From 1a3fc2c402810bf336882e695abd1678dbc8d279 Mon Sep 17 00:00:00 2001 From: Juergen Gross Date: Thu, 8 Jun 2017 16:03:42 +0200 Subject: [PATCH] xen: avoid deadlock in xenbus driver 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: [] xenbus_dev_queue_reply+0x3c/0x230 [ 248.017163] [ 248.017163] but task is already holding lock: [ 248.023096] (xb_write_mutex){+.+...}, at: [] 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: # 4.11 Reported-by: Andre Przywara Signed-off-by: Juergen Gross Tested-by: Andre Przywara Signed-off-by: Juergen Gross --- 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);