There has been a report about a deadlock in the xenbus driver:
[ 247.979498] ======================================================
[ 247.985688] WARNING: possible circular locking dependency detected
[ 247.991882] 4.12.0-rc4-00022-gc4b25c0 #575 Not tainted
[ 247.997040] ------------------------------------------------------
[ 248.003232] xenbus/91 is trying to acquire lock:
[ 248.007875] (&u->msgbuffer_mutex){+.+.+.}, at: [<ffff00000863e904>]
xenbus_dev_queue_reply+0x3c/0x230
[ 248.017163]
[ 248.017163] but task is already holding lock:
[ 248.023096] (xb_write_mutex){+.+...}, at: [<ffff00000863a940>]
xenbus_thread+0x5f0/0x798
[ 248.031267]
[ 248.031267] which lock already depends on the new lock.
[ 248.031267]
[ 248.039615]
[ 248.039615] the existing dependency chain (in reverse order) is:
[ 248.047176]
[ 248.047176] -> #1 (xb_write_mutex){+.+...}:
[ 248.052943] __lock_acquire+0x1728/0x1778
[ 248.057498] lock_acquire+0xc4/0x288
[ 248.061630] __mutex_lock+0x84/0x868
[ 248.065755] mutex_lock_nested+0x3c/0x50
[ 248.070227] xs_send+0x164/0x1f8
[ 248.074015] xenbus_dev_request_and_reply+0x6c/0x88
[ 248.079427] xenbus_file_write+0x260/0x420
[ 248.084073] __vfs_write+0x48/0x138
[ 248.088113] vfs_write+0xa8/0x1b8
[ 248.091983] SyS_write+0x54/0xb0
[ 248.095768] el0_svc_naked+0x24/0x28
[ 248.099897]
[ 248.099897] -> #0 (&u->msgbuffer_mutex){+.+.+.}:
[ 248.106088] print_circular_bug+0x80/0x2e0
[ 248.110730] __lock_acquire+0x1768/0x1778
[ 248.115288] lock_acquire+0xc4/0x288
[ 248.119417] __mutex_lock+0x84/0x868
[ 248.123545] mutex_lock_nested+0x3c/0x50
[ 248.128016] xenbus_dev_queue_reply+0x3c/0x230
[ 248.133005] xenbus_thread+0x788/0x798
[ 248.137306] kthread+0x110/0x140
[ 248.141087] ret_from_fork+0x10/0x40
It is rather easy to avoid by dropping xb_write_mutex before calling
xenbus_dev_queue_reply().
Fixes: fd8aa9095a95c02dcc35540a263267c29b8fda9d ("xen: optimize xenbus
driver for multiple concurrent xenstore accesses").
Cc: <stable@xxxxxxxxxxxxxxx> # 4.11
Reported-by: Andre Przywara <andre.przywara@xxxxxxx>
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
drivers/xen/xenbus/xenbus_comms.c | 21 ++++++++++-----------
1 file changed, 10 insertions(+), 11 deletions(-)
diff --git a/drivers/xen/xenbus/xenbus_comms.c b/drivers/xen/xenbus/xenbus_comms.c
index 856ada5d39c9..5b081a01779d 100644
--- a/drivers/xen/xenbus/xenbus_comms.c
+++ b/drivers/xen/xenbus/xenbus_comms.c
@@ -299,17 +299,7 @@ static int process_msg(void)
mutex_lock(&xb_write_mutex);
list_for_each_entry(req, &xs_reply_list, list) {
if (req->msg.req_id == state.msg.req_id) {
- if (req->state == xb_req_state_wait_reply) {
- req->msg.type = state.msg.type;
- req->msg.len = state.msg.len;
- req->body = state.body;
- req->state = xb_req_state_got_reply;
- list_del(&req->list);
- req->cb(req);
- } else {
- list_del(&req->list);
- kfree(req);
- }
+ list_del(&req->list);
err = 0;
break;
}
@@ -317,6 +307,15 @@ static int process_msg(void)
mutex_unlock(&xb_write_mutex);
if (err)
goto out;
+
+ if (req->state == xb_req_state_wait_reply) {
+ req->msg.type = state.msg.type;
+ req->msg.len = state.msg.len;
+ req->body = state.body;
+ req->state = xb_req_state_got_reply;
+ req->cb(req);
+ } else
+ kfree(req);
}
mutex_unlock(&xs_response_mutex);
--
2.12.3