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Date:   Thu, 06 Feb 2020 10:59:49 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Jakub Sitnicki <jakub@...udflare.com>, bpf@...r.kernel.org
Cc:     netdev@...r.kernel.org, kernel-team@...udflare.com,
        John Fastabend <john.fastabend@...il.com>
Subject: RE: [PATCH bpf 1/3] bpf, sockmap: Don't sleep while holding RCU lock
 on tear-down

Jakub Sitnicki wrote:
> rcu_read_lock is needed to protect access to psock inside sock_map_unref
> when tearing down the map. However, we can't afford to sleep in lock_sock
> while in RCU read-side critical section. Grab the RCU lock only after we
> have locked the socket.
> 
> This fixes RCU warnings triggerable on a VM with 1 vCPU when free'ing a
> sockmap/sockhash that contains at least one socket:
> 
> | =============================
> | WARNING: suspicious RCU usage
> | 5.5.0-04005-g8fc91b972b73 #450 Not tainted
> | -----------------------------
> | include/linux/rcupdate.h:272 Illegal context switch in RCU read-side critical section!
> |
> | other info that might help us debug this:
> |
> |
> | rcu_scheduler_active = 2, debug_locks = 1
> | 4 locks held by kworker/0:1/62:
> |  #0: ffff88813b019748 ((wq_completion)events){+.+.}, at: process_one_work+0x1d7/0x5e0
> |  #1: ffffc900000abe50 ((work_completion)(&map->work)){+.+.}, at: process_one_work+0x1d7/0x5e0
> |  #2: ffffffff82065d20 (rcu_read_lock){....}, at: sock_map_free+0x5/0x170
> |  #3: ffff8881368c5df8 (&stab->lock){+...}, at: sock_map_free+0x64/0x170
> |
> | stack backtrace:
> | CPU: 0 PID: 62 Comm: kworker/0:1 Not tainted 5.5.0-04005-g8fc91b972b73 #450
> | Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20190727_073836-buildvm-ppc64le-16.ppc.fedoraproject.org-3.fc31 04/01/2014
> | Workqueue: events bpf_map_free_deferred
> | Call Trace:
> |  dump_stack+0x71/0xa0
> |  ___might_sleep+0x105/0x190
> |  lock_sock_nested+0x28/0x90
> |  sock_map_free+0x95/0x170
> |  bpf_map_free_deferred+0x58/0x80
> |  process_one_work+0x260/0x5e0
> |  worker_thread+0x4d/0x3e0
> |  kthread+0x108/0x140
> |  ? process_one_work+0x5e0/0x5e0
> |  ? kthread_park+0x90/0x90
> |  ret_from_fork+0x3a/0x50
> 
> | =============================
> | WARNING: suspicious RCU usage
> | 5.5.0-04005-g8fc91b972b73-dirty #452 Not tainted
> | -----------------------------
> | include/linux/rcupdate.h:272 Illegal context switch in RCU read-side critical section!
> |
> | other info that might help us debug this:
> |
> |
> | rcu_scheduler_active = 2, debug_locks = 1
> | 4 locks held by kworker/0:1/62:
> |  #0: ffff88813b019748 ((wq_completion)events){+.+.}, at: process_one_work+0x1d7/0x5e0
> |  #1: ffffc900000abe50 ((work_completion)(&map->work)){+.+.}, at: process_one_work+0x1d7/0x5e0
> |  #2: ffffffff82065d20 (rcu_read_lock){....}, at: sock_hash_free+0x5/0x1d0
> |  #3: ffff888139966e00 (&htab->buckets[i].lock){+...}, at: sock_hash_free+0x92/0x1d0
> |
> | stack backtrace:
> | CPU: 0 PID: 62 Comm: kworker/0:1 Not tainted 5.5.0-04005-g8fc91b972b73-dirty #452
> | Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20190727_073836-buildvm-ppc64le-16.ppc.fedoraproject.org-3.fc31 04/01/2014
> | Workqueue: events bpf_map_free_deferred
> | Call Trace:
> |  dump_stack+0x71/0xa0
> |  ___might_sleep+0x105/0x190
> |  lock_sock_nested+0x28/0x90
> |  sock_hash_free+0xec/0x1d0
> |  bpf_map_free_deferred+0x58/0x80
> |  process_one_work+0x260/0x5e0
> |  worker_thread+0x4d/0x3e0
> |  kthread+0x108/0x140
> |  ? process_one_work+0x5e0/0x5e0
> |  ? kthread_park+0x90/0x90
> |  ret_from_fork+0x3a/0x50
> 
> Fixes: 7e81a3530206 ("bpf: Sockmap, ensure sock lock held during tear down")
> Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
> ---
>  net/core/sock_map.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)

Thanks! Fixes for fixes agh.

Acked-by: John Fastabend <john.fastabend@...il.com>

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