cregit-Linux how code gets into the kernel

Release 4.16 lib/errseq.c

Directory: lib
// SPDX-License-Identifier: GPL-2.0
#include <linux/err.h>
#include <linux/bug.h>
#include <linux/atomic.h>
#include <linux/errseq.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 */


/* 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; }


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EXPORT_SYMBOL(errseq_set); /** * errseq_sample() - Grab current errseq_t value. * @eseq: Pointer to errseq_t to be sampled. * * This function allows callers to sample an errseq_t value, marking it as * "seen" if required. * * Return: The current errseq value. */
errseq_t errseq_sample(errseq_t *eseq) { errseq_t old = READ_ONCE(*eseq); errseq_t new = old; /* * For the common case of no errors ever having been set, we can skip * marking the SEEN bit. Once an error has been set, the value will * never go back to zero. */ if (old != 0) { new |= ERRSEQ_SEEN; if (old != new) cmpxchg(eseq, old, new); } return new; }


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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); }


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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; }


Jeff Layton80100.00%1100.00%


Overall Contributors

Jeff Layton37298.67%250.00%
Matthew Wilcox41.06%125.00%
Greg Kroah-Hartman10.27%125.00%
Directory: lib
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