cxl: fix nested locking hang during EEH hotplug

Commit 14a3ae34bfd0 ("cxl: Prevent read/write to AFU config space while AFU
not configured") introduced a rwsem to fix an invalid memory access that
occurred when someone attempts to access the config space of an AFU on a
vPHB whilst the AFU is deconfigured, such as during EEH recovery.

It turns out that it's possible to run into a nested locking issue when EEH
recovery fails and a full device hotplug is required.
cxl_pci_error_detected() deconfigures the AFU, taking a writer lock on
configured_rwsem. When EEH recovery fails, the EEH code calls
pci_hp_remove_devices() to remove the device, which in turn calls
cxl_remove() -> cxl_pci_remove_afu() -> pci_deconfigure_afu(), which tries
to grab the writer lock that's already held.

Standard rwsem semantics don't express what we really want to do here and
don't allow for nested locking. Fix this by replacing the rwsem with an
atomic_t which we can control more finely. Allow the AFU to be locked
multiple times so long as there are no readers.

Fixes: 14a3ae34bfd0 ("cxl: Prevent read/write to AFU config space while AFU not configured")
Cc: stable@vger.kernel.org # v4.9+
Signed-off-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Acked-by: Frederic Barrat <fbarrat@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

1 files changed, 3 insertions, 2 deletions

diff --git a/drivers/misc/cxl/cxl.h b/drivers/misc/cxl/cxl.h
index 6c722d96b775..79e60ec70bd3 100644
--- a/drivers/misc/cxl/cxl.h
+++ b/drivers/misc/cxl/cxl.h
@@ -418,8 +418,9 @@ struct cxl_afu {
 	struct dentry *debugfs;
 	struct mutex contexts_lock;
 	spinlock_t afu_cntl_lock;
-	/* Used to block access to AFU config space while deconfigured */
-	struct rw_semaphore configured_rwsem;
+
+	/* -1: AFU deconfigured/locked, >= 0: number of readers */
+	atomic_t configured_state;
 
 	/* AFU error buffer fields and bin attribute for sysfs */
 	u64 eb_len, eb_offset;