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Subject[PATCHv2 1/5] nvme: fix lockdep warning in nvme_mpath_clear_current_path
Date
When running blktest's nvme/005 with a lockdep enabled kernel the test
case fails due to the following lockdep splat in dmesg:

[ 18.206166] =============================
[ 18.207286] WARNING: suspicious RCU usage
[ 18.208417] 4.17.0-rc5 #881 Not tainted
[ 18.209487] -----------------------------
[ 18.210612] drivers/nvme/host/nvme.h:457 suspicious rcu_dereference_check() usage!
[ 18.213486]
[ 18.213486] other info that might help us debug this:
[ 18.213486]
[ 18.214745]
[ 18.214745] rcu_scheduler_active = 2, debug_locks = 1
[ 18.215798] 3 locks held by kworker/u32:5/1102:
[ 18.216535] #0: (ptrval) ((wq_completion)"nvme-wq"){+.+.}, at: process_one_work+0x152/0x5c0
[ 18.217983] #1: (ptrval) ((work_completion)(&ctrl->scan_work)){+.+.}, at: process_one_work+0x152/0x5c0
[ 18.219584] #2: (ptrval) (&subsys->lock#2){+.+.}, at: nvme_ns_remove+0x43/0x1c0 [nvme_core]
[ 18.221037]
[ 18.221037] stack backtrace:
[ 18.221721] CPU: 12 PID: 1102 Comm: kworker/u32:5 Not tainted 4.17.0-rc5 #881
[ 18.222830] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.0.0-prebuilt.qemu-project.org 04/01/2014
[ 18.224451] Workqueue: nvme-wq nvme_scan_work [nvme_core]
[ 18.225308] Call Trace:
[ 18.225704] dump_stack+0x78/0xb3
[ 18.226224] nvme_ns_remove+0x1a3/0x1c0 [nvme_core]
[ 18.226975] nvme_validate_ns+0x87/0x850 [nvme_core]
[ 18.227749] ? blk_queue_exit+0x69/0x110
[ 18.228358] ? blk_queue_exit+0x81/0x110
[ 18.228960] ? direct_make_request+0x1a0/0x1a0
[ 18.229649] nvme_scan_work+0x212/0x2d0 [nvme_core]
[ 18.230411] process_one_work+0x1d8/0x5c0
[ 18.231037] ? process_one_work+0x152/0x5c0
[ 18.231705] worker_thread+0x45/0x3e0
[ 18.232282] kthread+0x101/0x140
[ 18.232788] ? process_one_work+0x5c0/0x5c0

The only caller of nvme_mpath_clear_current_path() is nvme_ns_remove()
which holds the subsys lock so it's likely a false positive, but when
using rcu_access_pointer(), we're telling rcu and lockdep that we're
only after the pointer falue.

Fixes: 32acab3181c7 ("nvme: implement multipath access to nvme subsystems")
Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Suggested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

---
Changes to v1:
- Change rcu_dereference_protected() to rc_access_pointer() (Paul)
---
drivers/nvme/host/nvme.h | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
index 17d2f7cf3fed..af2bb6bc984d 100644
--- a/drivers/nvme/host/nvme.h
+++ b/drivers/nvme/host/nvme.h
@@ -22,6 +22,7 @@
#include <linux/lightnvm.h>
#include <linux/sed-opal.h>
#include <linux/fault-inject.h>
+#include <linux/rcupdate.h>

extern unsigned int nvme_io_timeout;
#define NVME_IO_TIMEOUT (nvme_io_timeout * HZ)
@@ -454,7 +455,7 @@ static inline void nvme_mpath_clear_current_path(struct nvme_ns *ns)
{
struct nvme_ns_head *head = ns->head;

- if (head && ns == srcu_dereference(head->current_path, &head->srcu))
+ if (head && ns == rcu_access_pointer(head->current_path))
rcu_assign_pointer(head->current_path, NULL);
}
struct nvme_ns *nvme_find_path(struct nvme_ns_head *head);
--
2.16.3
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