lib/errseq.c

   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 #include <linux/log2.h>
   7
   8 /*
   9  * An errseq_t is a way of recording errors in one place, and allowing any
  10  * number of "subscribers" to tell whether it has changed since a previous
  11  * point where it was sampled.
  12  *
  13  * It's implemented as an unsigned 32-bit value. The low order bits are
  14  * designated to hold an error code (between 0 and -MAX_ERRNO). The upper bits
  15  * are used as a counter. This is done with atomics instead of locking so that
  16  * these functions can be called from any context.
  17  *
  18  * The general idea is for consumers to sample an errseq_t value. That value
  19  * can later be used to tell whether any new errors have occurred since that
  20  * sampling was done.
  21  *
  22  * Note that there is a risk of collisions if new errors are being recorded
  23  * frequently, since we have so few bits to use as a counter.
  24  *
  25  * To mitigate this, one bit is used as a flag to tell whether the value has
  26  * been sampled since a new value was recorded. That allows us to avoid bumping
  27  * the counter if no one has sampled it since the last time an error was
  28  * recorded.
  29  *
  30  * A new errseq_t should always be zeroed out.  A errseq_t value of all zeroes
  31  * is the special (but common) case where there has never been an error. An all
  32  * zero value thus serves as the "epoch" if one wishes to know whether there
  33  * has ever been an error set since it was first initialized.
  34  */
  35
  36 /* The low bits are designated for error code (max of MAX_ERRNO) */
  37 #define ERRSEQ_SHIFT            ilog2(MAX_ERRNO + 1)
  38
  39 /* This bit is used as a flag to indicate whether the value has been seen */
  40 #define ERRSEQ_SEEN             (1 << ERRSEQ_SHIFT)
  41
  42 /* The lowest bit of the counter */
  43 #define ERRSEQ_CTR_INC          (1 << (ERRSEQ_SHIFT + 1))
  44
  45 /**
  46  * errseq_set - set a errseq_t for later reporting
  47  * @eseq: errseq_t field that should be set
  48  * @err: error to set (must be between -1 and -MAX_ERRNO)
  49  *
  50  * This function sets the error in @eseq, and increments the sequence counter
  51  * if the last sequence was sampled at some point in the past.
  52  *
  53  * Any error set will always overwrite an existing error.
  54  *
  55  * Return: The previous value, primarily for debugging purposes. The
  56  * return value should not be used as a previously sampled value in later
  57  * calls as it will not have the SEEN flag set.
  58  */
  59 errseq_t errseq_set(errseq_t *eseq, int err)
  60 {
  61         errseq_t cur, old;
  62
  63         /* MAX_ERRNO must be able to serve as a mask */
  64         BUILD_BUG_ON_NOT_POWER_OF_2(MAX_ERRNO + 1);
  65
  66         /*
  67          * Ensure the error code actually fits where we want it to go. If it
  68          * doesn't then just throw a warning and don't record anything. We
  69          * also don't accept zero here as that would effectively clear a
  70          * previous error.
  71          */
  72         old = READ_ONCE(*eseq);
  73
  74         if (WARN(unlikely(err == 0 || (unsigned int)-err > MAX_ERRNO),
  75                                 "err = %d\n", err))
  76                 return old;
  77
  78         for (;;) {
  79                 errseq_t new;
  80
  81                 /* Clear out error bits and set new error */
  82                 new = (old & ~(MAX_ERRNO|ERRSEQ_SEEN)) | -err;
  83
  84                 /* Only increment if someone has looked at it */
  85                 if (old & ERRSEQ_SEEN)
  86                         new += ERRSEQ_CTR_INC;
  87
  88                 /* If there would be no change, then call it done */
  89                 if (new == old) {
  90                         cur = new;
  91                         break;
  92                 }
  93
  94                 /* Try to swap the new value into place */
  95                 cur = cmpxchg(eseq, old, new);
  96
  97                 /*
  98                  * Call it success if we did the swap or someone else beat us
  99                  * to it for the same value.
 100                  */
 101                 if (likely(cur == old || cur == new))
 102                         break;
 103
 104                 /* Raced with an update, try again */
 105                 old = cur;
 106         }
 107         return cur;
 108 }
 109 EXPORT_SYMBOL(errseq_set);
 110
 111 /**
 112  * errseq_sample() - Grab current errseq_t value.
 113  * @eseq: Pointer to errseq_t to be sampled.
 114  *
 115  * This function allows callers to initialise their errseq_t variable.
 116  * If the error has been "seen", new callers will not see an old error.
 117  * If there is an unseen error in @eseq, the caller of this function will
 118  * see it the next time it checks for an error.
 119  *
 120  * Context: Any context.
 121  * Return: The current errseq value.
 122  */
 123 errseq_t errseq_sample(errseq_t *eseq)
 124 {
 125         errseq_t old = READ_ONCE(*eseq);
 126
 127         /* If nobody has seen this error yet, then we can be the first. */
 128         if (!(old & ERRSEQ_SEEN))
 129                 old = 0;
 130         return old;
 131 }
 132 EXPORT_SYMBOL(errseq_sample);
 133
 134 /**
 135  * errseq_check() - Has an error occurred since a particular sample point?
 136  * @eseq: Pointer to errseq_t value to be checked.
 137  * @since: Previously-sampled errseq_t from which to check.
 138  *
 139  * Grab the value that eseq points to, and see if it has changed @since
 140  * the given value was sampled. The @since value is not advanced, so there
 141  * is no need to mark the value as seen.
 142  *
 143  * Return: The latest error set in the errseq_t or 0 if it hasn't changed.
 144  */
 145 int errseq_check(errseq_t *eseq, errseq_t since)
 146 {
 147         errseq_t cur = READ_ONCE(*eseq);
 148
 149         if (likely(cur == since))
 150                 return 0;
 151         return -(cur & MAX_ERRNO);
 152 }
 153 EXPORT_SYMBOL(errseq_check);
 154
 155 /**
 156  * errseq_check_and_advance() - Check an errseq_t and advance to current value.
 157  * @eseq: Pointer to value being checked and reported.
 158  * @since: Pointer to previously-sampled errseq_t to check against and advance.
 159  *
 160  * Grab the eseq value, and see whether it matches the value that @since
 161  * points to. If it does, then just return 0.
 162  *
 163  * If it doesn't, then the value has changed. Set the "seen" flag, and try to
 164  * swap it into place as the new eseq value. Then, set that value as the new
 165  * "since" value, and return whatever the error portion is set to.
 166  *
 167  * Note that no locking is provided here for concurrent updates to the "since"
 168  * value. The caller must provide that if necessary. Because of this, callers
 169  * may want to do a lockless errseq_check before taking the lock and calling
 170  * this.
 171  *
 172  * Return: Negative errno if one has been stored, or 0 if no new error has
 173  * occurred.
 174  */
 175 int errseq_check_and_advance(errseq_t *eseq, errseq_t *since)
 176 {
 177         int err = 0;
 178         errseq_t old, new;
 179
 180         /*
 181          * Most callers will want to use the inline wrapper to check this,
 182          * so that the common case of no error is handled without needing
 183          * to take the lock that protects the "since" value.
 184          */
 185         old = READ_ONCE(*eseq);
 186         if (old != *since) {
 187                 /*
 188                  * Set the flag and try to swap it into place if it has
 189                  * changed.
 190                  *
 191                  * We don't care about the outcome of the swap here. If the
 192                  * swap doesn't occur, then it has either been updated by a
 193                  * writer who is altering the value in some way (updating
 194                  * counter or resetting the error), or another reader who is
 195                  * just setting the "seen" flag. Either outcome is OK, and we
 196                  * can advance "since" and return an error based on what we
 197                  * have.
 198                  */
 199                 new = old | ERRSEQ_SEEN;
 200                 if (new != old)
 201                         cmpxchg(eseq, old, new);
 202                 *since = new;
 203                 err = -(new & MAX_ERRNO);
 204         }
 205         return err;
 206 }
 207 EXPORT_SYMBOL(errseq_check_and_advance);