Branch data     Line data    Source code

       1                 :            : // SPDX-License-Identifier: GPL-2.0
       2                 :            : #include <linux/err.h>
       3                 :            : #include <linux/bug.h>
       4                 :            : #include <linux/atomic.h>
       5                 :            : #include <linux/errseq.h>
       6                 :            : 
       7                 :            : /*
       8                 :            :  * An errseq_t is a way of recording errors in one place, and allowing any
       9                 :            :  * number of "subscribers" to tell whether it has changed since a previous
      10                 :            :  * point where it was sampled.
      11                 :            :  *
      12                 :            :  * It's implemented as an unsigned 32-bit value. The low order bits are
      13                 :            :  * designated to hold an error code (between 0 and -MAX_ERRNO). The upper bits
      14                 :            :  * are used as a counter. This is done with atomics instead of locking so that
      15                 :            :  * these functions can be called from any context.
      16                 :            :  *
      17                 :            :  * The general idea is for consumers to sample an errseq_t value. That value
      18                 :            :  * can later be used to tell whether any new errors have occurred since that
      19                 :            :  * sampling was done.
      20                 :            :  *
      21                 :            :  * Note that there is a risk of collisions if new errors are being recorded
      22                 :            :  * frequently, since we have so few bits to use as a counter.
      23                 :            :  *
      24                 :            :  * To mitigate this, one bit is used as a flag to tell whether the value has
      25                 :            :  * been sampled since a new value was recorded. That allows us to avoid bumping
      26                 :            :  * the counter if no one has sampled it since the last time an error was
      27                 :            :  * recorded.
      28                 :            :  *
      29                 :            :  * A new errseq_t should always be zeroed out.  A errseq_t value of all zeroes
      30                 :            :  * is the special (but common) case where there has never been an error. An all
      31                 :            :  * zero value thus serves as the "epoch" if one wishes to know whether there
      32                 :            :  * has ever been an error set since it was first initialized.
      33                 :            :  */
      34                 :            : 
      35                 :            : /* The low bits are designated for error code (max of MAX_ERRNO) */
      36                 :            : #define ERRSEQ_SHIFT            ilog2(MAX_ERRNO + 1)
      37                 :            : 
      38                 :            : /* This bit is used as a flag to indicate whether the value has been seen */
      39                 :            : #define ERRSEQ_SEEN             (1 << ERRSEQ_SHIFT)
      40                 :            : 
      41                 :            : /* The lowest bit of the counter */
      42                 :            : #define ERRSEQ_CTR_INC          (1 << (ERRSEQ_SHIFT + 1))
      43                 :            : 
      44                 :            : /**
      45                 :            :  * errseq_set - set a errseq_t for later reporting
      46                 :            :  * @eseq: errseq_t field that should be set
      47                 :            :  * @err: error to set (must be between -1 and -MAX_ERRNO)
      48                 :            :  *
      49                 :            :  * This function sets the error in @eseq, and increments the sequence counter
      50                 :            :  * if the last sequence was sampled at some point in the past.
      51                 :            :  *
      52                 :            :  * Any error set will always overwrite an existing error.
      53                 :            :  *
      54                 :            :  * Return: The previous value, primarily for debugging purposes. The
      55                 :            :  * return value should not be used as a previously sampled value in later
      56                 :            :  * calls as it will not have the SEEN flag set.
      57                 :            :  */
      58                 :          0 : errseq_t errseq_set(errseq_t *eseq, int err)
      59                 :            : {
      60                 :            :         errseq_t cur, old;
      61                 :            : 
      62                 :            :         /* MAX_ERRNO must be able to serve as a mask */
      63                 :            :         BUILD_BUG_ON_NOT_POWER_OF_2(MAX_ERRNO + 1);
      64                 :            : 
      65                 :            :         /*
      66                 :            :          * Ensure the error code actually fits where we want it to go. If it
      67                 :            :          * doesn't then just throw a warning and don't record anything. We
      68                 :            :          * also don't accept zero here as that would effectively clear a
      69                 :            :          * previous error.
      70                 :            :          */
      71                 :            :         old = READ_ONCE(*eseq);
      72                 :            : 
      73                 :          0 :         if (WARN(unlikely(err == 0 || (unsigned int)-err > MAX_ERRNO),
      74                 :            :                                 "err = %d\n", err))
      75                 :            :                 return old;
      76                 :            : 
      77                 :            :         for (;;) {
      78                 :            :                 errseq_t new;
      79                 :            : 
      80                 :            :                 /* Clear out error bits and set new error */
      81                 :          0 :                 new = (old & ~(MAX_ERRNO|ERRSEQ_SEEN)) | -err;
      82                 :            : 
      83                 :            :                 /* Only increment if someone has looked at it */
      84                 :          0 :                 if (old & ERRSEQ_SEEN)
      85                 :          0 :                         new += ERRSEQ_CTR_INC;
      86                 :            : 
      87                 :            :                 /* If there would be no change, then call it done */
      88                 :          0 :                 if (new == old) {
      89                 :          0 :                         cur = new;
      90                 :          0 :                         break;
      91                 :            :                 }
      92                 :            : 
      93                 :            :                 /* Try to swap the new value into place */
      94                 :          0 :                 cur = cmpxchg(eseq, old, new);
      95                 :            : 
      96                 :            :                 /*
      97                 :            :                  * Call it success if we did the swap or someone else beat us
      98                 :            :                  * to it for the same value.
      99                 :            :                  */
     100                 :          0 :                 if (likely(cur == old || cur == new))
     101                 :            :                         break;
     102                 :            : 
     103                 :            :                 /* Raced with an update, try again */
     104                 :            :                 old = cur;
     105                 :            :         }
     106                 :          0 :         return cur;
     107                 :            : }
     108                 :            : EXPORT_SYMBOL(errseq_set);
     109                 :            : 
     110                 :            : /**
     111                 :            :  * errseq_sample() - Grab current errseq_t value.
     112                 :            :  * @eseq: Pointer to errseq_t to be sampled.
     113                 :            :  *
     114                 :            :  * This function allows callers to initialise their errseq_t variable.
     115                 :            :  * If the error has been "seen", new callers will not see an old error.
     116                 :            :  * If there is an unseen error in @eseq, the caller of this function will
     117                 :            :  * see it the next time it checks for an error.
     118                 :            :  *
     119                 :            :  * Context: Any context.
     120                 :            :  * Return: The current errseq value.
     121                 :            :  */
     122                 :          3 : errseq_t errseq_sample(errseq_t *eseq)
     123                 :            : {
     124                 :            :         errseq_t old = READ_ONCE(*eseq);
     125                 :            : 
     126                 :            :         /* If nobody has seen this error yet, then we can be the first. */
     127                 :          3 :         if (!(old & ERRSEQ_SEEN))
     128                 :            :                 old = 0;
     129                 :          3 :         return old;
     130                 :            : }
     131                 :            : EXPORT_SYMBOL(errseq_sample);
     132                 :            : 
     133                 :            : /**
     134                 :            :  * errseq_check() - Has an error occurred since a particular sample point?
     135                 :            :  * @eseq: Pointer to errseq_t value to be checked.
     136                 :            :  * @since: Previously-sampled errseq_t from which to check.
     137                 :            :  *
     138                 :            :  * Grab the value that eseq points to, and see if it has changed @since
     139                 :            :  * the given value was sampled. The @since value is not advanced, so there
     140                 :            :  * is no need to mark the value as seen.
     141                 :            :  *
     142                 :            :  * Return: The latest error set in the errseq_t or 0 if it hasn't changed.
     143                 :            :  */
     144                 :          3 : int errseq_check(errseq_t *eseq, errseq_t since)
     145                 :            : {
     146                 :            :         errseq_t cur = READ_ONCE(*eseq);
     147                 :            : 
     148                 :          3 :         if (likely(cur == since))
     149                 :            :                 return 0;
     150                 :          0 :         return -(cur & MAX_ERRNO);
     151                 :            : }
     152                 :            : EXPORT_SYMBOL(errseq_check);
     153                 :            : 
     154                 :            : /**
     155                 :            :  * errseq_check_and_advance() - Check an errseq_t and advance to current value.
     156                 :            :  * @eseq: Pointer to value being checked and reported.
     157                 :            :  * @since: Pointer to previously-sampled errseq_t to check against and advance.
     158                 :            :  *
     159                 :            :  * Grab the eseq value, and see whether it matches the value that @since
     160                 :            :  * points to. If it does, then just return 0.
     161                 :            :  *
     162                 :            :  * If it doesn't, then the value has changed. Set the "seen" flag, and try to
     163                 :            :  * swap it into place as the new eseq value. Then, set that value as the new
     164                 :            :  * "since" value, and return whatever the error portion is set to.
     165                 :            :  *
     166                 :            :  * Note that no locking is provided here for concurrent updates to the "since"
     167                 :            :  * value. The caller must provide that if necessary. Because of this, callers
     168                 :            :  * may want to do a lockless errseq_check before taking the lock and calling
     169                 :            :  * this.
     170                 :            :  *
     171                 :            :  * Return: Negative errno if one has been stored, or 0 if no new error has
     172                 :            :  * occurred.
     173                 :            :  */
     174                 :          0 : int errseq_check_and_advance(errseq_t *eseq, errseq_t *since)
     175                 :            : {
     176                 :            :         int err = 0;
     177                 :            :         errseq_t old, new;
     178                 :            : 
     179                 :            :         /*
     180                 :            :          * Most callers will want to use the inline wrapper to check this,
     181                 :            :          * so that the common case of no error is handled without needing
     182                 :            :          * to take the lock that protects the "since" value.
     183                 :            :          */
     184                 :            :         old = READ_ONCE(*eseq);
     185                 :          0 :         if (old != *since) {
     186                 :            :                 /*
     187                 :            :                  * Set the flag and try to swap it into place if it has
     188                 :            :                  * changed.
     189                 :            :                  *
     190                 :            :                  * We don't care about the outcome of the swap here. If the
     191                 :            :                  * swap doesn't occur, then it has either been updated by a
     192                 :            :                  * writer who is altering the value in some way (updating
     193                 :            :                  * counter or resetting the error), or another reader who is
     194                 :            :                  * just setting the "seen" flag. Either outcome is OK, and we
     195                 :            :                  * can advance "since" and return an error based on what we
     196                 :            :                  * have.
     197                 :            :                  */
     198                 :          0 :                 new = old | ERRSEQ_SEEN;
     199                 :          0 :                 if (new != old)
     200                 :          0 :                         cmpxchg(eseq, old, new);
     201                 :          0 :                 *since = new;
     202                 :          0 :                 err = -(new & MAX_ERRNO);
     203                 :            :         }
     204                 :          0 :         return err;
     205                 :            : }
     206                 :            : EXPORT_SYMBOL(errseq_check_and_advance);