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 7 /* 8 * An errseq_t is a way of recording errors in one place, and allowing any 9 * number of "subscribers" to tell whether it has changed since a previous 10 * point where it was sampled. 11 * 12 * It's implemented as an unsigned 32-bit value. The low order bits are 13 * designated to hold an error code (between 0 and -MAX_ERRNO). The upper bits 14 * are used as a counter. This is done with atomics instead of locking so that 15 * these functions can be called from any context. 16 * 17 * The general idea is for consumers to sample an errseq_t value. That value 18 * can later be used to tell whether any new errors have occurred since that 19 * sampling was done. 20 * 21 * Note that there is a risk of collisions if new errors are being recorded 22 * frequently, since we have so few bits to use as a counter. 23 * 24 * To mitigate this, one bit is used as a flag to tell whether the value has 25 * been sampled since a new value was recorded. That allows us to avoid bumping 26 * the counter if no one has sampled it since the last time an error was 27 * recorded. 28 * 29 * A new errseq_t should always be zeroed out. A errseq_t value of all zeroes 30 * is the special (but common) case where there has never been an error. An all 31 * zero value thus serves as the "epoch" if one wishes to know whether there 32 * has ever been an error set since it was first initialized. 33 */ 34 35 /* The low bits are designated for error code (max of MAX_ERRNO) */ 36 #define ERRSEQ_SHIFT ilog2(MAX_ERRNO + 1) 37 38 /* This bit is used as a flag to indicate whether the value has been seen */ 39 #define ERRSEQ_SEEN (1 << ERRSEQ_SHIFT) 40 41 /* The lowest bit of the counter */ 42 #define ERRSEQ_CTR_INC (1 << (ERRSEQ_SHIFT + 1)) 43 44 /** 45 * errseq_set - set a errseq_t for later reporting 46 * @eseq: errseq_t field that should be set 47 * @err: error to set (must be between -1 and -MAX_ERRNO) 48 * 49 * This function sets the error in @eseq, and increments the sequence counter 50 * if the last sequence was sampled at some point in the past. 51 * 52 * Any error set will always overwrite an existing error. 53 * 54 * Return: The previous value, primarily for debugging purposes. The 55 * return value should not be used as a previously sampled value in later 56 * calls as it will not have the SEEN flag set. 57 */ 58 errseq_t errseq_set(errseq_t *eseq, int err) 59 { 60 errseq_t cur, old; 61 62 /* MAX_ERRNO must be able to serve as a mask */ 63 BUILD_BUG_ON_NOT_POWER_OF_2(MAX_ERRNO + 1); 64 65 /* 66 * Ensure the error code actually fits where we want it to go. If it 67 * doesn't then just throw a warning and don't record anything. We 68 * also don't accept zero here as that would effectively clear a 69 * previous error. 70 */ 71 old = READ_ONCE(*eseq); 72 73 if (WARN(unlikely(err == 0 || (unsigned int)-err > MAX_ERRNO), 74 "err = %d\n", err)) 75 return old; 76 77 for (;;) { 78 errseq_t new; 79 80 /* Clear out error bits and set new error */ 81 new = (old & ~(MAX_ERRNO|ERRSEQ_SEEN)) | -err; 82 83 /* Only increment if someone has looked at it */ 84 if (old & ERRSEQ_SEEN) 85 new += ERRSEQ_CTR_INC; 86 87 /* If there would be no change, then call it done */ 88 if (new == old) { 89 cur = new; 90 break; 91 } 92 93 /* Try to swap the new value into place */ 94 cur = cmpxchg(eseq, old, new); 95 96 /* 97 * Call it success if we did the swap or someone else beat us 98 * to it for the same value. 99 */ 100 if (likely(cur == old || cur == new)) 101 break; 102 103 /* Raced with an update, try again */ 104 old = cur; 105 } 106 return cur; 107 } 108 EXPORT_SYMBOL(errseq_set); 109 110 /** 111 * errseq_sample() - Grab current errseq_t value. 112 * @eseq: Pointer to errseq_t to be sampled. 113 * 114 * This function allows callers to sample an errseq_t value, marking it as 115 * "seen" if required. 116 * 117 * Return: The current errseq value. 118 */ 119 errseq_t errseq_sample(errseq_t *eseq) 120 { 121 errseq_t old = READ_ONCE(*eseq); 122 errseq_t new = old; 123 124 /* 125 * For the common case of no errors ever having been set, we can skip 126 * marking the SEEN bit. Once an error has been set, the value will 127 * never go back to zero. 128 */ 129 if (old != 0) { 130 new |= ERRSEQ_SEEN; 131 if (old != new) 132 cmpxchg(eseq, old, new); 133 } 134 return new; 135 } 136 EXPORT_SYMBOL(errseq_sample); 137 138 /** 139 * errseq_check() - Has an error occurred since a particular sample point? 140 * @eseq: Pointer to errseq_t value to be checked. 141 * @since: Previously-sampled errseq_t from which to check. 142 * 143 * Grab the value that eseq points to, and see if it has changed @since 144 * the given value was sampled. The @since value is not advanced, so there 145 * is no need to mark the value as seen. 146 * 147 * Return: The latest error set in the errseq_t or 0 if it hasn't changed. 148 */ 149 int errseq_check(errseq_t *eseq, errseq_t since) 150 { 151 errseq_t cur = READ_ONCE(*eseq); 152 153 if (likely(cur == since)) 154 return 0; 155 return -(cur & MAX_ERRNO); 156 } 157 EXPORT_SYMBOL(errseq_check); 158 159 /** 160 * errseq_check_and_advance() - Check an errseq_t and advance to current value. 161 * @eseq: Pointer to value being checked and reported. 162 * @since: Pointer to previously-sampled errseq_t to check against and advance. 163 * 164 * Grab the eseq value, and see whether it matches the value that @since 165 * points to. If it does, then just return 0. 166 * 167 * If it doesn't, then the value has changed. Set the "seen" flag, and try to 168 * swap it into place as the new eseq value. Then, set that value as the new 169 * "since" value, and return whatever the error portion is set to. 170 * 171 * Note that no locking is provided here for concurrent updates to the "since" 172 * value. The caller must provide that if necessary. Because of this, callers 173 * may want to do a lockless errseq_check before taking the lock and calling 174 * this. 175 * 176 * Return: Negative errno if one has been stored, or 0 if no new error has 177 * occurred. 178 */ 179 int errseq_check_and_advance(errseq_t *eseq, errseq_t *since) 180 { 181 int err = 0; 182 errseq_t old, new; 183 184 /* 185 * Most callers will want to use the inline wrapper to check this, 186 * so that the common case of no error is handled without needing 187 * to take the lock that protects the "since" value. 188 */ 189 old = READ_ONCE(*eseq); 190 if (old != *since) { 191 /* 192 * Set the flag and try to swap it into place if it has 193 * changed. 194 * 195 * We don't care about the outcome of the swap here. If the 196 * swap doesn't occur, then it has either been updated by a 197 * writer who is altering the value in some way (updating 198 * counter or resetting the error), or another reader who is 199 * just setting the "seen" flag. Either outcome is OK, and we 200 * can advance "since" and return an error based on what we 201 * have. 202 */ 203 new = old | ERRSEQ_SEEN; 204 if (new != old) 205 cmpxchg(eseq, old, new); 206 *since = new; 207 err = -(new & MAX_ERRNO); 208 } 209 return err; 210 } 211 EXPORT_SYMBOL(errseq_check_and_advance); 212