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
   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  * We do return the latest value here, primarily for debugging purposes. The
  55  * return value should not be used as a previously sampled value in later calls
  56  * as it will not have the SEEN flag set.
  57  */
  58 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         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                 new = (old & ~(MAX_ERRNO|ERRSEQ_SEEN)) | -err;
  82
  83                 /* Only increment if someone has looked at it */
  84                 if (old & ERRSEQ_SEEN)
  85                         new += ERRSEQ_CTR_INC;
  86
  87                 /* If there would be no change, then call it done */
  88                 if (new == old) {
  89                         cur = new;
  90                         break;
  91                 }
  92
  93                 /* Try to swap the new value into place */
  94                 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                 if (likely(cur == old || cur == new))
 101                         break;
 102
 103                 /* Raced with an update, try again */
 104                 old = cur;
 105         }
 106         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 sample an errseq_t value, marking it as
 115  * "seen" if required.
 116  */
 117 errseq_t errseq_sample(errseq_t *eseq)
 118 {
 119         errseq_t old = READ_ONCE(*eseq);
 120         errseq_t new = old;
 121
 122         /*
 123          * For the common case of no errors ever having been set, we can skip
 124          * marking the SEEN bit. Once an error has been set, the value will
 125          * never go back to zero.
 126          */
 127         if (old != 0) {
 128                 new |= ERRSEQ_SEEN;
 129                 if (old != new)
 130                         cmpxchg(eseq, old, new);
 131         }
 132         return new;
 133 }
 134 EXPORT_SYMBOL(errseq_sample);
 135
 136 /**
 137  * errseq_check - has an error occurred since a particular sample point?
 138  * @eseq: pointer to errseq_t value to be checked
 139  * @since: previously-sampled errseq_t from which to check
 140  *
 141  * Grab the value that eseq points to, and see if it has changed "since"
 142  * the given value was sampled. The "since" value is not advanced, so there
 143  * is no need to mark the value as seen.
 144  *
 145  * Returns the latest error set in the errseq_t or 0 if it hasn't changed.
 146  */
 147 int errseq_check(errseq_t *eseq, errseq_t since)
 148 {
 149         errseq_t cur = READ_ONCE(*eseq);
 150
 151         if (likely(cur == since))
 152                 return 0;
 153         return -(cur & MAX_ERRNO);
 154 }
 155 EXPORT_SYMBOL(errseq_check);
 156
 157 /**
 158  * errseq_check_and_advance - check an errseq_t and advance to current value
 159  * @eseq: pointer to value being checked and reported
 160  * @since: pointer to previously-sampled errseq_t to check against and advance
 161  *
 162  * Grab the eseq value, and see whether it matches the value that "since"
 163  * points to. If it does, then just return 0.
 164  *
 165  * If it doesn't, then the value has changed. Set the "seen" flag, and try to
 166  * swap it into place as the new eseq value. Then, set that value as the new
 167  * "since" value, and return whatever the error portion is set to.
 168  *
 169  * Note that no locking is provided here for concurrent updates to the "since"
 170  * value. The caller must provide that if necessary. Because of this, callers
 171  * may want to do a lockless errseq_check before taking the lock and calling
 172  * this.
 173  */
 174 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         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                 new = old | ERRSEQ_SEEN;
 199                 if (new != old)
 200                         cmpxchg(eseq, old, new);
 201                 *since = new;
 202                 err = -(new & MAX_ERRNO);
 203         }
 204         return err;
 205 }
 206 EXPORT_SYMBOL(errseq_check_and_advance);