lib/errseq.c

   1 #include <linux/err.h>
   2 #include <linux/bug.h>
   3 #include <linux/atomic.h>
   4 #include <linux/errseq.h>
   5
   6 /*
   7  * An errseq_t is a way of recording errors in one place, and allowing any
   8  * number of "subscribers" to tell whether it has changed since a previous
   9  * point where it was sampled.
  10  *
  11  * It's implemented as an unsigned 32-bit value. The low order bits are
  12  * designated to hold an error code (between 0 and -MAX_ERRNO). The upper bits
  13  * are used as a counter. This is done with atomics instead of locking so that
  14  * these functions can be called from any context.
  15  *
  16  * The general idea is for consumers to sample an errseq_t value. That value
  17  * can later be used to tell whether any new errors have occurred since that
  18  * sampling was done.
  19  *
  20  * Note that there is a risk of collisions if new errors are being recorded
  21  * frequently, since we have so few bits to use as a counter.
  22  *
  23  * To mitigate this, one bit is used as a flag to tell whether the value has
  24  * been sampled since a new value was recorded. That allows us to avoid bumping
  25  * the counter if no one has sampled it since the last time an error was
  26  * recorded.
  27  *
  28  * A new errseq_t should always be zeroed out.  A errseq_t value of all zeroes
  29  * is the special (but common) case where there has never been an error. An all
  30  * zero value thus serves as the "epoch" if one wishes to know whether there
  31  * has ever been an error set since it was first initialized.
  32  */
  33
  34 /* The low bits are designated for error code (max of MAX_ERRNO) */
  35 #define ERRSEQ_SHIFT            ilog2(MAX_ERRNO + 1)
  36
  37 /* This bit is used as a flag to indicate whether the value has been seen */
  38 #define ERRSEQ_SEEN             (1 << ERRSEQ_SHIFT)
  39
  40 /* The lowest bit of the counter */
  41 #define ERRSEQ_CTR_INC          (1 << (ERRSEQ_SHIFT + 1))
  42
  43 /**
  44  * errseq_set - set a errseq_t for later reporting
  45  * @eseq: errseq_t field that should be set
  46  * @err: error to set (must be between -1 and -MAX_ERRNO)
  47  *
  48  * This function sets the error in *eseq, and increments the sequence counter
  49  * if the last sequence was sampled at some point in the past.
  50  *
  51  * Any error set will always overwrite an existing error.
  52  *
  53  * We do return the latest value here, primarily for debugging purposes. The
  54  * return value should not be used as a previously sampled value in later calls
  55  * as it will not have the SEEN flag set.
  56  */
  57 errseq_t errseq_set(errseq_t *eseq, int err)
  58 {
  59         errseq_t cur, old;
  60
  61         /* MAX_ERRNO must be able to serve as a mask */
  62         BUILD_BUG_ON_NOT_POWER_OF_2(MAX_ERRNO + 1);
  63
  64         /*
  65          * Ensure the error code actually fits where we want it to go. If it
  66          * doesn't then just throw a warning and don't record anything. We
  67          * also don't accept zero here as that would effectively clear a
  68          * previous error.
  69          */
  70         old = READ_ONCE(*eseq);
  71
  72         if (WARN(unlikely(err == 0 || (unsigned int)-err > MAX_ERRNO),
  73                                 "err = %d\n", err))
  74                 return old;
  75
  76         for (;;) {
  77                 errseq_t new;
  78
  79                 /* Clear out error bits and set new error */
  80                 new = (old & ~(MAX_ERRNO|ERRSEQ_SEEN)) | -err;
  81
  82                 /* Only increment if someone has looked at it */
  83                 if (old & ERRSEQ_SEEN)
  84                         new += ERRSEQ_CTR_INC;
  85
  86                 /* If there would be no change, then call it done */
  87                 if (new == old) {
  88                         cur = new;
  89                         break;
  90                 }
  91
  92                 /* Try to swap the new value into place */
  93                 cur = cmpxchg(eseq, old, new);
  94
  95                 /*
  96                  * Call it success if we did the swap or someone else beat us
  97                  * to it for the same value.
  98                  */
  99                 if (likely(cur == old || cur == new))
 100                         break;
 101
 102                 /* Raced with an update, try again */
 103                 old = cur;
 104         }
 105         return cur;
 106 }
 107 EXPORT_SYMBOL(errseq_set);
 108
 109 /**
 110  * errseq_sample - grab current errseq_t value
 111  * @eseq: pointer to errseq_t to be sampled
 112  *
 113  * This function allows callers to sample an errseq_t value, marking it as
 114  * "seen" if required.
 115  */
 116 errseq_t errseq_sample(errseq_t *eseq)
 117 {
 118         errseq_t old = READ_ONCE(*eseq);
 119         errseq_t new = old;
 120
 121         /*
 122          * For the common case of no errors ever having been set, we can skip
 123          * marking the SEEN bit. Once an error has been set, the value will
 124          * never go back to zero.
 125          */
 126         if (old != 0) {
 127                 new |= ERRSEQ_SEEN;
 128                 if (old != new)
 129                         cmpxchg(eseq, old, new);
 130         }
 131         return new;
 132 }
 133 EXPORT_SYMBOL(errseq_sample);
 134
 135 /**
 136  * errseq_check - has an error occurred since a particular sample point?
 137  * @eseq: pointer to errseq_t value to be checked
 138  * @since: previously-sampled errseq_t from which to check
 139  *
 140  * Grab the value that eseq points to, and see if it has changed "since"
 141  * the given value was sampled. The "since" value is not advanced, so there
 142  * is no need to mark the value as seen.
 143  *
 144  * Returns the latest error set in the errseq_t or 0 if it hasn't changed.
 145  */
 146 int errseq_check(errseq_t *eseq, errseq_t since)
 147 {
 148         errseq_t cur = READ_ONCE(*eseq);
 149
 150         if (likely(cur == since))
 151                 return 0;
 152         return -(cur & MAX_ERRNO);
 153 }
 154 EXPORT_SYMBOL(errseq_check);
 155
 156 /**
 157  * errseq_check_and_advance - check an errseq_t and advance to current value
 158  * @eseq: pointer to value being checked and reported
 159  * @since: pointer to previously-sampled errseq_t to check against and advance
 160  *
 161  * Grab the eseq value, and see whether it matches the value that "since"
 162  * points to. If it does, then just return 0.
 163  *
 164  * If it doesn't, then the value has changed. Set the "seen" flag, and try to
 165  * swap it into place as the new eseq value. Then, set that value as the new
 166  * "since" value, and return whatever the error portion is set to.
 167  *
 168  * Note that no locking is provided here for concurrent updates to the "since"
 169  * value. The caller must provide that if necessary. Because of this, callers
 170  * may want to do a lockless errseq_check before taking the lock and calling
 171  * this.
 172  */
 173 int errseq_check_and_advance(errseq_t *eseq, errseq_t *since)
 174 {
 175         int err = 0;
 176         errseq_t old, new;
 177
 178         /*
 179          * Most callers will want to use the inline wrapper to check this,
 180          * so that the common case of no error is handled without needing
 181          * to take the lock that protects the "since" value.
 182          */
 183         old = READ_ONCE(*eseq);
 184         if (old != *since) {
 185                 /*
 186                  * Set the flag and try to swap it into place if it has
 187                  * changed.
 188                  *
 189                  * We don't care about the outcome of the swap here. If the
 190                  * swap doesn't occur, then it has either been updated by a
 191                  * writer who is altering the value in some way (updating
 192                  * counter or resetting the error), or another reader who is
 193                  * just setting the "seen" flag. Either outcome is OK, and we
 194                  * can advance "since" and return an error based on what we
 195                  * have.
 196                  */
 197                 new = old | ERRSEQ_SEEN;
 198                 if (new != old)
 199                         cmpxchg(eseq, old, new);
 200                 *since = new;
 201                 err = -(new & MAX_ERRNO);
 202         }
 203         return err;
 204 }
 205 EXPORT_SYMBOL(errseq_check_and_advance);