cxl/hdm: Fix a benign lockdep splat

The new helper "cxl_num_decoders_committed()" added a lockdep assertion
to validate that port->commit_end is protected against modification.
That assertion fires in init_hdm_decoder() where it is initializing
port->commit_end. Given that it is both accessing and writing that
property it obstensibly needs the lock.

In practice, CXL decoder commit rules (must commit in order) and the
in-order discovery of device decoders makes the manipulation of
->commit_end in init_hdm_decoder() safe. However, rather than rely on
the subtle rules of CXL hardware, just make the implementation obviously
correct from a software perspective.

The Fixes: tag is only for cleaning up a lockdep splat, there is no
functional issue addressed by this fix.

Fixes: 458ba8189cb4 ("cxl: Add cxl_decoders_committed() helper")
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/170025232811.2147250.16376901801315194121.stgit@djiang5-mobl3
Acked-by: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

diff --git a/drivers/cxl/core/hdm.c b/drivers/cxl/core/hdm.c
index 1cc9be8..529baa8 100644 (file)
--- a/drivers/cxl/core/hdm.c
+++ b/drivers/cxl/core/hdm.c
@@ -839,6 +839,8 @@ static int init_hdm_decoder(struct cxl_port *port, struct cxl_decoder *cxld,
                        cxld->target_type = CXL_DECODER_HOSTONLYMEM;
                else
                        cxld->target_type = CXL_DECODER_DEVMEM;
+
+               guard(rwsem_write)(&cxl_region_rwsem);
                if (cxld->id != cxl_num_decoders_committed(port)) {
                        dev_warn(&port->dev,
                                 "decoder%d.%d: Committed out of order\n",