184cbadadSJeff Layton #include <linux/err.h> 284cbadadSJeff Layton #include <linux/bug.h> 384cbadadSJeff Layton #include <linux/atomic.h> 484cbadadSJeff Layton #include <linux/errseq.h> 584cbadadSJeff Layton 684cbadadSJeff Layton /* 784cbadadSJeff Layton * An errseq_t is a way of recording errors in one place, and allowing any 884cbadadSJeff Layton * number of "subscribers" to tell whether it has changed since a previous 984cbadadSJeff Layton * point where it was sampled. 1084cbadadSJeff Layton * 1184cbadadSJeff Layton * It's implemented as an unsigned 32-bit value. The low order bits are 1284cbadadSJeff Layton * designated to hold an error code (between 0 and -MAX_ERRNO). The upper bits 1384cbadadSJeff Layton * are used as a counter. This is done with atomics instead of locking so that 1484cbadadSJeff Layton * these functions can be called from any context. 1584cbadadSJeff Layton * 1684cbadadSJeff Layton * The general idea is for consumers to sample an errseq_t value. That value 1784cbadadSJeff Layton * can later be used to tell whether any new errors have occurred since that 1884cbadadSJeff Layton * sampling was done. 1984cbadadSJeff Layton * 2084cbadadSJeff Layton * Note that there is a risk of collisions if new errors are being recorded 2184cbadadSJeff Layton * frequently, since we have so few bits to use as a counter. 2284cbadadSJeff Layton * 2384cbadadSJeff Layton * To mitigate this, one bit is used as a flag to tell whether the value has 2484cbadadSJeff Layton * been sampled since a new value was recorded. That allows us to avoid bumping 2584cbadadSJeff Layton * the counter if no one has sampled it since the last time an error was 2684cbadadSJeff Layton * recorded. 2784cbadadSJeff Layton * 2884cbadadSJeff Layton * A new errseq_t should always be zeroed out. A errseq_t value of all zeroes 2984cbadadSJeff Layton * is the special (but common) case where there has never been an error. An all 3084cbadadSJeff Layton * zero value thus serves as the "epoch" if one wishes to know whether there 3184cbadadSJeff Layton * has ever been an error set since it was first initialized. 3284cbadadSJeff Layton */ 3384cbadadSJeff Layton 3484cbadadSJeff Layton /* The low bits are designated for error code (max of MAX_ERRNO) */ 3584cbadadSJeff Layton #define ERRSEQ_SHIFT ilog2(MAX_ERRNO + 1) 3684cbadadSJeff Layton 3784cbadadSJeff Layton /* This bit is used as a flag to indicate whether the value has been seen */ 3884cbadadSJeff Layton #define ERRSEQ_SEEN (1 << ERRSEQ_SHIFT) 3984cbadadSJeff Layton 4084cbadadSJeff Layton /* The lowest bit of the counter */ 4184cbadadSJeff Layton #define ERRSEQ_CTR_INC (1 << (ERRSEQ_SHIFT + 1)) 4284cbadadSJeff Layton 4384cbadadSJeff Layton /** 44*3acdfd28SJeff Layton * errseq_set - set a errseq_t for later reporting 4584cbadadSJeff Layton * @eseq: errseq_t field that should be set 46*3acdfd28SJeff Layton * @err: error to set (must be between -1 and -MAX_ERRNO) 4784cbadadSJeff Layton * 4884cbadadSJeff Layton * This function sets the error in *eseq, and increments the sequence counter 4984cbadadSJeff Layton * if the last sequence was sampled at some point in the past. 5084cbadadSJeff Layton * 5184cbadadSJeff Layton * Any error set will always overwrite an existing error. 5284cbadadSJeff Layton * 53*3acdfd28SJeff Layton * We do return the latest value here, primarily for debugging purposes. The 54*3acdfd28SJeff Layton * return value should not be used as a previously sampled value in later calls 55*3acdfd28SJeff Layton * as it will not have the SEEN flag set. 5684cbadadSJeff Layton */ 57*3acdfd28SJeff Layton errseq_t errseq_set(errseq_t *eseq, int err) 5884cbadadSJeff Layton { 5984cbadadSJeff Layton errseq_t cur, old; 6084cbadadSJeff Layton 6184cbadadSJeff Layton /* MAX_ERRNO must be able to serve as a mask */ 6284cbadadSJeff Layton BUILD_BUG_ON_NOT_POWER_OF_2(MAX_ERRNO + 1); 6384cbadadSJeff Layton 6484cbadadSJeff Layton /* 6584cbadadSJeff Layton * Ensure the error code actually fits where we want it to go. If it 6684cbadadSJeff Layton * doesn't then just throw a warning and don't record anything. We 6784cbadadSJeff Layton * also don't accept zero here as that would effectively clear a 6884cbadadSJeff Layton * previous error. 6984cbadadSJeff Layton */ 7084cbadadSJeff Layton old = READ_ONCE(*eseq); 7184cbadadSJeff Layton 7284cbadadSJeff Layton if (WARN(unlikely(err == 0 || (unsigned int)-err > MAX_ERRNO), 7384cbadadSJeff Layton "err = %d\n", err)) 7484cbadadSJeff Layton return old; 7584cbadadSJeff Layton 7684cbadadSJeff Layton for (;;) { 7784cbadadSJeff Layton errseq_t new; 7884cbadadSJeff Layton 7984cbadadSJeff Layton /* Clear out error bits and set new error */ 8084cbadadSJeff Layton new = (old & ~(MAX_ERRNO|ERRSEQ_SEEN)) | -err; 8184cbadadSJeff Layton 8284cbadadSJeff Layton /* Only increment if someone has looked at it */ 8384cbadadSJeff Layton if (old & ERRSEQ_SEEN) 8484cbadadSJeff Layton new += ERRSEQ_CTR_INC; 8584cbadadSJeff Layton 8684cbadadSJeff Layton /* If there would be no change, then call it done */ 8784cbadadSJeff Layton if (new == old) { 8884cbadadSJeff Layton cur = new; 8984cbadadSJeff Layton break; 9084cbadadSJeff Layton } 9184cbadadSJeff Layton 9284cbadadSJeff Layton /* Try to swap the new value into place */ 9384cbadadSJeff Layton cur = cmpxchg(eseq, old, new); 9484cbadadSJeff Layton 9584cbadadSJeff Layton /* 9684cbadadSJeff Layton * Call it success if we did the swap or someone else beat us 9784cbadadSJeff Layton * to it for the same value. 9884cbadadSJeff Layton */ 9984cbadadSJeff Layton if (likely(cur == old || cur == new)) 10084cbadadSJeff Layton break; 10184cbadadSJeff Layton 10284cbadadSJeff Layton /* Raced with an update, try again */ 10384cbadadSJeff Layton old = cur; 10484cbadadSJeff Layton } 10584cbadadSJeff Layton return cur; 10684cbadadSJeff Layton } 107*3acdfd28SJeff Layton EXPORT_SYMBOL(errseq_set); 10884cbadadSJeff Layton 10984cbadadSJeff Layton /** 11084cbadadSJeff Layton * errseq_sample - grab current errseq_t value 11184cbadadSJeff Layton * @eseq: pointer to errseq_t to be sampled 11284cbadadSJeff Layton * 11384cbadadSJeff Layton * This function allows callers to sample an errseq_t value, marking it as 11484cbadadSJeff Layton * "seen" if required. 11584cbadadSJeff Layton */ 11684cbadadSJeff Layton errseq_t errseq_sample(errseq_t *eseq) 11784cbadadSJeff Layton { 11884cbadadSJeff Layton errseq_t old = READ_ONCE(*eseq); 11984cbadadSJeff Layton errseq_t new = old; 12084cbadadSJeff Layton 12184cbadadSJeff Layton /* 12284cbadadSJeff Layton * For the common case of no errors ever having been set, we can skip 12384cbadadSJeff Layton * marking the SEEN bit. Once an error has been set, the value will 12484cbadadSJeff Layton * never go back to zero. 12584cbadadSJeff Layton */ 12684cbadadSJeff Layton if (old != 0) { 12784cbadadSJeff Layton new |= ERRSEQ_SEEN; 12884cbadadSJeff Layton if (old != new) 12984cbadadSJeff Layton cmpxchg(eseq, old, new); 13084cbadadSJeff Layton } 13184cbadadSJeff Layton return new; 13284cbadadSJeff Layton } 13384cbadadSJeff Layton EXPORT_SYMBOL(errseq_sample); 13484cbadadSJeff Layton 13584cbadadSJeff Layton /** 13684cbadadSJeff Layton * errseq_check - has an error occurred since a particular sample point? 13784cbadadSJeff Layton * @eseq: pointer to errseq_t value to be checked 13884cbadadSJeff Layton * @since: previously-sampled errseq_t from which to check 13984cbadadSJeff Layton * 14084cbadadSJeff Layton * Grab the value that eseq points to, and see if it has changed "since" 14184cbadadSJeff Layton * the given value was sampled. The "since" value is not advanced, so there 14284cbadadSJeff Layton * is no need to mark the value as seen. 14384cbadadSJeff Layton * 14484cbadadSJeff Layton * Returns the latest error set in the errseq_t or 0 if it hasn't changed. 14584cbadadSJeff Layton */ 14684cbadadSJeff Layton int errseq_check(errseq_t *eseq, errseq_t since) 14784cbadadSJeff Layton { 14884cbadadSJeff Layton errseq_t cur = READ_ONCE(*eseq); 14984cbadadSJeff Layton 15084cbadadSJeff Layton if (likely(cur == since)) 15184cbadadSJeff Layton return 0; 15284cbadadSJeff Layton return -(cur & MAX_ERRNO); 15384cbadadSJeff Layton } 15484cbadadSJeff Layton EXPORT_SYMBOL(errseq_check); 15584cbadadSJeff Layton 15684cbadadSJeff Layton /** 15784cbadadSJeff Layton * errseq_check_and_advance - check an errseq_t and advance to current value 15884cbadadSJeff Layton * @eseq: pointer to value being checked and reported 15984cbadadSJeff Layton * @since: pointer to previously-sampled errseq_t to check against and advance 16084cbadadSJeff Layton * 16184cbadadSJeff Layton * Grab the eseq value, and see whether it matches the value that "since" 16284cbadadSJeff Layton * points to. If it does, then just return 0. 16384cbadadSJeff Layton * 16484cbadadSJeff Layton * If it doesn't, then the value has changed. Set the "seen" flag, and try to 16584cbadadSJeff Layton * swap it into place as the new eseq value. Then, set that value as the new 16684cbadadSJeff Layton * "since" value, and return whatever the error portion is set to. 16784cbadadSJeff Layton * 16884cbadadSJeff Layton * Note that no locking is provided here for concurrent updates to the "since" 16984cbadadSJeff Layton * value. The caller must provide that if necessary. Because of this, callers 17084cbadadSJeff Layton * may want to do a lockless errseq_check before taking the lock and calling 17184cbadadSJeff Layton * this. 17284cbadadSJeff Layton */ 17384cbadadSJeff Layton int errseq_check_and_advance(errseq_t *eseq, errseq_t *since) 17484cbadadSJeff Layton { 17584cbadadSJeff Layton int err = 0; 17684cbadadSJeff Layton errseq_t old, new; 17784cbadadSJeff Layton 17884cbadadSJeff Layton /* 17984cbadadSJeff Layton * Most callers will want to use the inline wrapper to check this, 18084cbadadSJeff Layton * so that the common case of no error is handled without needing 18184cbadadSJeff Layton * to take the lock that protects the "since" value. 18284cbadadSJeff Layton */ 18384cbadadSJeff Layton old = READ_ONCE(*eseq); 18484cbadadSJeff Layton if (old != *since) { 18584cbadadSJeff Layton /* 18684cbadadSJeff Layton * Set the flag and try to swap it into place if it has 18784cbadadSJeff Layton * changed. 18884cbadadSJeff Layton * 18984cbadadSJeff Layton * We don't care about the outcome of the swap here. If the 19084cbadadSJeff Layton * swap doesn't occur, then it has either been updated by a 19184cbadadSJeff Layton * writer who is altering the value in some way (updating 19284cbadadSJeff Layton * counter or resetting the error), or another reader who is 19384cbadadSJeff Layton * just setting the "seen" flag. Either outcome is OK, and we 19484cbadadSJeff Layton * can advance "since" and return an error based on what we 19584cbadadSJeff Layton * have. 19684cbadadSJeff Layton */ 19784cbadadSJeff Layton new = old | ERRSEQ_SEEN; 19884cbadadSJeff Layton if (new != old) 19984cbadadSJeff Layton cmpxchg(eseq, old, new); 20084cbadadSJeff Layton *since = new; 20184cbadadSJeff Layton err = -(new & MAX_ERRNO); 20284cbadadSJeff Layton } 20384cbadadSJeff Layton return err; 20484cbadadSJeff Layton } 20584cbadadSJeff Layton EXPORT_SYMBOL(errseq_check_and_advance); 206