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	Version: linux-6.0.1 spark-3.0.0 hadoop-3.2.0-src hadoop-3.3.0-src spark-2.4.7 linux-4.15.13 hadoop-2.7.4-src Revert   Change

 // SPDX-License-Identifier: GPL-2.0
 #include <linux/err.h>
 #include <linux/bug.h>
 #include <linux/atomic.h>
 #include <linux/errseq.h>
 #include <linux/log2.h>
 
 /*
  * An errseq_t is a way of recording errors in one place, and allowing any
  * number of "subscribers" to tell whether it has changed since a previous
  * point where it was sampled.
  *
  * It's implemented as an unsigned 32-bit value. The low order bits are
  * designated to hold an error code (between 0 and -MAX_ERRNO). The upper bits
  * are used as a counter. This is done with atomics instead of locking so that
  * these functions can be called from any context.
  *
  * The general idea is for consumers to sample an errseq_t value. That value
  * can later be used to tell whether any new errors have occurred since that
  * sampling was done.
  *
  * Note that there is a risk of collisions if new errors are being recorded
  * frequently, since we have so few bits to use as a counter.
  *
  * To mitigate this, one bit is used as a flag to tell whether the value has
  * been sampled since a new value was recorded. That allows us to avoid bumping
  * the counter if no one has sampled it since the last time an error was
  * recorded.
  *
  * A new errseq_t should always be zeroed out.  A errseq_t value of all zeroes
  * is the special (but common) case where there has never been an error. An all
  * zero value thus serves as the "epoch" if one wishes to know whether there
  * has ever been an error set since it was first initialized.
  */
 
 /* The low bits are designated for error code (max of MAX_ERRNO) */
 #define ERRSEQ_SHIFT        ilog2(MAX_ERRNO + 1)
 
 /* This bit is used as a flag to indicate whether the value has been seen */
 #define ERRSEQ_SEEN     (1 << ERRSEQ_SHIFT)
 
 /* The lowest bit of the counter */
 #define ERRSEQ_CTR_INC      (1 << (ERRSEQ_SHIFT + 1))
 
 /**
  * errseq_set - set a errseq_t for later reporting
  * @eseq: errseq_t field that should be set
  * @err: error to set (must be between -1 and -MAX_ERRNO)
  *
  * This function sets the error in @eseq, and increments the sequence counter
  * if the last sequence was sampled at some point in the past.
  *
  * Any error set will always overwrite an existing error.
  *
  * Return: The previous value, primarily for debugging purposes. The
  * return value should not be used as a previously sampled value in later
  * calls as it will not have the SEEN flag set.
  */
 errseq_t errseq_set(errseq_t *eseq, int err)
 {
     errseq_t cur, old;
 
     /* MAX_ERRNO must be able to serve as a mask */
     BUILD_BUG_ON_NOT_POWER_OF_2(MAX_ERRNO + 1);
 
     /*
      * Ensure the error code actually fits where we want it to go. If it
      * doesn't then just throw a warning and don't record anything. We
      * also don't accept zero here as that would effectively clear a
      * previous error.
      */
     old = READ_ONCE(*eseq);
 
     if (WARN(unlikely(err == 0 || (unsigned int)-err > MAX_ERRNO),
                 "err = %d\n", err))
         return old;
 
     for (;;) {
         errseq_t new;
 
         /* Clear out error bits and set new error */
         new = (old & ~(MAX_ERRNO|ERRSEQ_SEEN)) | -err;
 
         /* Only increment if someone has looked at it */
         if (old & ERRSEQ_SEEN)
             new += ERRSEQ_CTR_INC;
 
         /* If there would be no change, then call it done */
         if (new == old) {
             cur = new;
             break;
         }
 
         /* Try to swap the new value into place */
         cur = cmpxchg(eseq, old, new);
 
         /*
          * Call it success if we did the swap or someone else beat us
          * to it for the same value.
          */
         if (likely(cur == old || cur == new))
             break;
 
         /* Raced with an update, try again */
         old = cur;
     }
     return cur;
 }
 EXPORT_SYMBOL(errseq_set);
 
 /**
  * errseq_sample() - Grab current errseq_t value.
  * @eseq: Pointer to errseq_t to be sampled.
  *
  * This function allows callers to initialise their errseq_t variable.
  * If the error has been "seen", new callers will not see an old error.
  * If there is an unseen error in @eseq, the caller of this function will
  * see it the next time it checks for an error.
  *
  * Context: Any context.
  * Return: The current errseq value.
  */
 errseq_t errseq_sample(errseq_t *eseq)
 {
     errseq_t old = READ_ONCE(*eseq);
 
     /* If nobody has seen this error yet, then we can be the first. */
     if (!(old & ERRSEQ_SEEN))
         old = 0;
     return old;
 }
 EXPORT_SYMBOL(errseq_sample);
 
 /**
  * errseq_check() - Has an error occurred since a particular sample point?
  * @eseq: Pointer to errseq_t value to be checked.
  * @since: Previously-sampled errseq_t from which to check.
  *
  * Grab the value that eseq points to, and see if it has changed @since
  * the given value was sampled. The @since value is not advanced, so there
  * is no need to mark the value as seen.
  *
  * Return: The latest error set in the errseq_t or 0 if it hasn't changed.
  */
 int errseq_check(errseq_t *eseq, errseq_t since)
 {
     errseq_t cur = READ_ONCE(*eseq);
 
     if (likely(cur == since))
         return 0;
     return -(cur & MAX_ERRNO);
 }
 EXPORT_SYMBOL(errseq_check);
 
 /**
  * errseq_check_and_advance() - Check an errseq_t and advance to current value.
  * @eseq: Pointer to value being checked and reported.
  * @since: Pointer to previously-sampled errseq_t to check against and advance.
  *
  * Grab the eseq value, and see whether it matches the value that @since
  * points to. If it does, then just return 0.
  *
  * If it doesn't, then the value has changed. Set the "seen" flag, and try to
  * swap it into place as the new eseq value. Then, set that value as the new
  * "since" value, and return whatever the error portion is set to.
  *
  * Note that no locking is provided here for concurrent updates to the "since"
  * value. The caller must provide that if necessary. Because of this, callers
  * may want to do a lockless errseq_check before taking the lock and calling
  * this.
  *
  * Return: Negative errno if one has been stored, or 0 if no new error has
  * occurred.
  */
 int errseq_check_and_advance(errseq_t *eseq, errseq_t *since)
 {
     int err = 0;
     errseq_t old, new;
 
     /*
      * Most callers will want to use the inline wrapper to check this,
      * so that the common case of no error is handled without needing
      * to take the lock that protects the "since" value.
      */
     old = READ_ONCE(*eseq);
     if (old != *since) {
         /*
          * Set the flag and try to swap it into place if it has
          * changed.
          *
          * We don't care about the outcome of the swap here. If the
          * swap doesn't occur, then it has either been updated by a
          * writer who is altering the value in some way (updating
          * counter or resetting the error), or another reader who is
          * just setting the "seen" flag. Either outcome is OK, and we
          * can advance "since" and return an error based on what we
          * have.
          */
         new = old | ERRSEQ_SEEN;
         if (new != old)
             cmpxchg(eseq, old, new);
         *since = new;
         err = -(new & MAX_ERRNO);
     }
     return err;
 }
 EXPORT_SYMBOL(errseq_check_and_advance);

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