1 #include <linux/err.h> 2 #include <linux/bug.h> 3 #include <linux/atomic.h> 4 #include <linux/errseq.h> 5 6 /* 7 * An errseq_t is a way of recording errors in one place, and allowing any 8 * number of "subscribers" to tell whether it has changed since a previous 9 * point where it was sampled. 10 * 11 * It's implemented as an unsigned 32-bit value. The low order bits are 12 * designated to hold an error code (between 0 and -MAX_ERRNO). The upper bits 13 * are used as a counter. This is done with atomics instead of locking so that 14 * these functions can be called from any context. 15 * 16 * The general idea is for consumers to sample an errseq_t value. That value 17 * can later be used to tell whether any new errors have occurred since that 18 * sampling was done. 19 * 20 * Note that there is a risk of collisions if new errors are being recorded 21 * frequently, since we have so few bits to use as a counter. 22 * 23 * To mitigate this, one bit is used as a flag to tell whether the value has 24 * been sampled since a new value was recorded. That allows us to avoid bumping 25 * the counter if no one has sampled it since the last time an error was 26 * recorded. 27 * 28 * A new errseq_t should always be zeroed out. A errseq_t value of all zeroes 29 * is the special (but common) case where there has never been an error. An all 30 * zero value thus serves as the "epoch" if one wishes to know whether there 31 * has ever been an error set since it was first initialized. 32 */ 33 34 /* The low bits are designated for error code (max of MAX_ERRNO) */ 35 #define ERRSEQ_SHIFT ilog2(MAX_ERRNO + 1) 36 37 /* This bit is used as a flag to indicate whether the value has been seen */ 38 #define ERRSEQ_SEEN (1 << ERRSEQ_SHIFT) 39 40 /* The lowest bit of the counter */ 41 #define ERRSEQ_CTR_INC (1 << (ERRSEQ_SHIFT + 1)) 42 43 /** 44 * errseq_set - set a errseq_t for later reporting 45 * @eseq: errseq_t field that should be set 46 * @err: error to set (must be between -1 and -MAX_ERRNO) 47 * 48 * This function sets the error in *eseq, and increments the sequence counter 49 * if the last sequence was sampled at some point in the past. 50 * 51 * Any error set will always overwrite an existing error. 52 * 53 * We do return the latest value here, primarily for debugging purposes. The 54 * return value should not be used as a previously sampled value in later calls 55 * as it will not have the SEEN flag set. 56 */ 57 errseq_t errseq_set(errseq_t *eseq, int err) 58 { 59 errseq_t cur, old; 60 61 /* MAX_ERRNO must be able to serve as a mask */ 62 BUILD_BUG_ON_NOT_POWER_OF_2(MAX_ERRNO + 1); 63 64 /* 65 * Ensure the error code actually fits where we want it to go. If it 66 * doesn't then just throw a warning and don't record anything. We 67 * also don't accept zero here as that would effectively clear a 68 * previous error. 69 */ 70 old = READ_ONCE(*eseq); 71 72 if (WARN(unlikely(err == 0 || (unsigned int)-err > MAX_ERRNO), 73 "err = %d\n", err)) 74 return old; 75 76 for (;;) { 77 errseq_t new; 78 79 /* Clear out error bits and set new error */ 80 new = (old & ~(MAX_ERRNO|ERRSEQ_SEEN)) | -err; 81 82 /* Only increment if someone has looked at it */ 83 if (old & ERRSEQ_SEEN) 84 new += ERRSEQ_CTR_INC; 85 86 /* If there would be no change, then call it done */ 87 if (new == old) { 88 cur = new; 89 break; 90 } 91 92 /* Try to swap the new value into place */ 93 cur = cmpxchg(eseq, old, new); 94 95 /* 96 * Call it success if we did the swap or someone else beat us 97 * to it for the same value. 98 */ 99 if (likely(cur == old || cur == new)) 100 break; 101 102 /* Raced with an update, try again */ 103 old = cur; 104 } 105 return cur; 106 } 107 EXPORT_SYMBOL(errseq_set); 108 109 /** 110 * errseq_sample - grab current errseq_t value 111 * @eseq: pointer to errseq_t to be sampled 112 * 113 * This function allows callers to sample an errseq_t value, marking it as 114 * "seen" if required. 115 */ 116 errseq_t errseq_sample(errseq_t *eseq) 117 { 118 errseq_t old = READ_ONCE(*eseq); 119 errseq_t new = old; 120 121 /* 122 * For the common case of no errors ever having been set, we can skip 123 * marking the SEEN bit. Once an error has been set, the value will 124 * never go back to zero. 125 */ 126 if (old != 0) { 127 new |= ERRSEQ_SEEN; 128 if (old != new) 129 cmpxchg(eseq, old, new); 130 } 131 return new; 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 * Returns 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 & MAX_ERRNO); 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 int errseq_check_and_advance(errseq_t *eseq, errseq_t *since) 174 { 175 int err = 0; 176 errseq_t old, new; 177 178 /* 179 * Most callers will want to use the inline wrapper to check this, 180 * so that the common case of no error is handled without needing 181 * to take the lock that protects the "since" value. 182 */ 183 old = READ_ONCE(*eseq); 184 if (old != *since) { 185 /* 186 * Set the flag and try to swap it into place if it has 187 * changed. 188 * 189 * We don't care about the outcome of the swap here. If the 190 * swap doesn't occur, then it has either been updated by a 191 * writer who is altering the value in some way (updating 192 * counter or resetting the error), or another reader who is 193 * just setting the "seen" flag. Either outcome is OK, and we 194 * can advance "since" and return an error based on what we 195 * have. 196 */ 197 new = old | ERRSEQ_SEEN; 198 if (new != old) 199 cmpxchg(eseq, old, new); 200 *since = new; 201 err = -(new & MAX_ERRNO); 202 } 203 return err; 204 } 205 EXPORT_SYMBOL(errseq_check_and_advance); 206