On Tue, May 09 2017, Jeff Layton wrote: > An errseq_t is a way of recording errors in one place, and allowing any > number of "subscribers" to tell whether an error has been set again > since a previous time. > > It's implemented as an unsigned 32-bit value that is managed with atomic > operations. The low order bits are designated to hold an error code > (max size of MAX_ERRNO). The upper bits are used as a counter. > > The API works with consumers sampling an errseq_t value at a particular > point in time. Later, that value can be used to tell whether new errors > have been set since that time. > > Note that there is a 1 in 512k risk of collisions here 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 it > was last bumped. > > Later patches will build on this infrastructure to change how writeback > errors are tracked in the kernel. > > Signed-off-by: Jeff Layton <jlay...@redhat.com>
I like that this is a separate lib/*.c - nicely structured too. Reviewed-by: NeilBrown <ne...@suse.com> Thanks, NeilBrown > --- > include/linux/errseq.h | 19 +++++ > lib/Makefile | 2 +- > lib/errseq.c | 199 > +++++++++++++++++++++++++++++++++++++++++++++++++ > 3 files changed, 219 insertions(+), 1 deletion(-) > create mode 100644 include/linux/errseq.h > create mode 100644 lib/errseq.c > > diff --git a/include/linux/errseq.h b/include/linux/errseq.h > new file mode 100644 > index 000000000000..0d2555f310cd > --- /dev/null > +++ b/include/linux/errseq.h > @@ -0,0 +1,19 @@ > +#ifndef _LINUX_ERRSEQ_H > +#define _LINUX_ERRSEQ_H > + > +/* See lib/errseq.c for more info */ > + > +typedef u32 errseq_t; > + > +void __errseq_set(errseq_t *eseq, int err); > +static inline void errseq_set(errseq_t *eseq, int err) > +{ > + /* Optimize for the common case of no error */ > + if (unlikely(err)) > + __errseq_set(eseq, err); > +} > + > +errseq_t errseq_sample(errseq_t *eseq); > +int errseq_check(errseq_t *eseq, errseq_t since); > +int errseq_check_and_advance(errseq_t *eseq, errseq_t *since); > +#endif > diff --git a/lib/Makefile b/lib/Makefile > index 320ac46a8725..2423afef40f7 100644 > --- a/lib/Makefile > +++ b/lib/Makefile > @@ -41,7 +41,7 @@ obj-y += bcd.o div64.o sort.o parser.o debug_locks.o > random32.o \ > gcd.o lcm.o list_sort.o uuid.o flex_array.o iov_iter.o clz_ctz.o \ > bsearch.o find_bit.o llist.o memweight.o kfifo.o \ > percpu-refcount.o percpu_ida.o rhashtable.o reciprocal_div.o \ > - once.o refcount.o > + once.o refcount.o errseq.o > obj-y += string_helpers.o > obj-$(CONFIG_TEST_STRING_HELPERS) += test-string_helpers.o > obj-y += hexdump.o > diff --git a/lib/errseq.c b/lib/errseq.c > new file mode 100644 > index 000000000000..0f8b4ed460f0 > --- /dev/null > +++ b/lib/errseq.c > @@ -0,0 +1,199 @@ > +#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 an arbitrary > + * time of their choosing. > + * > + * 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 at a > + * particular point in time. Later, that value can be used to tell whether > any > + * new errors have occurred since that time. > + * > + * 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 "ones" bit for 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 > + * > + * 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. > + * > + * Most callers will want to use the errseq_set inline wrapper to efficiently > + * handle the common case where err is 0. > + */ > +void __errseq_set(errseq_t *eseq, int err) > +{ > + errseq_t 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. > + */ > + if (WARN(unlikely(err == 0 || (unsigned int)-err > MAX_ERRNO), > + "err = %d\n", err)) > + return; > + > + old = READ_ONCE(*eseq); > + for (;;) { > + errseq_t new, cur; > + > + /* 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) > + 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; > + } > +} > +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 point in time? > + * @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 it to the > current value > + * @eseq: pointer to value being checked 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 lock. > + */ > + 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 bumping the seq count anyway, 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); > -- > 2.9.3 > > -- > To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in > the body of a message to majord...@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html
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