Re: [PATCH net v3 2/2] virtio_net: fix a spurious deadlock issue

[Heng Qi <hengqi@linux.alibaba.com>]

On Thu, 30 May 2024 10:34:07 +0200, Paolo Abeni <pabeni@redhat.com> wrote:
> 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@linux.alibaba.com>
> 
> This would have deserved a changelog after the commit message.

I declared the changelog in the cover-letter, but I can initiate a new
RESEND version with a changelog in this patch if you want :)

> 
> The patch LGTM (for obvious reasons ;), but it deserves an explicit ack
> from Jason and/or Michael

Thanks.

> 
> Cheers,
> 
> Paolo
>