Re: [PATCH] cxl/port: Prevent out-of-order decoder allocation

[Dan Williams <dan.j.williams@intel.com>]

Zijun Hu wrote:
> On 2024/10/15 08:06, Dan Williams wrote:
> > With the recent change to allow out-of-order decoder de-commit it
> > highlights a need to strengthen the in-order decoder commit guarantees.
> > As it stands match_free_decoder() ensures that if 2 regions are racing
> > decoder allocations the one that wins the race will get the lower id
> > decoder, but that still leaves the race to *commit* the decoder.
> > 
> > Rather than have this complicated case of "reserved in-order, but may
> > still commit out-of-order", just arrange for the reservation order to
> > match the commit-order. In other words, prevent subsequent allocations
> > until the last reservation is committed.
> > 
> > This precludes overlapping region creation events and requires the
> > previous regionN to either move forward to the decoder commit stage or
> > drop its reservation before regionN+1 can move forward. That is,
> > provided that regionN and regionN+1 decode through the same switch port.
> > 
> > As a side effect this allows match_free_decoder() to drop its dependency
> > on needing write access to the device_find_child() @data parameter [1].
> > 
> > Reported-by: Zijun Hu <zijun_hu@icloud.com>
> > Closes: http://lore.kernel.org/20240905-const_dfc_prepare-v4-0-4180e1d5a244@quicinc.com [1]
> > Cc: Davidlohr Bueso <dave@stgolabs.net>
> > Cc: Vishal Verma <vishal.l.verma@intel.com>
> > Cc: Alison Schofield <alison.schofield@intel.com>
> > Cc: Jonathan Cameron <jonathan.cameron@huawei.com>
> > Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> > ---
> > This patch is incremental to "cxl: Initialization and shutdown fixes"
> > 
> > [2]: http://lore.kernel.org/172862483180.2150669.5564474284074502692.stgit@dwillia2-xfh.jf.intel.com
> > 
> >  drivers/cxl/core/region.c |   43 +++++++++++++++++++++++++++++++++----------
> >  1 file changed, 33 insertions(+), 10 deletions(-)
> > 
> > diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> > index 3478d2058303..dff618c708dc 100644
> > --- a/drivers/cxl/core/region.c
> > +++ b/drivers/cxl/core/region.c
[..]
> 
> Above checking seems meaningless since there are only one CLDX with ID
> port->commit_end + 1.
> 
> logical of below my proposal maybe what you want.
> https://lore.kernel.org/all/4239bfd4-d5fe-4ac8-a087-9e1584765e61@icloud.com/

No. Consider that port->commit_end + 1 may walk off the end of available
decoders.