On 6/12/19 9:01 AM, Christoph Hellwig wrote: > On Tue, Jun 11, 2019 at 10:56:42PM -0400, John Dorminy wrote: >> I believe the second of these might, but is not guaranteed to, >> preserve the first error observed in a child; I believe if you want to >> definitely save the first error you need an atomic. > > Is there any reason not to simply use a cmpxchg? Yes, it is a > relatively expensive operation, but once we are chaining bios we are out > of the super hot path anyway. We do something similar in xfs and iomap > already. > Agree. Thing is, we need to check if the parent status is NULL, _and_ the parent status might be modified asynchronously. So even a READ_ONCE() wouldn't cut it, as it would tell us that the parent status _was_ NULL, not that the parent status _is_ NULL by the time we're setting it. So cmpxchg() is it. Cheers, Hannes -- Dr. Hannes Reinecke Teamlead Storage & Networking hare@xxxxxxx +49 911 74053 688 SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: Felix Imendörffer, Mary Higgins, Sri Rasiah HRB 21284 (AG Nürnberg)
