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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 : */ 59 0 : errseq_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 0 : old = READ_ONCE(*eseq); 73 : 74 0 : 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 0 : new = (old & ~(MAX_ERRNO|ERRSEQ_SEEN)) | -err; 83 : 84 : /* Only increment if someone has looked at it */ 85 0 : if (old & ERRSEQ_SEEN) 86 0 : new += ERRSEQ_CTR_INC; 87 : 88 : /* If there would be no change, then call it done */ 89 0 : if (new == old) { 90 : cur = new; 91 : break; 92 : } 93 : 94 : /* Try to swap the new value into place */ 95 0 : 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 0 : if (likely(cur == old || cur == new)) 102 : break; 103 : 104 : /* Raced with an update, try again */ 105 : old = cur; 106 : } 107 : return cur; 108 : } 109 : EXPORT_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 : */ 123 0 : errseq_t errseq_sample(errseq_t *eseq) 124 : { 125 0 : errseq_t old = READ_ONCE(*eseq); 126 : 127 : /* If nobody has seen this error yet, then we can be the first. */ 128 0 : if (!(old & ERRSEQ_SEEN)) 129 0 : old = 0; 130 0 : return old; 131 : } 132 : EXPORT_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 : */ 145 0 : int errseq_check(errseq_t *eseq, errseq_t since) 146 : { 147 0 : errseq_t cur = READ_ONCE(*eseq); 148 : 149 0 : if (likely(cur == since)) 150 : return 0; 151 0 : return -(cur & MAX_ERRNO); 152 : } 153 : EXPORT_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 : */ 175 0 : int errseq_check_and_advance(errseq_t *eseq, errseq_t *since) 176 : { 177 0 : 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 0 : old = READ_ONCE(*eseq); 186 0 : 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 0 : new = old | ERRSEQ_SEEN; 200 0 : if (new != old) 201 0 : cmpxchg(eseq, old, new); 202 0 : *since = new; 203 0 : err = -(new & MAX_ERRNO); 204 : } 205 0 : return err; 206 : } 207 : EXPORT_SYMBOL(errseq_check_and_advance);
