On Tue, 2024-05-28 at 21:41 +0800, Heng Qi wrote:
> When the following snippet is run, lockdep will report a deadlock[1].
>
> /* Acquire all queues dim_locks */
> for (i = 0; i < vi->max_queue_pairs; i++)
> mutex_lock(&vi->rq[i].dim_lock);
>
> There's no deadlock here because the vq locks are always taken
> in the same order, but lockdep can not figure it out. So refactoring
> the code to alleviate the problem.
>
> [1]
> ========================================================
> WARNING: possible recursive locking detected
> 6.9.0-rc7+ #319 Not tainted
> --------------------------------------------
> ethtool/962 is trying to acquire lock:
>
> but task is already holding lock:
>
> other info that might help us debug this:
> Possible unsafe locking scenario:
>
> CPU0
> ----
> lock(&vi->rq[i].dim_lock);
> lock(&vi->rq[i].dim_lock);
>
> *** DEADLOCK ***
>
> May be due to missing lock nesting notation
>
> 3 locks held by ethtool/962:
> #0: ffffffff82dbaab0 (cb_lock){++++}-{3:3}, at: genl_rcv+0x19/0x40
> #1: ffffffff82dad0a8 (rtnl_mutex){+.+.}-{3:3}, at:
> ethnl_default_set_doit+0xbe/0x1e0
>
> stack backtrace:
> CPU: 6 PID: 962 Comm: ethtool Not tainted 6.9.0-rc7+ #319
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
> rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
> Call Trace:
> <TASK>
> dump_stack_lvl+0x79/0xb0
> check_deadlock+0x130/0x220
> __lock_acquire+0x861/0x990
> lock_acquire.part.0+0x72/0x1d0
> ? lock_acquire+0xf8/0x130
> __mutex_lock+0x71/0xd50
> virtnet_set_coalesce+0x151/0x190
> __ethnl_set_coalesce.isra.0+0x3f8/0x4d0
> ethnl_set_coalesce+0x34/0x90
> ethnl_default_set_doit+0xdd/0x1e0
> genl_family_rcv_msg_doit+0xdc/0x130
> genl_family_rcv_msg+0x154/0x230
> ? __pfx_ethnl_default_set_doit+0x10/0x10
> genl_rcv_msg+0x4b/0xa0
> ? __pfx_genl_rcv_msg+0x10/0x10
> netlink_rcv_skb+0x5a/0x110
> genl_rcv+0x28/0x40
> netlink_unicast+0x1af/0x280
> netlink_sendmsg+0x20e/0x460
> __sys_sendto+0x1fe/0x210
> ? find_held_lock+0x2b/0x80
> ? do_user_addr_fault+0x3a2/0x8a0
> ? __lock_release+0x5e/0x160
> ? do_user_addr_fault+0x3a2/0x8a0
> ? lock_release+0x72/0x140
> ? do_user_addr_fault+0x3a7/0x8a0
> __x64_sys_sendto+0x29/0x30
> do_syscall_64+0x78/0x180
> entry_SYSCALL_64_after_hwframe+0x76/0x7e
>
> Fixes: 4d4ac2ececd3 ("virtio_net: Add a lock for per queue RX coalesce")
> Signed-off-by: Heng Qi <hengqi@xxxxxxxxxxxxxxxxx>
This would have deserved a changelog after the commit message.
The patch LGTM (for obvious reasons ;), but it deserves an explicit ack
from Jason and/or Michael
Cheers,
Paolo
