SEEK_END:  first = f->filp_ino->i_size; break;
        default:        return(EINVAL);
  }
  /* Check for overflow. */
  if (((long)flock.l_start > 0) && ((first + flock.l_start) < first))
        return(EINVAL);
  if (((long)flock.l_start < 0) && ((first + flock.l_start) > first))
        return(EINVAL);
  first = first + flock.l_start;
  last = first + flock.l_len - 1;
  if (flock.l_len == 0) last = MAX_FILE_POS;
  if (last < first) return(EINVAL);

  /* Check if this region conflicts with any existing lock. */
  empty = (struct file_lock *) 0;
  for (flp = &file_lock[0]; flp < & file_lock[NR_LOCKS]; flp++) {
        if (flp->lock_type == 0) {
                if (empty == (struct file_lock *) 0) empty = flp;
                continue;       /* 0 means unused slot */
        }
        if (flp->lock_inode != f->filp_ino) continue;   /* different file */
        if (last < flp->lock_first) continue;   /* new one is in front */
        if (first > flp->lock_last) continue;   /* new one is afterwards */
        if (ltype == F_RDLCK && flp->lock_type == F_RDLCK) continue;
        if (ltype != F_UNLCK && flp->lock_pid == fp->fp_pid) continue;
  
        /* There might be a conflict.  Process it. */
        conflict = 1;
        if (req == F_GETLK) break;

        /* If we are trying to set a lock, it just failed. */
        if (ltype == F_RDLCK || ltype == F_WRLCK) {
                if (req == F_SETLK) {
                        /* For F_SETLK, just report back failure. */
                        return(EAGAIN);
                } else {
                        /* For F_SETLKW, suspend the process. */
                        suspend(XLOCK);
                        return(SUSPEND);
                }
        }

        /* We are clearing a lock and we found something that overlaps. */
        unlocking = 1;
        if (first <= flp->lock_first && last >= flp->lock_last) {
                flp->lock_type = 0;     /* mark slot as unused */
                nr_locks--;             /* number of locks is now 1 less */
                continue;
        }

        /* Part of a locked region has been unlocked. */
        if (first <= flp->lock_first) {
                flp->lock_first = last + 1;
                continue;
        }

        if (last >= flp->lock_last) {
                flp->lock_last = first - 1;
                continue;
        }
        
        /* Bad luck. A lock has been split in two by unlocking the middle. */
        if (nr_locks == NR_LOCKS) return(ENOLCK);
        for (i = 0; i < NR_LOCKS; i++)
                if (file_lock[i].lock_type == 0) break;
        flp2 = &file_lock[i];
        flp2->lock_type = flp->lock_type;
        flp2->lock_pid = flp->lock_pid;
        flp2->lock_inode = flp->lock_inode;
        flp2->lock_first = last + 1;
        flp2->lock_last = flp->lock_last;
        flp->lock_last = first - 1;
        nr_locks++;
  }
  if (unlocking) lock_revive();

  if (req == F_GETLK) {
        if (conflict) {
                /* GETLK and conflict. Report on the conflicting lock. */
                flock.l_type = flp->lock_type;
                flock.l_whence = SEEK_SET;
                flock.l_start = flp->lock_first;
                flock.l_len = flp->lock_last - flp->lock_first + 1;
                flock.l_pid = flp->lock_pid;

        } else {
                /* It is GETLK and there is no conflict. */
                flock.l_type = F_UNLCK;
        }

        /* Copy the flock structure back to the caller. */
        r = sys_datacopy(FS_PROC_NR, (vir_bytes) &flock,
                who, (vir_bytes) user_flock, (phys_bytes) sizeof(flock));
        return(r);
  }

  if (ltype == F_UNLCK) return(OK);     /* unlocked a region with no locks */

  /* There is no conflict.  If space exists, store new lock in the table. */
  if (empty == (struct file_lock *) 0) return(ENOLCK);  /* table full */
  empty->lock_type = ltype;
  empty->lock_pid = fp->fp_pid;
  empty->lock_inode = f->filp_ino;
  empty->lock_first = first;
  empty->lock_last = last;
  nr_locks++;
  return(OK);
}

/*===========================================================================*
 *                              lock_revive                                  *
 *===========================================================================*/
PUBLIC void lock_revive()
{
/* Go find all the processes that are waiting for any kind of lock and 
 * revive them all.  The ones that are still blocked will block again when 
 * they run.  The others will complete.  This strategy is a space-time 
 * tradeoff.  Figuring out exactly which ones to unblock now would take 
 * extra code, and the only thing it would win would be some performance in 
 * extremely rare circumstances (namely, that somebody actually used 
 * locking).
 */

  int task;
  struct fproc *fptr;

  for (fptr = &fproc[INIT_PROC_NR + 1]; fptr < &fproc[NR_PROCS]; fptr++){
        task = -fptr->fp_task;
        if (fptr->fp_suspended == SUSPENDED && task == XLOCK) {
                revive( (int) (fptr - fproc), 0);
        }
  }
}