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