[PATCH] xen: avoid deadlock in xenbus driver

From: Juergen Gross
Date: Wed Jun 07 2017 - 12:24:21 EST


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 | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/xen/xenbus/xenbus_comms.c b/drivers/xen/xenbus/xenbus_comms.c
index 856ada5d39c9..a44bcdbf6533 100644
--- a/drivers/xen/xenbus/xenbus_comms.c
+++ b/drivers/xen/xenbus/xenbus_comms.c
@@ -305,18 +305,21 @@ static int process_msg(void)
req->body = state.body;
req->state = xb_req_state_got_reply;
list_del(&req->list);
+ mutex_unlock(&xb_write_mutex);
req->cb(req);
} else {
list_del(&req->list);
+ mutex_unlock(&xb_write_mutex);
kfree(req);
}
err = 0;
break;
}
}
- mutex_unlock(&xb_write_mutex);
- if (err)
+ if (err) {
+ mutex_unlock(&xb_write_mutex);
goto out;
+ }
}

mutex_unlock(&xs_response_mutex);
--
2.12.3