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 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 * Most callers will want to use the errseq_set inline wrapper to efficiently 54 * handle the common case where err is 0. 55 * 56 * We do return an errseq_t here, primarily for debugging purposes. The return 57 * value should not be used as a previously sampled value in later calls as it 58 * will not have the SEEN flag set. 59 */ 60 errseq_t __errseq_set(errseq_t *eseq, int err) 61 { 62 errseq_t cur, old; 63 64 /* MAX_ERRNO must be able to serve as a mask */ 65 BUILD_BUG_ON_NOT_POWER_OF_2(MAX_ERRNO + 1); 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 & ~(MAX_ERRNO|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 sample an errseq_t value, marking it as 117 * "seen" if required. 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 * Returns 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 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 & MAX_ERRNO); 205 } 206 return err; 207 } 208 EXPORT_SYMBOL(errseq_check_and_advance); 209