Date:   Wed, 2 Sep 2020 11:33:37 +0200
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Artem Savkov <asavkov@...hat.com>, gregkh@...uxfoundation.org
Cc:     threeearcat@...il.com, sergey.senozhatsky@...il.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] pty: do tty_flip_buffer_push without port->lock in
 pty_write

On 01. 09. 20, 14:01, Artem Savkov wrote:
> b6da31b2c07c "tty: Fix data race in tty_insert_flip_string_fixed_flag"
> puts tty_flip_buffer_push under port->lock introducing the following
> possible circular locking dependency:
> 
> [30129.876566] ======================================================
> [30129.876566] WARNING: possible circular locking dependency detected
> [30129.876567] 5.9.0-rc2+ #3 Tainted: G S      W
> [30129.876568] ------------------------------------------------------
> [30129.876568] sysrq.sh/1222 is trying to acquire lock:
> [30129.876569] ffffffff92c39480 (console_owner){....}-{0:0}, at: console_unlock+0x3fe/0xa90
> 
> [30129.876572] but task is already holding lock:
> [30129.876572] ffff888107cb9018 (&pool->lock/1){-.-.}-{2:2}, at: show_workqueue_state.cold.55+0x15b/0x6ca
> 
> [30129.876576] which lock already depends on the new lock.
> 
> [30129.876577] the existing dependency chain (in reverse order) is:
> 
> [30129.876578] -> #3 (&pool->lock/1){-.-.}-{2:2}:
> [30129.876581]        _raw_spin_lock+0x30/0x70
> [30129.876581]        __queue_work+0x1a3/0x10f0
> [30129.876582]        queue_work_on+0x78/0x80
> [30129.876582]        pty_write+0x165/0x1e0
> [30129.876583]        n_tty_write+0x47f/0xf00
> [30129.876583]        tty_write+0x3d6/0x8d0
> [30129.876584]        vfs_write+0x1a8/0x650
> [30129.876584]        redirected_tty_write+0x6b/0xb0
> [30129.876585]        do_iter_write+0x38d/0x600
> [30129.876586]        vfs_writev+0x172/0x2d0
> [30129.876586]        do_writev+0x100/0x280
> [30129.876587]        do_syscall_64+0x33/0x40
> [30129.876587]        entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> [30129.876588] -> #2 (&port->lock#2){-.-.}-{2:2}:
> [30129.876590]        _raw_spin_lock_irqsave+0x3b/0x80
> [30129.876591]        tty_port_tty_get+0x1d/0xb0
> [30129.876592]        tty_port_default_wakeup+0xb/0x30
> [30129.876592]        serial8250_tx_chars+0x3d6/0x970
> [30129.876593]        serial8250_handle_irq.part.12+0x216/0x380
> [30129.876593]        serial8250_default_handle_irq+0x82/0xe0
> [30129.876594]        serial8250_interrupt+0xdd/0x1b0
> [30129.876595]        __handle_irq_event_percpu+0xfc/0x850
> [30129.876595]        handle_irq_event_percpu+0x73/0x150
> [30129.876596]        handle_irq_event+0xa1/0x12d
> [30129.876596]        handle_edge_irq+0x201/0xa30
> [30129.876597]        asm_call_on_stack+0x12/0x20
> [30129.876597]        common_interrupt+0x108/0x1d0
> [30129.876598]        asm_common_interrupt+0x1e/0x40
> [30129.876599]        cpuidle_enter_state+0x116/0xe90
> [30129.876599]        cpuidle_enter+0x4a/0xa0
> [30129.876600]        do_idle+0x4d6/0x610
> [30129.876600]        cpu_startup_entry+0x19/0x1b
> [30129.876601]        start_secondary+0x2c6/0x3a0
> [30129.876602]        secondary_startup_64+0xb6/0xc0
> 
> [30129.876602] -> #1 (&port->lock){-.-.}-{2:2}:
> [30129.876605]        _raw_spin_lock_irqsave+0x3b/0x80
> [30129.876605]        serial8250_console_write+0x12d/0x900
> [30129.876606]        console_unlock+0x679/0xa90
> [30129.876606]        register_console+0x371/0x6e0
> [30129.876607]        univ8250_console_init+0x24/0x27
> [30129.876607]        console_init+0x2f9/0x45e
> [30129.876608]        start_kernel+0x28a/0x3e9
> [30129.876608]        secondary_startup_64+0xb6/0xc0
> 
> [30129.876609] -> #0 (console_owner){....}-{0:0}:
> [30129.876611]        __lock_acquire+0x2f70/0x4e90
> [30129.876612]        lock_acquire+0x1ac/0xad0
> [30129.876612]        console_unlock+0x460/0xa90
> [30129.876613]        vprintk_emit+0x130/0x420
> [30129.876613]        printk+0x9f/0xc5
> [30129.876614]        show_pwq+0x154/0x618
> [30129.876615]        show_workqueue_state.cold.55+0x193/0x6ca
> [30129.876615]        __handle_sysrq+0x244/0x460
> [30129.876616]        write_sysrq_trigger+0x48/0x4a
> [30129.876616]        proc_reg_write+0x1a6/0x240
> [30129.876617]        vfs_write+0x1a8/0x650
> [30129.876617]        ksys_write+0xf1/0x1c0
> [30129.876618]        do_syscall_64+0x33/0x40
> [30129.876619]        entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> [30129.876619] other info that might help us debug this:
> 
> [30129.876620] Chain exists of:
> [30129.876621]   console_owner --> &port->lock#2 --> &pool->lock/1
> 
> [30129.876625]  Possible unsafe locking scenario:
> 
> [30129.876626]        CPU0                    CPU1
> [30129.876626]        ----                    ----
> [30129.876627]   lock(&pool->lock/1);
> [30129.876628]                                lock(&port->lock#2);
> [30129.876630]                                lock(&pool->lock/1);
> [30129.876631]   lock(console_owner);
> 
> [30129.876633]  *** DEADLOCK ***
> 
> [30129.876634] 5 locks held by sysrq.sh/1222:
> [30129.876634]  #0: ffff8881d3ce0470 (sb_writers#3){.+.+}-{0:0}, at: vfs_write+0x359/0x650
> [30129.876637]  #1: ffffffff92c612c0 (rcu_read_lock){....}-{1:2}, at: __handle_sysrq+0x4d/0x460
> [30129.876640]  #2: ffffffff92c612c0 (rcu_read_lock){....}-{1:2}, at: show_workqueue_state+0x5/0xf0
> [30129.876642]  #3: ffff888107cb9018 (&pool->lock/1){-.-.}-{2:2}, at: show_workqueue_state.cold.55+0x15b/0x6ca
> [30129.876645]  #4: ffffffff92c39980 (console_lock){+.+.}-{0:0}, at: vprintk_emit+0x123/0x420
> 
> [30129.876648] stack backtrace:
> [30129.876649] CPU: 3 PID: 1222 Comm: sysrq.sh Tainted: G S      W         5.9.0-rc2+ #3
> [30129.876649] Hardware name: Intel Corporation 2012 Client Platform/Emerald Lake 2, BIOS ACRVMBY1.86C.0078.P00.1201161002 01/16/2012
> [30129.876650] Call Trace:
> [30129.876650]  dump_stack+0x9d/0xe0
> [30129.876651]  check_noncircular+0x34f/0x410
> [30129.876652]  ? print_circular_bug+0x360/0x360
> [30129.876652]  ? mark_lock+0x144/0x19e0
> [30129.876653]  ? sched_clock+0x5/0x10
> [30129.876653]  __lock_acquire+0x2f70/0x4e90
> [30129.876654]  ? lockdep_hardirqs_on_prepare+0x4e0/0x4e0
> [30129.876654]  ? sched_clock+0x5/0x10
> [30129.876655]  ? sched_clock_cpu+0x18/0x1d0
> [30129.876655]  ? find_held_lock+0x3a/0x1c0
> [30129.876656]  lock_acquire+0x1ac/0xad0
> [30129.876656]  ? console_unlock+0x3fe/0xa90
> [30129.876657]  ? lock_downgrade+0x730/0x730
> [30129.876657]  ? rcu_read_unlock+0x50/0x50
> [30129.876658]  console_unlock+0x460/0xa90
> [30129.876658]  ? console_unlock+0x3fe/0xa90
> [30129.876659]  ? __down_trylock_console_sem+0x76/0x80
> [30129.876660]  vprintk_emit+0x130/0x420
> [30129.876660]  printk+0x9f/0xc5
> [30129.876661]  ? kmsg_dump_rewind_nolock+0xd9/0xd9
> [30129.876661]  show_pwq+0x154/0x618
> [30129.876662]  show_workqueue_state.cold.55+0x193/0x6ca
> [30129.876662]  ? printk+0x9f/0xc5
> [30129.876663]  ? print_worker_info+0x260/0x260
> [30129.876663]  ? debug_show_all_locks+0x1f2/0x209
> [30129.876664]  __handle_sysrq+0x244/0x460
> [30129.876665]  write_sysrq_trigger+0x48/0x4a
> [30129.876665]  proc_reg_write+0x1a6/0x240
> [30129.876666]  vfs_write+0x1a8/0x650
> [30129.876666]  ksys_write+0xf1/0x1c0
> [30129.876667]  ? __ia32_sys_read+0xb0/0xb0
> [30129.876667]  ? syscall_enter_from_user_mode+0x2a/0x2b0
> [30129.876668]  do_syscall_64+0x33/0x40
> [30129.876669]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> [30129.876669] RIP: 0033:0x7f0446ab28a8
> [30129.876671] Code: 89 02 48 c7 c0 ff ff ff ff eb b3 0f 1f 80 00 00 00 00 f3 0f 1e fa 48 8d 05 b5 4c 2d 00 8b 00 85 c0 75 17 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 58 c3 0f 1f 80 00 00 00 00 41 54 49 89 d4 55
> [30129.876671] RSP: 002b:00007fff991890c8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
> [30129.876672] RAX: ffffffffffffffda RBX: 0000000000000002 RCX: 00007f0446ab28a8
> [30129.876673] RDX: 0000000000000002 RSI: 000055887dae8e00 RDI: 0000000000000001
> [30129.876674] RBP: 000055887dae8e00 R08: 000000000000000a R09: 00007f0446b42d40
> [30129.876674] R10: 000000000000000a R11: 0000000000000246 R12: 00007f0446d836c0
> [30129.876675] R13: 0000000000000002 R14: 00007f0446d7e880 R15: 0000000000000002
> 
> It looks like the commit was aimed to protect tty_insert_flip_string and
> there is no need for tty_flip_buffer_push to be under this lock.

Looks sensible. But could you trim the log by removing at least the "?"
lines from the stacktrace? And the registers dumps above are not as
useful too? Also the tails of most of the stack traces are not
interesting too -- like everything behind __handle_irq_event_percpu,
vfs_write, etc.

Acked-by: Jiri Slaby <jirislaby@...nel.org>

> Fixes: b6da31b2c07c ("tty: Fix data race in tty_insert_flip_string_fixed_flag")
> Signed-off-by: Artem Savkov <asavkov@...hat.com>
> ---
>  drivers/tty/pty.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
> index 00099a8439d2..c6a1d8c4e689 100644
> --- a/drivers/tty/pty.c
> +++ b/drivers/tty/pty.c
> @@ -120,10 +120,10 @@ static int pty_write(struct tty_struct *tty, const unsigned char *buf, int c)
>  		spin_lock_irqsave(&to->port->lock, flags);
>  		/* Stuff the data into the input queue of the other end */
>  		c = tty_insert_flip_string(to->port, buf, c);
> +		spin_unlock_irqrestore(&to->port->lock, flags);
>  		/* And shovel */
>  		if (c)
>  			tty_flip_buffer_push(to->port);
> -		spin_unlock_irqrestore(&to->port->lock, flags);
>  	}
>  	return c;
>  }
> 


-- 
js

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