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1// SPDX-License-Identifier: GPL-2.02#include <linux/err.h>3#include <linux/bug.h>4#include <linux/atomic.h>5#include <linux/errseq.h>6#include <linux/log2.h>7 8/*9 * An errseq_t is a way of recording errors in one place, and allowing any10 * number of "subscribers" to tell whether it has changed since a previous11 * point where it was sampled.12 *13 * It's implemented as an unsigned 32-bit value. The low order bits are14 * designated to hold an error code (between 0 and -MAX_ERRNO). The upper bits15 * are used as a counter. This is done with atomics instead of locking so that16 * these functions can be called from any context.17 *18 * The general idea is for consumers to sample an errseq_t value. That value19 * can later be used to tell whether any new errors have occurred since that20 * sampling was done.21 *22 * Note that there is a risk of collisions if new errors are being recorded23 * frequently, since we have so few bits to use as a counter.24 *25 * To mitigate this, one bit is used as a flag to tell whether the value has26 * been sampled since a new value was recorded. That allows us to avoid bumping27 * the counter if no one has sampled it since the last time an error was28 * recorded.29 *30 * A new errseq_t should always be zeroed out. A errseq_t value of all zeroes31 * is the special (but common) case where there has never been an error. An all32 * zero value thus serves as the "epoch" if one wishes to know whether there33 * has ever been an error set since it was first initialized.34 */35 36/* The low bits are designated for error code (max of MAX_ERRNO) */37#define ERRSEQ_SHIFT ilog2(MAX_ERRNO + 1)38 39/* This bit is used as a flag to indicate whether the value has been seen */40#define ERRSEQ_SEEN (1 << ERRSEQ_SHIFT)41 42/* The lowest bit of the counter */43#define ERRSEQ_CTR_INC (1 << (ERRSEQ_SHIFT + 1))44 45/**46 * errseq_set - set a errseq_t for later reporting47 * @eseq: errseq_t field that should be set48 * @err: error to set (must be between -1 and -MAX_ERRNO)49 *50 * This function sets the error in @eseq, and increments the sequence counter51 * if the last sequence was sampled at some point in the past.52 *53 * Any error set will always overwrite an existing error.54 *55 * Return: The previous value, primarily for debugging purposes. The56 * return value should not be used as a previously sampled value in later57 * calls as it will not have the SEEN flag set.58 */59errseq_t errseq_set(errseq_t *eseq, int err)60{61 errseq_t cur, old;62 63 /* MAX_ERRNO must be able to serve as a mask */64 BUILD_BUG_ON_NOT_POWER_OF_2(MAX_ERRNO + 1);65 66 /*67 * Ensure the error code actually fits where we want it to go. If it68 * doesn't then just throw a warning and don't record anything. We69 * also don't accept zero here as that would effectively clear a70 * previous error.71 */72 old = READ_ONCE(*eseq);73 74 if (WARN(unlikely(err == 0 || (unsigned int)-err > MAX_ERRNO),75 "err = %d\n", err))76 return old;77 78 for (;;) {79 errseq_t new;80 81 /* Clear out error bits and set new error */82 new = (old & ~(MAX_ERRNO|ERRSEQ_SEEN)) | -err;83 84 /* Only increment if someone has looked at it */85 if (old & ERRSEQ_SEEN)86 new += ERRSEQ_CTR_INC;87 88 /* If there would be no change, then call it done */89 if (new == old) {90 cur = new;91 break;92 }93 94 /* Try to swap the new value into place */95 cur = cmpxchg(eseq, old, new);96 97 /*98 * Call it success if we did the swap or someone else beat us99 * to it for the same value.100 */101 if (likely(cur == old || cur == new))102 break;103 104 /* Raced with an update, try again */105 old = cur;106 }107 return cur;108}109EXPORT_SYMBOL(errseq_set);110 111/**112 * errseq_sample() - Grab current errseq_t value.113 * @eseq: Pointer to errseq_t to be sampled.114 *115 * This function allows callers to initialise their errseq_t variable.116 * If the error has been "seen", new callers will not see an old error.117 * If there is an unseen error in @eseq, the caller of this function will118 * see it the next time it checks for an error.119 *120 * Context: Any context.121 * Return: The current errseq value.122 */123errseq_t errseq_sample(errseq_t *eseq)124{125 errseq_t old = READ_ONCE(*eseq);126 127 /* If nobody has seen this error yet, then we can be the first. */128 if (!(old & ERRSEQ_SEEN))129 old = 0;130 return old;131}132EXPORT_SYMBOL(errseq_sample);133 134/**135 * errseq_check() - Has an error occurred since a particular sample point?136 * @eseq: Pointer to errseq_t value to be checked.137 * @since: Previously-sampled errseq_t from which to check.138 *139 * Grab the value that eseq points to, and see if it has changed @since140 * the given value was sampled. The @since value is not advanced, so there141 * is no need to mark the value as seen.142 *143 * Return: The latest error set in the errseq_t or 0 if it hasn't changed.144 */145int errseq_check(errseq_t *eseq, errseq_t since)146{147 errseq_t cur = READ_ONCE(*eseq);148 149 if (likely(cur == since))150 return 0;151 return -(cur & MAX_ERRNO);152}153EXPORT_SYMBOL(errseq_check);154 155/**156 * errseq_check_and_advance() - Check an errseq_t and advance to current value.157 * @eseq: Pointer to value being checked and reported.158 * @since: Pointer to previously-sampled errseq_t to check against and advance.159 *160 * Grab the eseq value, and see whether it matches the value that @since161 * points to. If it does, then just return 0.162 *163 * If it doesn't, then the value has changed. Set the "seen" flag, and try to164 * swap it into place as the new eseq value. Then, set that value as the new165 * "since" value, and return whatever the error portion is set to.166 *167 * Note that no locking is provided here for concurrent updates to the "since"168 * value. The caller must provide that if necessary. Because of this, callers169 * may want to do a lockless errseq_check before taking the lock and calling170 * this.171 *172 * Return: Negative errno if one has been stored, or 0 if no new error has173 * occurred.174 */175int errseq_check_and_advance(errseq_t *eseq, errseq_t *since)176{177 int err = 0;178 errseq_t old, new;179 180 /*181 * Most callers will want to use the inline wrapper to check this,182 * so that the common case of no error is handled without needing183 * to take the lock that protects the "since" value.184 */185 old = READ_ONCE(*eseq);186 if (old != *since) {187 /*188 * Set the flag and try to swap it into place if it has189 * changed.190 *191 * We don't care about the outcome of the swap here. If the192 * swap doesn't occur, then it has either been updated by a193 * writer who is altering the value in some way (updating194 * counter or resetting the error), or another reader who is195 * just setting the "seen" flag. Either outcome is OK, and we196 * can advance "since" and return an error based on what we197 * have.198 */199 new = old | ERRSEQ_SEEN;200 if (new != old)201 cmpxchg(eseq, old, new);202 *since = new;203 err = -(new & MAX_ERRNO);204 }205 return err;206}207EXPORT_SYMBOL(errseq_check_and_advance);208