#include <linux/err.h>#include <linux/bug.h>#include <linux/atomic.h>#include <linux/errseq.h> /* * An errseq_t is a way of recording errors in one place, and allowing any * number of "subscribers" to tell whether it has changed since a previous * point where it was sampled. * * It's implemented as an unsigned 32-bit value. The low order bits are * designated to hold an error code (between 0 and -MAX_ERRNO). The upper bits * are used as a counter. This is done with atomics instead of locking so that * these functions can be called from any context. * * The general idea is for consumers to sample an errseq_t value. That value * can later be used to tell whether any new errors have occurred since that * sampling was done. * * Note that there is a risk of collisions if new errors are being recorded * frequently, since we have so few bits to use as a counter. * * To mitigate this, one bit is used as a flag to tell whether the value has * been sampled since a new value was recorded. That allows us to avoid bumping * the counter if no one has sampled it since the last time an error was * recorded. * * A new errseq_t should always be zeroed out.  A errseq_t value of all zeroes * is the special (but common) case where there has never been an error. An all * zero value thus serves as the "epoch" if one wishes to know whether there * has ever been an error set since it was first initialized. */ /* The low bits are designated for error code (max of MAX_ERRNO) */#define ERRSEQ_SHIFT		ilog2(MAX_ERRNO + 1) /* This bit is used as a flag to indicate whether the value has been seen */#define ERRSEQ_SEEN		(1 << ERRSEQ_SHIFT) /* The lowest bit of the counter */#define ERRSEQ_CTR_INC		(1 << (ERRSEQ_SHIFT + 1)) /** * errseq_set - set a errseq_t for later reporting * @eseq: errseq_t field that should be set * @err: error to set (must be between -1 and -MAX_ERRNO) * * This function sets the error in *eseq, and increments the sequence counter * if the last sequence was sampled at some point in the past. * * Any error set will always overwrite an existing error. * * We do return the latest value here, primarily for debugging purposes. The * return value should not be used as a previously sampled value in later calls * as it will not have the SEEN flag set. */errseq_t errseq_set(errseq_t *eseq, int err){	errseq_t cur, old; 	/* MAX_ERRNO must be able to serve as a mask */	BUILD_BUG_ON_NOT_POWER_OF_2(MAX_ERRNO + 1); 	/*	 * Ensure the error code actually fits where we want it to go. If it	 * doesn't then just throw a warning and don't record anything. We	 * also don't accept zero here as that would effectively clear a	 * previous error.	 */	old = READ_ONCE(*eseq); 	if (WARN(unlikely(err == 0 || (unsigned int)-err > MAX_ERRNO),				"err = %d\n", err))		return old; 	for (;;) {		errseq_t new; 		/* Clear out error bits and set new error */		new = (old & ~(MAX_ERRNO|ERRSEQ_SEEN)) | -err; 		/* Only increment if someone has looked at it */		if (old & ERRSEQ_SEEN)			new += ERRSEQ_CTR_INC; 		/* If there would be no change, then call it done */		if (new == old) {			cur = new;			break;		} 		/* Try to swap the new value into place */		cur = cmpxchg(eseq, old, new); 		/*		 * Call it success if we did the swap or someone else beat us		 * to it for the same value.		 */		if (likely(cur == old || cur == new))			break; 		/* Raced with an update, try again */		old = cur;	}	return cur;}EXPORT_SYMBOL(errseq_set); /** * errseq_sample - grab current errseq_t value * @eseq: pointer to errseq_t to be sampled * * This function allows callers to sample an errseq_t value, marking it as * "seen" if required. */errseq_t errseq_sample(errseq_t *eseq){	errseq_t old = READ_ONCE(*eseq);	errseq_t new = old; 	/*	 * For the common case of no errors ever having been set, we can skip	 * marking the SEEN bit. Once an error has been set, the value will	 * never go back to zero.	 */	if (old != 0) {		new |= ERRSEQ_SEEN;		if (old != new)			cmpxchg(eseq, old, new);	}	return new;}EXPORT_SYMBOL(errseq_sample); /** * errseq_check - has an error occurred since a particular sample point? * @eseq: pointer to errseq_t value to be checked * @since: previously-sampled errseq_t from which to check * * Grab the value that eseq points to, and see if it has changed "since" * the given value was sampled. The "since" value is not advanced, so there * is no need to mark the value as seen. * * Returns the latest error set in the errseq_t or 0 if it hasn't changed. */int errseq_check(errseq_t *eseq, errseq_t since){	errseq_t cur = READ_ONCE(*eseq); 	if (likely(cur == since))		return 0;	return -(cur & MAX_ERRNO);}EXPORT_SYMBOL(errseq_check); /** * errseq_check_and_advance - check an errseq_t and advance to current value * @eseq: pointer to value being checked and reported * @since: pointer to previously-sampled errseq_t to check against and advance * * Grab the eseq value, and see whether it matches the value that "since" * points to. If it does, then just return 0. * * If it doesn't, then the value has changed. Set the "seen" flag, and try to * swap it into place as the new eseq value. Then, set that value as the new * "since" value, and return whatever the error portion is set to. * * Note that no locking is provided here for concurrent updates to the "since" * value. The caller must provide that if necessary. Because of this, callers * may want to do a lockless errseq_check before taking the lock and calling * this. */int errseq_check_and_advance(errseq_t *eseq, errseq_t *since){	int err = 0;	errseq_t old, new; 	/*	 * Most callers will want to use the inline wrapper to check this,	 * so that the common case of no error is handled without needing	 * to take the lock that protects the "since" value.	 */	old = READ_ONCE(*eseq);	if (old != *since) {		/*		 * Set the flag and try to swap it into place if it has		 * changed.		 *		 * We don't care about the outcome of the swap here. If the		 * swap doesn't occur, then it has either been updated by a		 * writer who is altering the value in some way (updating		 * counter or resetting the error), or another reader who is		 * just setting the "seen" flag. Either outcome is OK, and we		 * can advance "since" and return an error based on what we		 * have.		 */		new = old | ERRSEQ_SEEN;		if (new != old)			cmpxchg(eseq, old, new);		*since = new;		err = -(new & MAX_ERRNO);	}	return err;}EXPORT_SYMBOL(errseq_check_and_advance);