The Design and Implementation of the FreeBSD Operating System, Second Edition
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FreeBSD/Linux Kernel Cross Reference
sys/kern/kern_acct.c

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    1 /*-
    2  * Copyright (c) 1982, 1986, 1989, 1993
    3  *      The Regents of the University of California.  All rights reserved.
    4  * (c) UNIX System Laboratories, Inc.
    5  * Copyright (c) 2005 Robert N. M. Watson
    6  * All rights reserved.
    7  *
    8  * All or some portions of this file are derived from material licensed
    9  * to the University of California by American Telephone and Telegraph
   10  * Co. or Unix System Laboratories, Inc. and are reproduced herein with
   11  * the permission of UNIX System Laboratories, Inc.
   12  *
   13  * Redistribution and use in source and binary forms, with or without
   14  * modification, are permitted provided that the following conditions
   15  * are met:
   16  * 1. Redistributions of source code must retain the above copyright
   17  *    notice, this list of conditions and the following disclaimer.
   18  * 2. Redistributions in binary form must reproduce the above copyright
   19  *    notice, this list of conditions and the following disclaimer in the
   20  *    documentation and/or other materials provided with the distribution.
   21  * 4. Neither the name of the University nor the names of its contributors
   22  *    may be used to endorse or promote products derived from this software
   23  *    without specific prior written permission.
   24  *
   25  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
   26  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   27  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   28  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
   29  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   30  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   31  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   32  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   33  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   34  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   35  * SUCH DAMAGE.
   36  *
   37  * Copyright (c) 1994 Christopher G. Demetriou
   38  *
   39  * Redistribution and use in source and binary forms, with or without
   40  * modification, are permitted provided that the following conditions
   41  * are met:
   42  * 1. Redistributions of source code must retain the above copyright
   43  *    notice, this list of conditions and the following disclaimer.
   44  * 2. Redistributions in binary form must reproduce the above copyright
   45  *    notice, this list of conditions and the following disclaimer in the
   46  *    documentation and/or other materials provided with the distribution.
   47  * 3. All advertising materials mentioning features or use of this software
   48  *    must display the following acknowledgement:
   49  *      This product includes software developed by the University of
   50  *      California, Berkeley and its contributors.
   51  * 4. Neither the name of the University nor the names of its contributors
   52  *    may be used to endorse or promote products derived from this software
   53  *    without specific prior written permission.
   54  *
   55  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
   56  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   57  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   58  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
   59  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   60  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   61  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   62  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   63  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   64  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   65  * SUCH DAMAGE.
   66  *
   67  *      @(#)kern_acct.c 8.1 (Berkeley) 6/14/93
   68  */
   69 
   70 #include <sys/cdefs.h>
   71 __FBSDID("$FreeBSD: releng/11.1/sys/kern/kern_acct.c 311957 2017-01-12 01:09:15Z kib $");
   72 
   73 #include <sys/param.h>
   74 #include <sys/systm.h>
   75 #include <sys/acct.h>
   76 #include <sys/fcntl.h>
   77 #include <sys/kernel.h>
   78 #include <sys/kthread.h>
   79 #include <sys/limits.h>
   80 #include <sys/lock.h>
   81 #include <sys/malloc.h>
   82 #include <sys/mount.h>
   83 #include <sys/mutex.h>
   84 #include <sys/namei.h>
   85 #include <sys/priv.h>
   86 #include <sys/proc.h>
   87 #include <sys/resourcevar.h>
   88 #include <sys/sched.h>
   89 #include <sys/sx.h>
   90 #include <sys/sysctl.h>
   91 #include <sys/sysent.h>
   92 #include <sys/syslog.h>
   93 #include <sys/sysproto.h>
   94 #include <sys/tty.h>
   95 #include <sys/vnode.h>
   96 
   97 #include <security/mac/mac_framework.h>
   98 
   99 /*
  100  * The routines implemented in this file are described in:
  101  *      Leffler, et al.: The Design and Implementation of the 4.3BSD
  102  *          UNIX Operating System (Addison Welley, 1989)
  103  * on pages 62-63.
  104  * On May 2007 the historic 3 bits base 8 exponent, 13 bit fraction
  105  * compt_t representation described in the above reference was replaced
  106  * with that of IEEE-754 floats.
  107  *
  108  * Arguably, to simplify accounting operations, this mechanism should
  109  * be replaced by one in which an accounting log file (similar to /dev/klog)
  110  * is read by a user process, etc.  However, that has its own problems.
  111  */
  112 
  113 /* Floating point definitions from <float.h>. */
  114 #define FLT_MANT_DIG    24              /* p */
  115 #define FLT_MAX_EXP     128             /* emax */
  116 
  117 /*
  118  * Internal accounting functions.
  119  * The former's operation is described in Leffler, et al., and the latter
  120  * was provided by UCB with the 4.4BSD-Lite release
  121  */
  122 static uint32_t encode_timeval(struct timeval);
  123 static uint32_t encode_long(long);
  124 static void     acctwatch(void);
  125 static void     acct_thread(void *);
  126 static int      acct_disable(struct thread *, int);
  127 
  128 /*
  129  * Accounting vnode pointer, saved vnode pointer, and flags for each.
  130  * acct_sx protects against changes to the active vnode and credentials
  131  * while accounting records are being committed to disk.
  132  */
  133 static int               acct_configured;
  134 static int               acct_suspended;
  135 static struct vnode     *acct_vp;
  136 static struct ucred     *acct_cred;
  137 static struct plimit    *acct_limit;
  138 static int               acct_flags;
  139 static struct sx         acct_sx;
  140 
  141 SX_SYSINIT(acct, &acct_sx, "acct_sx");
  142 
  143 /*
  144  * State of the accounting kthread.
  145  */
  146 static int               acct_state;
  147 
  148 #define ACCT_RUNNING    1       /* Accounting kthread is running. */
  149 #define ACCT_EXITREQ    2       /* Accounting kthread should exit. */
  150 
  151 /*
  152  * Values associated with enabling and disabling accounting
  153  */
  154 static int acctsuspend = 2;     /* stop accounting when < 2% free space left */
  155 SYSCTL_INT(_kern, OID_AUTO, acct_suspend, CTLFLAG_RW,
  156         &acctsuspend, 0, "percentage of free disk space below which accounting stops");
  157 
  158 static int acctresume = 4;      /* resume when free space risen to > 4% */
  159 SYSCTL_INT(_kern, OID_AUTO, acct_resume, CTLFLAG_RW,
  160         &acctresume, 0, "percentage of free disk space above which accounting resumes");
  161 
  162 static int acctchkfreq = 15;    /* frequency (in seconds) to check space */
  163 
  164 static int
  165 sysctl_acct_chkfreq(SYSCTL_HANDLER_ARGS)
  166 {
  167         int error, value;
  168 
  169         /* Write out the old value. */
  170         error = SYSCTL_OUT(req, &acctchkfreq, sizeof(int));
  171         if (error || req->newptr == NULL)
  172                 return (error);
  173 
  174         /* Read in and verify the new value. */
  175         error = SYSCTL_IN(req, &value, sizeof(int));
  176         if (error)
  177                 return (error);
  178         if (value <= 0)
  179                 return (EINVAL);
  180         acctchkfreq = value;
  181         return (0);
  182 }
  183 SYSCTL_PROC(_kern, OID_AUTO, acct_chkfreq, CTLTYPE_INT|CTLFLAG_RW,
  184     &acctchkfreq, 0, sysctl_acct_chkfreq, "I",
  185     "frequency for checking the free space");
  186 
  187 SYSCTL_INT(_kern, OID_AUTO, acct_configured, CTLFLAG_RD, &acct_configured, 0,
  188         "Accounting configured or not");
  189 
  190 SYSCTL_INT(_kern, OID_AUTO, acct_suspended, CTLFLAG_RD, &acct_suspended, 0,
  191         "Accounting suspended or not");
  192 
  193 /*
  194  * Accounting system call.  Written based on the specification and previous
  195  * implementation done by Mark Tinguely.
  196  */
  197 int
  198 sys_acct(struct thread *td, struct acct_args *uap)
  199 {
  200         struct nameidata nd;
  201         int error, flags, i, replacing;
  202 
  203         error = priv_check(td, PRIV_ACCT);
  204         if (error)
  205                 return (error);
  206 
  207         /*
  208          * If accounting is to be started to a file, open that file for
  209          * appending and make sure it's a 'normal'.
  210          */
  211         if (uap->path != NULL) {
  212                 NDINIT(&nd, LOOKUP, NOFOLLOW | AUDITVNODE1,
  213                     UIO_USERSPACE, uap->path, td);
  214                 flags = FWRITE | O_APPEND;
  215                 error = vn_open(&nd, &flags, 0, NULL);
  216                 if (error)
  217                         return (error);
  218                 NDFREE(&nd, NDF_ONLY_PNBUF);
  219 #ifdef MAC
  220                 error = mac_system_check_acct(td->td_ucred, nd.ni_vp);
  221                 if (error) {
  222                         VOP_UNLOCK(nd.ni_vp, 0);
  223                         vn_close(nd.ni_vp, flags, td->td_ucred, td);
  224                         return (error);
  225                 }
  226 #endif
  227                 VOP_UNLOCK(nd.ni_vp, 0);
  228                 if (nd.ni_vp->v_type != VREG) {
  229                         vn_close(nd.ni_vp, flags, td->td_ucred, td);
  230                         return (EACCES);
  231                 }
  232 #ifdef MAC
  233         } else {
  234                 error = mac_system_check_acct(td->td_ucred, NULL);
  235                 if (error)
  236                         return (error);
  237 #endif
  238         }
  239 
  240         /*
  241          * Disallow concurrent access to the accounting vnode while we swap
  242          * it out, in order to prevent access after close.
  243          */
  244         sx_xlock(&acct_sx);
  245 
  246         /*
  247          * Don't log spurious disable/enable messages if we are
  248          * switching from one accounting file to another due to log
  249          * rotation.
  250          */
  251         replacing = (acct_vp != NULL && uap->path != NULL);
  252 
  253         /*
  254          * If accounting was previously enabled, kill the old space-watcher,
  255          * close the file, and (if no new file was specified, leave).  Reset
  256          * the suspended state regardless of whether accounting remains
  257          * enabled.
  258          */
  259         acct_suspended = 0;
  260         if (acct_vp != NULL)
  261                 error = acct_disable(td, !replacing);
  262         if (uap->path == NULL) {
  263                 if (acct_state & ACCT_RUNNING) {
  264                         acct_state |= ACCT_EXITREQ;
  265                         wakeup(&acct_state);
  266                 }
  267                 sx_xunlock(&acct_sx);
  268                 return (error);
  269         }
  270 
  271         /*
  272          * Create our own plimit object without limits. It will be assigned
  273          * to exiting processes.
  274          */
  275         acct_limit = lim_alloc();
  276         for (i = 0; i < RLIM_NLIMITS; i++)
  277                 acct_limit->pl_rlimit[i].rlim_cur =
  278                     acct_limit->pl_rlimit[i].rlim_max = RLIM_INFINITY;
  279 
  280         /*
  281          * Save the new accounting file vnode, and schedule the new
  282          * free space watcher.
  283          */
  284         acct_vp = nd.ni_vp;
  285         acct_cred = crhold(td->td_ucred);
  286         acct_flags = flags;
  287         if (acct_state & ACCT_RUNNING)
  288                 acct_state &= ~ACCT_EXITREQ;
  289         else {
  290                 /*
  291                  * Try to start up an accounting kthread.  We may start more
  292                  * than one, but if so the extras will commit suicide as
  293                  * soon as they start up.
  294                  */
  295                 error = kproc_create(acct_thread, NULL, NULL, 0, 0,
  296                     "accounting");
  297                 if (error) {
  298                         (void) acct_disable(td, 0);
  299                         sx_xunlock(&acct_sx);
  300                         log(LOG_NOTICE, "Unable to start accounting thread\n");
  301                         return (error);
  302                 }
  303         }
  304         acct_configured = 1;
  305         sx_xunlock(&acct_sx);
  306         if (!replacing)
  307                 log(LOG_NOTICE, "Accounting enabled\n");
  308         return (error);
  309 }
  310 
  311 /*
  312  * Disable currently in-progress accounting by closing the vnode, dropping
  313  * our reference to the credential, and clearing the vnode's flags.
  314  */
  315 static int
  316 acct_disable(struct thread *td, int logging)
  317 {
  318         int error;
  319 
  320         sx_assert(&acct_sx, SX_XLOCKED);
  321         error = vn_close(acct_vp, acct_flags, acct_cred, td);
  322         crfree(acct_cred);
  323         lim_free(acct_limit);
  324         acct_configured = 0;
  325         acct_vp = NULL;
  326         acct_cred = NULL;
  327         acct_flags = 0;
  328         if (logging)
  329                 log(LOG_NOTICE, "Accounting disabled\n");
  330         return (error);
  331 }
  332 
  333 /*
  334  * Write out process accounting information, on process exit.
  335  * Data to be written out is specified in Leffler, et al.
  336  * and are enumerated below.  (They're also noted in the system
  337  * "acct.h" header file.)
  338  */
  339 int
  340 acct_process(struct thread *td)
  341 {
  342         struct acctv2 acct;
  343         struct timeval ut, st, tmp;
  344         struct plimit *oldlim;
  345         struct proc *p;
  346         struct rusage ru;
  347         int t, ret;
  348 
  349         /*
  350          * Lockless check of accounting condition before doing the hard
  351          * work.
  352          */
  353         if (acct_vp == NULL || acct_suspended)
  354                 return (0);
  355 
  356         sx_slock(&acct_sx);
  357 
  358         /*
  359          * If accounting isn't enabled, don't bother.  Have to check again
  360          * once we own the lock in case we raced with disabling of accounting
  361          * by another thread.
  362          */
  363         if (acct_vp == NULL || acct_suspended) {
  364                 sx_sunlock(&acct_sx);
  365                 return (0);
  366         }
  367 
  368         p = td->td_proc;
  369 
  370         /*
  371          * Get process accounting information.
  372          */
  373 
  374         sx_slock(&proctree_lock);
  375         PROC_LOCK(p);
  376 
  377         /* (1) The terminal from which the process was started */
  378         if ((p->p_flag & P_CONTROLT) && p->p_pgrp->pg_session->s_ttyp)
  379                 acct.ac_tty = tty_udev(p->p_pgrp->pg_session->s_ttyp);
  380         else
  381                 acct.ac_tty = NODEV;
  382         sx_sunlock(&proctree_lock);
  383 
  384         /* (2) The name of the command that ran */
  385         bcopy(p->p_comm, acct.ac_comm, sizeof acct.ac_comm);
  386 
  387         /* (3) The amount of user and system time that was used */
  388         rufetchcalc(p, &ru, &ut, &st);
  389         acct.ac_utime = encode_timeval(ut);
  390         acct.ac_stime = encode_timeval(st);
  391 
  392         /* (4) The elapsed time the command ran (and its starting time) */
  393         getboottime(&tmp);
  394         timevaladd(&tmp, &p->p_stats->p_start);
  395         acct.ac_btime = tmp.tv_sec;
  396         microuptime(&tmp);
  397         timevalsub(&tmp, &p->p_stats->p_start);
  398         acct.ac_etime = encode_timeval(tmp);
  399 
  400         /* (5) The average amount of memory used */
  401         tmp = ut;
  402         timevaladd(&tmp, &st);
  403         /* Convert tmp (i.e. u + s) into hz units to match ru_i*. */
  404         t = tmp.tv_sec * hz + tmp.tv_usec / tick;
  405         if (t)
  406                 acct.ac_mem = encode_long((ru.ru_ixrss + ru.ru_idrss +
  407                     + ru.ru_isrss) / t);
  408         else
  409                 acct.ac_mem = 0;
  410 
  411         /* (6) The number of disk I/O operations done */
  412         acct.ac_io = encode_long(ru.ru_inblock + ru.ru_oublock);
  413 
  414         /* (7) The UID and GID of the process */
  415         acct.ac_uid = p->p_ucred->cr_ruid;
  416         acct.ac_gid = p->p_ucred->cr_rgid;
  417 
  418         /* (8) The boolean flags that tell how the process terminated, etc. */
  419         acct.ac_flagx = p->p_acflag;
  420 
  421         /* Setup ancillary structure fields. */
  422         acct.ac_flagx |= ANVER;
  423         acct.ac_zero = 0;
  424         acct.ac_version = 2;
  425         acct.ac_len = acct.ac_len2 = sizeof(acct);
  426 
  427         /*
  428          * Eliminate rlimits (file size limit in particular).
  429          */
  430         oldlim = p->p_limit;
  431         p->p_limit = lim_hold(acct_limit);
  432         PROC_UNLOCK(p);
  433         lim_free(oldlim);
  434 
  435         /*
  436          * Write the accounting information to the file.
  437          */
  438         ret = vn_rdwr(UIO_WRITE, acct_vp, (caddr_t)&acct, sizeof (acct),
  439             (off_t)0, UIO_SYSSPACE, IO_APPEND|IO_UNIT, acct_cred, NOCRED,
  440             NULL, td);
  441         sx_sunlock(&acct_sx);
  442         return (ret);
  443 }
  444 
  445 /* FLOAT_CONVERSION_START (Regression testing; don't remove this line.) */
  446 
  447 /* Convert timevals and longs into IEEE-754 bit patterns. */
  448 
  449 /* Mantissa mask (MSB is implied, so subtract 1). */
  450 #define MANT_MASK ((1 << (FLT_MANT_DIG - 1)) - 1)
  451 
  452 /*
  453  * We calculate integer values to a precision of approximately
  454  * 28 bits.
  455  * This is high-enough precision to fill the 24 float bits
  456  * and low-enough to avoid overflowing the 32 int bits.
  457  */
  458 #define CALC_BITS 28
  459 
  460 /* log_2(1000000). */
  461 #define LOG2_1M 20
  462 
  463 /*
  464  * Convert the elements of a timeval into a 32-bit word holding
  465  * the bits of a IEEE-754 float.
  466  * The float value represents the timeval's value in microsecond units.
  467  */
  468 static uint32_t
  469 encode_timeval(struct timeval tv)
  470 {
  471         int log2_s;
  472         int val, exp;   /* Unnormalized value and exponent */
  473         int norm_exp;   /* Normalized exponent */
  474         int shift;
  475 
  476         /*
  477          * First calculate value and exponent to about CALC_BITS precision.
  478          * Note that the following conditionals have been ordered so that
  479          * the most common cases appear first.
  480          */
  481         if (tv.tv_sec == 0) {
  482                 if (tv.tv_usec == 0)
  483                         return (0);
  484                 exp = 0;
  485                 val = tv.tv_usec;
  486         } else {
  487                 /*
  488                  * Calculate the value to a precision of approximately
  489                  * CALC_BITS.
  490                  */
  491                 log2_s = fls(tv.tv_sec) - 1;
  492                 if (log2_s + LOG2_1M < CALC_BITS) {
  493                         exp = 0;
  494                         val = 1000000 * tv.tv_sec + tv.tv_usec;
  495                 } else {
  496                         exp = log2_s + LOG2_1M - CALC_BITS;
  497                         val = (unsigned int)(((uint64_t)1000000 * tv.tv_sec +
  498                             tv.tv_usec) >> exp);
  499                 }
  500         }
  501         /* Now normalize and pack the value into an IEEE-754 float. */
  502         norm_exp = fls(val) - 1;
  503         shift = FLT_MANT_DIG - norm_exp - 1;
  504 #ifdef ACCT_DEBUG
  505         printf("val=%d exp=%d shift=%d log2(val)=%d\n",
  506             val, exp, shift, norm_exp);
  507         printf("exp=%x mant=%x\n", FLT_MAX_EXP - 1 + exp + norm_exp,
  508             ((shift > 0 ? (val << shift) : (val >> -shift)) & MANT_MASK));
  509 #endif
  510         return (((FLT_MAX_EXP - 1 + exp + norm_exp) << (FLT_MANT_DIG - 1)) |
  511             ((shift > 0 ? val << shift : val >> -shift) & MANT_MASK));
  512 }
  513 
  514 /*
  515  * Convert a non-negative long value into the bit pattern of
  516  * an IEEE-754 float value.
  517  */
  518 static uint32_t
  519 encode_long(long val)
  520 {
  521         int norm_exp;   /* Normalized exponent */
  522         int shift;
  523 
  524         if (val == 0)
  525                 return (0);
  526         if (val < 0) {
  527                 log(LOG_NOTICE,
  528                     "encode_long: negative value %ld in accounting record\n",
  529                     val);
  530                 val = LONG_MAX;
  531         }
  532         norm_exp = fls(val) - 1;
  533         shift = FLT_MANT_DIG - norm_exp - 1;
  534 #ifdef ACCT_DEBUG
  535         printf("val=%d shift=%d log2(val)=%d\n",
  536             val, shift, norm_exp);
  537         printf("exp=%x mant=%x\n", FLT_MAX_EXP - 1 + exp + norm_exp,
  538             ((shift > 0 ? (val << shift) : (val >> -shift)) & MANT_MASK));
  539 #endif
  540         return (((FLT_MAX_EXP - 1 + norm_exp) << (FLT_MANT_DIG - 1)) |
  541             ((shift > 0 ? val << shift : val >> -shift) & MANT_MASK));
  542 }
  543 
  544 /* FLOAT_CONVERSION_END (Regression testing; don't remove this line.) */
  545 
  546 /*
  547  * Periodically check the filesystem to see if accounting
  548  * should be turned on or off.  Beware the case where the vnode
  549  * has been vgone()'d out from underneath us, e.g. when the file
  550  * system containing the accounting file has been forcibly unmounted.
  551  */
  552 /* ARGSUSED */
  553 static void
  554 acctwatch(void)
  555 {
  556         struct statfs *sp;
  557 
  558         sx_assert(&acct_sx, SX_XLOCKED);
  559 
  560         /*
  561          * If accounting was disabled before our kthread was scheduled,
  562          * then acct_vp might be NULL.  If so, just ask our kthread to
  563          * exit and return.
  564          */
  565         if (acct_vp == NULL) {
  566                 acct_state |= ACCT_EXITREQ;
  567                 return;
  568         }
  569 
  570         /*
  571          * If our vnode is no longer valid, tear it down and signal the
  572          * accounting thread to die.
  573          */
  574         if (acct_vp->v_type == VBAD) {
  575                 (void) acct_disable(NULL, 1);
  576                 acct_state |= ACCT_EXITREQ;
  577                 return;
  578         }
  579 
  580         /*
  581          * Stopping here is better than continuing, maybe it will be VBAD
  582          * next time around.
  583          */
  584         sp = malloc(sizeof(struct statfs), M_STATFS, M_WAITOK);
  585         if (VFS_STATFS(acct_vp->v_mount, sp) < 0) {
  586                 free(sp, M_STATFS);
  587                 return;
  588         }
  589         if (acct_suspended) {
  590                 if (sp->f_bavail > (int64_t)(acctresume * sp->f_blocks /
  591                     100)) {
  592                         acct_suspended = 0;
  593                         log(LOG_NOTICE, "Accounting resumed\n");
  594                 }
  595         } else {
  596                 if (sp->f_bavail <= (int64_t)(acctsuspend * sp->f_blocks /
  597                     100)) {
  598                         acct_suspended = 1;
  599                         log(LOG_NOTICE, "Accounting suspended\n");
  600                 }
  601         }
  602         free(sp, M_STATFS);
  603 }
  604 
  605 /*
  606  * The main loop for the dedicated kernel thread that periodically calls
  607  * acctwatch().
  608  */
  609 static void
  610 acct_thread(void *dummy)
  611 {
  612         u_char pri;
  613 
  614         /* This is a low-priority kernel thread. */
  615         pri = PRI_MAX_KERN;
  616         thread_lock(curthread);
  617         sched_prio(curthread, pri);
  618         thread_unlock(curthread);
  619 
  620         /* If another accounting kthread is already running, just die. */
  621         sx_xlock(&acct_sx);
  622         if (acct_state & ACCT_RUNNING) {
  623                 sx_xunlock(&acct_sx);
  624                 kproc_exit(0);
  625         }
  626         acct_state |= ACCT_RUNNING;
  627 
  628         /* Loop until we are asked to exit. */
  629         while (!(acct_state & ACCT_EXITREQ)) {
  630 
  631                 /* Perform our periodic checks. */
  632                 acctwatch();
  633 
  634                 /*
  635                  * We check this flag again before sleeping since the
  636                  * acctwatch() might have shut down accounting and asked us
  637                  * to exit.
  638                  */
  639                 if (!(acct_state & ACCT_EXITREQ)) {
  640                         sx_sleep(&acct_state, &acct_sx, 0, "-",
  641                             acctchkfreq * hz);
  642                 }
  643         }
  644 
  645         /*
  646          * Acknowledge the exit request and shutdown.  We clear both the
  647          * exit request and running flags.
  648          */
  649         acct_state = 0;
  650         sx_xunlock(&acct_sx);
  651         kproc_exit(0);
  652 }

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