linux-kernel - [PATCH v3 2/4] dax/bus.c: fix locking for unregister_dax_dev / unregister_dax

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Message-Id: <20240430-vv-dax_abi_fixes-v3-2-e3dcd755774c@intel.com>
Date: Tue, 30 Apr 2024 11:44:24 -0600
From: Vishal Verma <vishal.l.verma@...el.com>
To: Dan Williams <dan.j.williams@...el.com>, 
 Dave Jiang <dave.jiang@...el.com>, 
 Alison Schofield <alison.schofield@...el.com>, 
 Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org, nvdimm@...ts.linux.dev, linux-cxl@...r.kernel.org, 
 linux-kernel@...r.kernel.org, Vishal Verma <vishal.l.verma@...el.com>
Subject: [PATCH v3 2/4] dax/bus.c: fix locking for unregister_dax_dev /
 unregister_dax_mapping paths

Commit c05ae9d85b47 ("dax/bus.c: replace driver-core lock usage by a local rwsem")
aimed to undo device_lock() abuses for protecting changes to dax-driver
internal data-structures like the dax_region resource tree to
device-dax-instance range structures. However, the device_lock() was
legitamately enforcing that devices to be deleted were not current
actively attached to any driver nor assigned any capacity from the
region.

As a result of the device_lock restoration in delete_store(), the
conditional locking in unregister_dev_dax() and unregister_dax_mapping()
can be removed.

Fixes: c05ae9d85b47 ("dax/bus.c: replace driver-core lock usage by a local rwsem")
Reported-by: Dan Williams <dan.j.williams@...el.com>
Reviewed-by: Dan Williams <dan.j.williams@...el.com>
Signed-off-by: Vishal Verma <vishal.l.verma@...el.com>
---
 drivers/dax/bus.c | 42 ++++++++----------------------------------
 1 file changed, 8 insertions(+), 34 deletions(-)

diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
index 7924dd542a13..e2c7354ce328 100644
--- a/drivers/dax/bus.c
+++ b/drivers/dax/bus.c
@@ -465,26 +465,17 @@ static void free_dev_dax_ranges(struct dev_dax *dev_dax)
 		trim_dev_dax_range(dev_dax);
 }
 
-static void __unregister_dev_dax(void *dev)
+static void unregister_dev_dax(void *dev)
 {
 	struct dev_dax *dev_dax = to_dev_dax(dev);
 
 	dev_dbg(dev, "%s\n", __func__);
 
+	down_write(&dax_region_rwsem);
 	kill_dev_dax(dev_dax);
 	device_del(dev);
 	free_dev_dax_ranges(dev_dax);
 	put_device(dev);
-}
-
-static void unregister_dev_dax(void *dev)
-{
-	if (rwsem_is_locked(&dax_region_rwsem))
-		return __unregister_dev_dax(dev);
-
-	if (WARN_ON_ONCE(down_write_killable(&dax_region_rwsem) != 0))
-		return;
-	__unregister_dev_dax(dev);
 	up_write(&dax_region_rwsem);
 }
 
@@ -560,15 +551,10 @@ static ssize_t delete_store(struct device *dev, struct device_attribute *attr,
 	if (!victim)
 		return -ENXIO;
 
-	rc = down_write_killable(&dax_region_rwsem);
-	if (rc)
-		return rc;
-	rc = down_write_killable(&dax_dev_rwsem);
-	if (rc) {
-		up_write(&dax_region_rwsem);
-		return rc;
-	}
+	device_lock(dev);
+	device_lock(victim);
 	dev_dax = to_dev_dax(victim);
+	down_write(&dax_dev_rwsem);
 	if (victim->driver || dev_dax_size(dev_dax))
 		rc = -EBUSY;
 	else {
@@ -589,11 +575,12 @@ static ssize_t delete_store(struct device *dev, struct device_attribute *attr,
 			rc = -EBUSY;
 	}
 	up_write(&dax_dev_rwsem);
+	device_unlock(victim);
 
 	/* won the race to invalidate the device, clean it up */
 	if (do_del)
 		devm_release_action(dev, unregister_dev_dax, victim);
-	up_write(&dax_region_rwsem);
+	device_unlock(dev);
 	put_device(victim);
 
 	return rc;
@@ -705,7 +692,7 @@ static void dax_mapping_release(struct device *dev)
 	put_device(parent);
 }
 
-static void __unregister_dax_mapping(void *data)
+static void unregister_dax_mapping(void *data)
 {
 	struct device *dev = data;
 	struct dax_mapping *mapping = to_dax_mapping(dev);
@@ -713,25 +700,12 @@ static void __unregister_dax_mapping(void *data)
 
 	dev_dbg(dev, "%s\n", __func__);
 
-	lockdep_assert_held_write(&dax_region_rwsem);
-
 	dev_dax->ranges[mapping->range_id].mapping = NULL;
 	mapping->range_id = -1;
 
 	device_unregister(dev);
 }
 
-static void unregister_dax_mapping(void *data)
-{
-	if (rwsem_is_locked(&dax_region_rwsem))
-		return __unregister_dax_mapping(data);
-
-	if (WARN_ON_ONCE(down_write_killable(&dax_region_rwsem) != 0))
-		return;
-	__unregister_dax_mapping(data);
-	up_write(&dax_region_rwsem);
-}
-
 static struct dev_dax_range *get_dax_range(struct device *dev)
 {
 	struct dax_mapping *mapping = to_dax_mapping(dev);

-- 
2.44.0