Re: [PATCH 4.14 022/284] SUNRPC: avoid race between mod_timer() and del_timer_sync()

[Eugeniu Rosca <erosca@de.adit-jv.com>]

Hello all,

On Mo, Apr 18, 2022 at 02:10:03 +0200, Greg Kroah-Hartman wrote:
> From: NeilBrown <neilb@suse.de>
> 
> commit 3848e96edf4788f772d83990022fa7023a233d83 upstream.
> 
> xprt_destory() claims XPRT_LOCKED and then calls del_timer_sync().
> Both xprt_unlock_connect() and xprt_release() call
>  ->release_xprt()
> which drops XPRT_LOCKED and *then* xprt_schedule_autodisconnect()
> which calls mod_timer().
> 
> This may result in mod_timer() being called *after* del_timer_sync().
> When this happens, the timer may fire long after the xprt has been freed,
> and run_timer_softirq() will probably crash.
> 
> The pairing of ->release_xprt() and xprt_schedule_autodisconnect() is
> always called under ->transport_lock.  So if we take ->transport_lock to
> call del_timer_sync(), we can be sure that mod_timer() will run first
> (if it runs at all).
> 
> Cc: stable@vger.kernel.org
> Signed-off-by: NeilBrown <neilb@suse.de>
> Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> ---
>  net/sunrpc/xprt.c |    7 +++++++
>  1 file changed, 7 insertions(+)
> 
> --- a/net/sunrpc/xprt.c
> +++ b/net/sunrpc/xprt.c
> @@ -1520,7 +1520,14 @@ static void xprt_destroy(struct rpc_xprt
>  	 */
>  	wait_on_bit_lock(&xprt->state, XPRT_LOCKED, TASK_UNINTERRUPTIBLE);
>  
> +	/*
> +	 * xprt_schedule_autodisconnect() can run after XPRT_LOCKED
> +	 * is cleared.  We use ->transport_lock to ensure the mod_timer()
> +	 * can only run *before* del_time_sync(), never after.
> +	 */
> +	spin_lock(&xprt->transport_lock);
>  	del_timer_sync(&xprt->timer);
> +	spin_unlock(&xprt->transport_lock);
>  
>  	/*
>  	 * Destroy sockets etc from the system workqueue so they can

This commit introduced the following warning [1][2] on the
v4.9, v4.14 and v4.19 stable trees, when booting Renesas H3ULCB
(and potentially other HW targets) from NFS.

Once in a while, the issue leads to the real freeze of the target.

The culprit commits have been identified via git bisecting (see [1]).

Additionally, it has been determined that what's missing for fixing
the issue on the stable trees are the two v5.3-rc1 mainline commits:

4f8943f8088348 ("SUNRPC: Replace direct task wakeups from softirq context")
b5e924191f8723 ("SUNRPC: Remove the bh-safe lock requirement on xprt->transport_lock")

However, attempting to port them to the stable trees leads to
significant amount of conflicts. Any idea if the culprit commit(s)
should better be reverted?

[1] https://gist.github.com/erosca/7b5f1dadd4172b38461478d38c1040b8
[2] Excerpt from [1]

[    7.548549] ================================
[    7.552827] WARNING: inconsistent lock state
[    7.557112] 4.14.292 #35 Not tainted
[    7.560694] --------------------------------
[    7.564973] inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage.
[    7.570994] swapper/0/0 [HC0[0]:SC1[3]:HE1:SE0] takes:
[    7.576142]  (&(&xprt->transport_lock)->rlock){+.?.}, at: [<ffff2000095a6a48>] xs_tcp_state_change+0x740/0xb24
[    7.586196] {SOFTIRQ-ON-W} state was registered at:
[    7.591093]   lock_acquire+0x724/0x790
[    7.594857]   _raw_spin_lock+0xec/0x12c
[    7.598706]   xprt_destroy+0xc8/0x214
[    7.602379]   xprt_put+0x34/0x40
[    7.605618]   rpc_new_client+0x1a8/0x8e8
[    7.609552]   rpc_create_xprt+0x124/0x300
[    7.613573]   rpc_create+0x234/0x410
[    7.617162]   nfs_create_rpc_client+0x33c/0x38c
[    7.621709]   nfs4_alloc_client+0x8f4/0xb5c
[    7.625904]   nfs_get_client+0x10d4/0x10e8
[    7.630013]   nfs4_set_client+0x1b0/0x254
[    7.634034]   nfs4_create_server+0x4c0/0x92c
[    7.638317]   nfs4_remote_mount+0x74/0xac
[    7.642339]   mount_fs+0x80/0x27c
[    7.645666]   vfs_kern_mount+0xe0/0x3b4
[    7.649514]   nfs_do_root_mount+0x8c/0xc8
[    7.653534]   nfs4_try_mount+0xdc/0x120
[    7.657382]   nfs_fs_mount+0x1b6c/0x2038
[    7.661315]   mount_fs+0x80/0x27c
[    7.664639]   vfs_kern_mount+0xe0/0x3b4
[    7.668488]   do_mount+0x1324/0x1c90
[    7.672074]   SyS_mount+0xc0/0xd0
[    7.675403]   mount_root+0xe4/0x1c0
[    7.678903]   prepare_namespace+0x174/0x184
[    7.683098]   kernel_init_freeable+0x5b4/0x674
[    7.687556]   kernel_init+0x18/0x138
[    7.691144]   ret_from_fork+0x10/0x18
[    7.694814] irq event stamp: 24322
[    7.698228] hardirqs last  enabled at (24322): [<ffff200009679f8c>] _raw_spin_unlock_irqrestore+0x100/0x108
[    7.707990] hardirqs last disabled at (24321): [<ffff200009679a38>] _raw_spin_lock_irqsave+0x30/0x138
[    7.717233] softirqs last  enabled at (24244): [<ffff2000080e79d8>] _local_bh_enable+0x78/0x84
[    7.725865] softirqs last disabled at (24245): [<ffff2000080e9ed4>] irq_exit+0x350/0x4e0
[    7.733969] 
[    7.733969] other info that might help us debug this:
[    7.740511]  Possible unsafe locking scenario:
[    7.740511] 
[    7.746442]        CPU0
[    7.748893]        ----
[    7.751343]   lock(&(&xprt->transport_lock)->rlock);
[    7.756323]   <Interrupt>
[    7.758947]     lock(&(&xprt->transport_lock)->rlock);
[    7.764100] 
[    7.764100]  *** DEADLOCK ***
[    7.764100] 
[    7.770038] 4 locks held by swapper/0/0:
[    7.773967]  #0:  (rcu_read_lock){....}, at: [<ffff200009318d1c>] netif_receive_skb_internal+0x1b0/0x99c
[    7.783487]  #1:  (rcu_read_lock){....}, at: [<ffff2000093e0b10>] ip_local_deliver_finish+0x1e4/0xae0
[    7.792743]  #2:  (slock-AF_INET-RPC/1){+.-.}, at: [<ffff2000094757cc>] tcp_v4_rcv+0x1a04/0x2694
[    7.801567]  #3:  (k-clock-AF_INET){++.-}, at: [<ffff2000095a633c>] xs_tcp_state_change+0x34/0xb24
[    7.810559] 
[    7.810559] stack backtrace:
[    7.814931] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.14.292 #35
[    7.821123] Hardware name: Renesas H3ULCB Kingfisher board based on r8a7795 ES2.0+ (DT)
[    7.829142] Call trace:
[    7.831599]  dump_backtrace+0x0/0x320
[    7.835272]  show_stack+0x24/0x30
[    7.838598]  dump_stack+0x150/0x1b8
[    7.842097]  print_usage_bug.part.23+0x5c4/0x724
[    7.846725]  mark_lock+0x940/0x113c
[    7.850223]  __lock_acquire+0x9a8/0x2e70
[    7.854156]  lock_acquire+0x724/0x790
[    7.857827]  _raw_spin_lock+0xec/0x12c
[    7.861586]  xs_tcp_state_change+0x740/0xb24
[    7.865869]  tcp_rcv_state_process+0x1330/0x3a34
[    7.870498]  tcp_v4_do_rcv+0x9c8/0x9dc
[    7.874256]  tcp_v4_rcv+0x1c5c/0x2694
[    7.877930]  ip_local_deliver_finish+0x770/0xae0
[    7.882560]  ip_local_deliver+0x1c0/0x528
[    7.886581]  ip_rcv_finish+0x770/0x1024
[    7.890427]  ip_rcv+0xa50/0xe5c
[    7.893579]  __netif_receive_skb_core+0x1adc/0x2164
[    7.898470]  __netif_receive_skb+0x1e0/0x1e8
[    7.902752]  netif_receive_skb_internal+0x6c8/0x99c
[    7.907642]  napi_gro_receive+0x79c/0x7dc
[    7.911666]  ravb_poll+0xc98/0x1594
[    7.915165]  napi_poll+0x260/0xb6c
[    7.918577]  net_rx_action+0x2fc/0x668
[    7.922336]  __do_softirq+0xec8/0x1620
[    7.926095]  irq_exit+0x350/0x4e0
[    7.929421]  __handle_domain_irq+0x124/0x1c0
[    7.933701]  gic_handle_irq+0x70/0xb0
[    7.937372]  el1_irq+0xb4/0x140
[    7.940523]  arch_cpu_idle+0x17c/0x7fc
[    7.944282]  default_idle_call+0x74/0x8c
[    7.948216]  do_idle+0x250/0x344
[    7.951453]  cpu_startup_entry+0x28/0x38
[    7.955387]  rest_init+0x5f0/0x604
[    7.958799]  start_kernel+0x5dc/0x60c

Best Regards,
Eugeniu Rosca