cxl/hdm: Fix dpa translation locking

The helper, cxl_dpa_resource_start(), snapshots the dpa-address of an
endpoint-decoder after acquiring the cxl_dpa_rwsem. However, it is
sufficient to assert that cxl_dpa_rwsem is held rather than acquire it
in the helper. Otherwise, it triggers multiple lockdep reports:

1/ Tracing callbacks are in an atomic context that can not acquire sleeping
locks:

    BUG: sleeping function called from invalid context at kernel/locking/rwsem.c:1525
    in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 1288, name: bash
    preempt_count: 2, expected: 0
    RCU nest depth: 0, expected: 0
    [..]
    Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS edk2-20230524-3.fc38 05/24/2023
    Call Trace:
     <TASK>
     dump_stack_lvl+0x71/0x90
     __might_resched+0x1b2/0x2c0
     down_read+0x1a/0x190
     cxl_dpa_resource_start+0x15/0x50 [cxl_core]
     cxl_trace_hpa+0x122/0x300 [cxl_core]
     trace_event_raw_event_cxl_poison+0x1c9/0x2d0 [cxl_core]

2/ The rwsem is already held in the inject poison path:

    WARNING: possible recursive locking detected
    6.7.0-rc2+ grate-driver#12 Tainted: G        W  OE    N
    --------------------------------------------
    bash/1288 is trying to acquire lock:
    ffffffffc05f73d0 (cxl_dpa_rwsem){++++}-{3:3}, at: cxl_dpa_resource_start+0x15/0x50 [cxl_core]

    but task is already holding lock:
    ffffffffc05f73d0 (cxl_dpa_rwsem){++++}-{3:3}, at: cxl_inject_poison+0x7d/0x1e0 [cxl_core]
    [..]
    Call Trace:
     <TASK>
     dump_stack_lvl+0x71/0x90
     __might_resched+0x1b2/0x2c0
     down_read+0x1a/0x190
     cxl_dpa_resource_start+0x15/0x50 [cxl_core]
     cxl_trace_hpa+0x122/0x300 [cxl_core]
     trace_event_raw_event_cxl_poison+0x1c9/0x2d0 [cxl_core]
     __traceiter_cxl_poison+0x5c/0x80 [cxl_core]
     cxl_inject_poison+0x1bc/0x1e0 [cxl_core]

This appears to have been an issue since the initial implementation and
uncovered by the new cxl-poison.sh test [1]. That test is now passing with
these changes.

Fixes: 28a3ae4 ("cxl/trace: Add an HPA to cxl_poison trace events")
Link: http://lore.kernel.org/r/e4f2716646918135ddbadf4146e92abb659de734.1700615159.git.alison.schofield@intel.com [1]
Cc: <[email protected]>
Cc: Alison Schofield <[email protected]>
Cc: Jonathan Cameron <[email protected]>
Cc: Dave Jiang <[email protected]>
Cc: Ira Weiny <[email protected]>
Signed-off-by: Dan Williams <[email protected]>

drivers/cxl/core/hdm.c

@@ -363,10 +363,9 @@ resource_size_t cxl_dpa_resource_start(struct cxl_endpoint_decoder *cxled)
 {
 	resource_size_t base = -1;
 
-	down_read(&cxl_dpa_rwsem);
+	lockdep_assert_held(&cxl_dpa_rwsem);
 	if (cxled->dpa_res)
 		base = cxled->dpa_res->start;
-	up_read(&cxl_dpa_rwsem);
 
 	return base;
 }

drivers/cxl/core/port.c

@@ -226,9 +226,9 @@ static ssize_t dpa_resource_show(struct device *dev, struct device_attribute *at
 			    char *buf)
 {
 	struct cxl_endpoint_decoder *cxled = to_cxl_endpoint_decoder(dev);
-	u64 base = cxl_dpa_resource_start(cxled);
 
-	return sysfs_emit(buf, "%#llx\n", base);
+	guard(rwsem_read)(&cxl_dpa_rwsem);
+	return sysfs_emit(buf, "%#llx\n", (u64)cxl_dpa_resource_start(cxled));
 }
 static DEVICE_ATTR_RO(dpa_resource);