On Mon, Jan 25, 2016 at 02:38:36PM -0500, Tejun Heo wrote: > On Mon, Jan 25, 2016 at 09:49:42AM +0100, Christoph Hellwig wrote: > > FYI, my use case was also related to percpu-ref. The percpu ref API > > is unfortunately really hard to use and will almost always involve > > a work queue due to the complex interaction between percpu_ref_kill > > and percpu_ref_exit. One thing that would help a lot of callers would > > That's interesting. Can you please elaborate on how kill and exit > interact to make things complex? That we need to first call kill to tear down the reference, then we get a release callback which is in the calling context of the last percpu_ref_put, but will need to call percpu_ref_exit from process context again. This means if any percpu_ref_put is from non-process context we will always need a work_struct or similar to schedule the final percpu_ref_exit. Except when.. > > be a percpu_ref_exit_sync that kills the ref and waits for all references > > to go away synchronously. > > That shouldn't be difficult to implement. One minor concern is that > it's almost guaranteed that there will be cases where the > synchronicity is exposed to userland. Anyways, can you please > describe the use case? We use this completion scheme where the percpu_ref_exit is done from the same context as the percpu_ref_kill which previously waits for the last reference drop. But for these cases exposing the synchronicity to the caller (including userland) actually is intentional. My use case is a new storage target, broadly similar to the SCSI target, which happens to exhibit the same behavior. In that case we only want to return from the teardown function when all I/O on a 'queue' of sorts has finished, for example during module removal. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
