On Tue 09-05-17 11:49:16, 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>
The patch looks good to me. Feel free to add: Reviewed-by: Jan Kara <j...@suse.cz> Just two nits below: ... > +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. > + */ I'm not sure which locking you are speaking about here. Is the comment stale? > + 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 "bumping the seq count anyway" part is not quite true. Writer may see ERRSEQ_SEEN not set and so just update the error code and leave seq count as is. But since you compare full errseq_t for equality, this works out as designed... > + * 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); Honza -- Jan Kara <j...@suse.com> SUSE Labs, CR ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot _______________________________________________ Jfs-discussion mailing list Jfs-discussion@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/jfs-discussion