1// SPDX-License-Identifier: GPL-2.0 2#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 any 10 * number of "subscribers" to tell whether it has changed since a previous 11 * point where it was sampled. 12 * 13 * It's implemented as an unsigned 32-bit value. The low order bits are 14 * designated to hold an error code (between 0 and -MAX_ERRNO). The upper bits 15 * are used as a counter. This is done with atomics instead of locking so that 16 * these functions can be called from any context. 17 * 18 * The general idea is for consumers to sample an errseq_t value. That value 19 * can later be used to tell whether any new errors have occurred since that 20 * sampling was done. 21 * 22 * Note that there is a risk of collisions if new errors are being recorded 23 * 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 has 26 * been sampled since a new value was recorded. That allows us to avoid bumping 27 * the counter if no one has sampled it since the last time an error was 28 * recorded. 29 * 30 * A new errseq_t should always be zeroed out. A errseq_t value of all zeroes 31 * is the special (but common) case where there has never been an error. An all 32 * zero value thus serves as the "epoch" if one wishes to know whether there 33 * 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 reporting 47 * @eseq: errseq_t field that should be set 48 * @err: error to set (must be between -1 and -MAX_ERRNO) 49 * 50 * This function sets the error in @eseq, and increments the sequence counter 51 * 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. The 56 * return value should not be used as a previously sampled value in later 57 * 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 it 68 * doesn't then just throw a warning and don't record anything. We 69 * also don't accept zero here as that would effectively clear a 70 * 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 us 99 * 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 will 118 * 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 @since 140 * the given value was sampled. The @since value is not advanced, so there 141 * 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 @since 161 * 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 to 164 * swap it into place as the new eseq value. Then, set that value as the new 165 * "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, callers 169 * may want to do a lockless errseq_check before taking the lock and calling 170 * this. 171 * 172 * Return: Negative errno if one has been stored, or 0 if no new error has 173 * 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 needing 183 * 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 has 189 * changed. 190 * 191 * We don't care about the outcome of the swap here. If the 192 * swap doesn't occur, then it has either been updated by a 193 * writer who is altering the value in some way (updating 194 * counter or resetting the error), or another reader who is 195 * just setting the "seen" flag. Either outcome is OK, and we 196 * can advance "since" and return an error based on what we 197 * 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