Re: [PATCH net v2 2/2] Revert "virtio_net: Add a lock for per queue RX coalesce"

[Jiri Pirko <jiri@resnulli.us>]

Thu, May 23, 2024 at 09:46:51AM CEST, hengqi@linux.alibaba.com wrote:
>This reverts commit 4d4ac2ececd3c42a08dd32a6e3a4aaf25f7efe44.
>
>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, and we
>can not make each lock a separate class because there can be more
>than MAX_LOCKDEP_SUBCLASSES of vqs.
>
>However, dropping the lock is harmless:
>  1. If dim is enabled, modifications made by dim worker to coalescing
>     params may cause the user's query results to be dirty data.
>  2. In scenarios (a) and (b), a spurious dim worker is scheduled,
>     but this can be handled correctly:
>     (a)
>       1. dim is on
>       2. net_dim call schedules a worker
>       3. dim is turning off
>       4. The worker checks that dim is off and then exits after
>          restoring dim's state.
>       5. The worker will not be scheduled until the next time dim is on.
>
>     (b)
>       1. dim is on
>       2. net_dim call schedules a worker
>       3. The worker checks that dim is on and keeps going
>       4. dim is turning off
>       5. The worker successfully configure this parameter to the device.
>       6. The worker will not be scheduled until the next time dim is on.
>
>[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>

Reviewed-by: Jiri Pirko <jiri@nvidia.com>