errseq: Add to documentation tree
- Move errseq.rst into core-api - Add errseq to the core-api index - Promote the header to a more prominent header type, otherwise we get three entries in the table of contents. - Reformat the table to look nicer and be a little more proportional in terms of horizontal width per bit (the SF bit is still disproportionately large, but there's no way to fix that). - Include errseq kernel-doc in the errseq.rst - Neaten some kernel-doc markup Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com> Reviewed-by: Jeff Layton <jlayton@redhat.com> Reviewed-by: Randy Dunlap <rdunlap@infradead.org> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Matthew Wilcox
Jonathan Corbet
parent
aa931b44ae
commit
14ebc28e07
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@ -1,5 +1,7 @@
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=====================
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The errseq_t datatype
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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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@ -21,12 +23,13 @@ a flag to tell whether the value has been sampled since a new value was
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recorded. That allows us to avoid bumping the counter if no one has
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sampled it since the last time an error was recorded.
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Thus we end up with a value that looks something like this::
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Thus we end up with a value that looks something like this:
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bit: 31..13 12 11..0
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+-----------------+----+----------------+
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| counter | SF | errno |
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+-----------------+----+----------------+
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+--------------------------------------+----+------------------------+
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| 31..13 | 12 | 11..0 |
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+--------------------------------------+----+------------------------+
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| counter | SF | errno |
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+--------------------------------------+----+------------------------+
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The general idea is for "watchers" to sample an errseq_t value and keep
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it as a running cursor. That value can later be used to tell whether
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@ -42,6 +45,7 @@ has ever been an error set since it was first initialized.
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API usage
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=========
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Let me tell you a story about a worker drone. Now, he's a good worker
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overall, but the company is a little...management heavy. He has to
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report to 77 supervisors today, and tomorrow the "big boss" is coming in
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@ -125,6 +129,7 @@ not usable by anyone else.
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Serializing errseq_t cursor updates
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===================================
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Note that the errseq_t API does not protect the errseq_t cursor during a
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check_and_advance_operation. Only the canonical error code is handled
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atomically. In a situation where more than one task might be using the
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@ -147,3 +152,8 @@ errseq_check_and_advance after taking the lock. e.g.::
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That avoids the spinlock in the common case where nothing has changed
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since the last time it was checked.
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Functions
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=========
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.. kernel-doc:: lib/errseq.c
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@ -22,6 +22,7 @@ Core utilities
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flexible-arrays
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librs
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genalloc
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errseq
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printk-formats
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Interfaces for kernel debugging
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@ -1,6 +1,6 @@
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/* SPDX-License-Identifier: GPL-2.0 */
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/*
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* See Documentation/errseq.rst and lib/errseq.c
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* See Documentation/core-api/errseq.rst and lib/errseq.c
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*/
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#ifndef _LINUX_ERRSEQ_H
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#define _LINUX_ERRSEQ_H
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37
lib/errseq.c
37
lib/errseq.c
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@ -46,14 +46,14 @@
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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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* 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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* We do return the latest value here, primarily for debugging purposes. The
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* return value should not be used as a previously sampled value in later calls
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* as it will not have the SEEN flag set.
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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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@ -108,11 +108,13 @@ errseq_t errseq_set(errseq_t *eseq, int err)
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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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* 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 sample an errseq_t value, marking it as
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* "seen" if required.
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*
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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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@ -134,15 +136,15 @@ errseq_t errseq_sample(errseq_t *eseq)
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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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* 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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* 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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* Returns the latest error set in the errseq_t or 0 if it hasn't changed.
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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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@ -155,11 +157,11 @@ int errseq_check(errseq_t *eseq, errseq_t since)
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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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* 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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* 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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@ -170,6 +172,9 @@ EXPORT_SYMBOL(errseq_check);
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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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