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 /* Leverage macro ERRSEQ_SEEN to define errno mask macro here */ 43 #define ERRNO_MASK (ERRSEQ_SEEN - 1) 44 45 /* The lowest bit of the counter */ 46 #define ERRSEQ_CTR_INC (1 << (ERRSEQ_SHIFT + 1)) 47 48 /** 49 * errseq_set - set a errseq_t for later reporting 50 * @eseq: errseq_t field that should be set 51 * @err: error to set (must be between -1 and -MAX_ERRNO) 52 * 53 * This function sets the error in @eseq, and increments the sequence counter 54 * if the last sequence was sampled at some point in the past. 55 * 56 * Any error set will always overwrite an existing error. 57 * 58 * Return: The previous value, primarily for debugging purposes. The 59 * return value should not be used as a previously sampled value in later 60 * calls as it will not have the SEEN flag set. 61 */ 62 errseq_t errseq_set(errseq_t *eseq, int err) 63 { 64 errseq_t cur, old; 65 66 67 /* 68 * Ensure the error code actually fits where we want it to go. If it 69 * doesn't then just throw a warning and don't record anything. We 70 * also don't accept zero here as that would effectively clear a 71 * previous error. 72 */ 73 old = READ_ONCE(*eseq); 74 75 if (WARN(unlikely(err == 0 || (unsigned int)-err > MAX_ERRNO), 76 "err = %d\n", err)) 77 return old; 78 79 for (;;) { 80 errseq_t new; 81 82 /* Clear out error bits and set new error */ 83 new = (old & ~(ERRNO_MASK | ERRSEQ_SEEN)) | -err; 84 85 /* Only increment if someone has looked at it */ 86 if (old & ERRSEQ_SEEN) 87 new += ERRSEQ_CTR_INC; 88 89 /* If there would be no change, then call it done */ 90 if (new == old) { 91 cur = new; 92 break; 93 } 94 95 /* Try to swap the new value into place */ 96 cur = cmpxchg(eseq, old, new); 97 98 /* 99 * Call it success if we did the swap or someone else beat us 100 * to it for the same value. 101 */ 102 if (likely(cur == old || cur == new)) 103 break; 104 105 /* Raced with an update, try again */ 106 old = cur; 107 } 108 return cur; 109 } 110 EXPORT_SYMBOL(errseq_set); 111 112 /** 113 * errseq_sample() - Grab current errseq_t value. 114 * @eseq: Pointer to errseq_t to be sampled. 115 * 116 * This function allows callers to initialise their errseq_t variable. 117 * If the error has been "seen", new callers will not see an old error. 118 * If there is an unseen error in @eseq, the caller of this function will 119 * see it the next time it checks for an error. 120 * 121 * Context: Any context. 122 * Return: The current errseq value. 123 */ 124 errseq_t errseq_sample(errseq_t *eseq) 125 { 126 errseq_t old = READ_ONCE(*eseq); 127 128 /* If nobody has seen this error yet, then we can be the first. */ 129 if (!(old & ERRSEQ_SEEN)) 130 old = 0; 131 return old; 132 } 133 EXPORT_SYMBOL(errseq_sample); 134 135 /** 136 * errseq_check() - Has an error occurred since a particular sample point? 137 * @eseq: Pointer to errseq_t value to be checked. 138 * @since: Previously-sampled errseq_t from which to check. 139 * 140 * Grab the value that eseq points to, and see if it has changed @since 141 * the given value was sampled. The @since value is not advanced, so there 142 * is no need to mark the value as seen. 143 * 144 * Return: The latest error set in the errseq_t or 0 if it hasn't changed. 145 */ 146 int errseq_check(errseq_t *eseq, errseq_t since) 147 { 148 errseq_t cur = READ_ONCE(*eseq); 149 150 if (likely(cur == since)) 151 return 0; 152 return -(cur & ERRNO_MASK); 153 } 154 EXPORT_SYMBOL(errseq_check); 155 156 /** 157 * errseq_check_and_advance() - Check an errseq_t and advance to current value. 158 * @eseq: Pointer to value being checked and reported. 159 * @since: Pointer to previously-sampled errseq_t to check against and advance. 160 * 161 * Grab the eseq value, and see whether it matches the value that @since 162 * points to. If it does, then just return 0. 163 * 164 * If it doesn't, then the value has changed. Set the "seen" flag, and try to 165 * swap it into place as the new eseq value. Then, set that value as the new 166 * "since" value, and return whatever the error portion is set to. 167 * 168 * Note that no locking is provided here for concurrent updates to the "since" 169 * value. The caller must provide that if necessary. Because of this, callers 170 * may want to do a lockless errseq_check before taking the lock and calling 171 * this. 172 * 173 * Return: Negative errno if one has been stored, or 0 if no new error has 174 * occurred. 175 */ 176 int errseq_check_and_advance(errseq_t *eseq, errseq_t *since) 177 { 178 int err = 0; 179 errseq_t old, new; 180 181 /* 182 * Most callers will want to use the inline wrapper to check this, 183 * so that the common case of no error is handled without needing 184 * to take the lock that protects the "since" value. 185 */ 186 old = READ_ONCE(*eseq); 187 if (old != *since) { 188 /* 189 * Set the flag and try to swap it into place if it has 190 * changed. 191 * 192 * We don't care about the outcome of the swap here. If the 193 * swap doesn't occur, then it has either been updated by a 194 * writer who is altering the value in some way (updating 195 * counter or resetting the error), or another reader who is 196 * just setting the "seen" flag. Either outcome is OK, and we 197 * can advance "since" and return an error based on what we 198 * have. 199 */ 200 new = old | ERRSEQ_SEEN; 201 if (new != old) 202 cmpxchg(eseq, old, new); 203 *since = new; 204 err = -(new & ERRNO_MASK); 205 } 206 return err; 207 } 208 EXPORT_SYMBOL(errseq_check_and_advance); 209