Locking in Si2157 and Si2168 drivers.

[Adam Baker <linux@baker-net.org.uk>]

Hi,

I think I've found a locking scenario that can potentially cause a
deadlock situation in these drivers.

I was trying to update the patch Antti Palosaari created to add support
for signal stats to work with current git
(https://patchwork.linuxtv.org/patch/25821/). I believe that the fact
that this accesses the tuner I2C bus more frequently makes it much
more likely to hit the issue but I suspect it exists but is very rare
in the published kernel.

When my version deadlocks I see call stacks for the two threads as


[<ffffffff814e3bd4>] ? schedule_preempt_disabled+0x24/0x70
[<ffffffff814e53ba>] ? __mutex_lock_slowpath+0x8a/0xf0
[<ffffffff814e5436>] ? mutex_lock+0x16/0x25
[<ffffffffa0722df7>] ? si2168_select+0x37/0x90 [si2168]
[<ffffffffa070702f>] ? i2c_mux_master_xfer+0x2f/0x80 [i2c_mux]
[<ffffffffa000c734>] ? __i2c_transfer+0x74/0x220 [i2c_core]
[<ffffffffa000c961>] ? i2c_transfer+0x81/0xc0 [i2c_core]
[<ffffffffa000c9d8>] ? i2c_master_send+0x38/0x50 [i2c_core]
[<ffffffffa07b1134>] ? si2157_cmd_execute+0x44/0xd0 [si2157]
[<ffffffff8101155e>] ? __switch_to+0xde/0x580
[<ffffffffa07b1797>] ? si2157_stat_work+0x37/0x90 [si2157]
[<ffffffff81078c0b>] ? process_one_work+0x14b/0x3c0
[<ffffffff810792a4>] ? worker_thread+0x114/0x470
[<ffffffff81079190>] ? rescuer_thread+0x310/0x310
[<ffffffff8107dbe5>] ? kthread+0xc5/0xe0
[<ffffffff8107db20>] ? kthread_create_on_node+0x170/0x170
[<ffffffff814e6f7c>] ? ret_from_fork+0x7c/0xb0
[<ffffffff8107db20>] ? kthread_create_on_node+0x170/0x170


[<ffffffff814e575f>] ? __rt_mutex_slowlock+0x3f/0xc0
[<ffffffff814e58e9>] ? rt_mutex_slowlock+0xa9/0x1e0
[<ffffffff8108686f>] ? ttwu_do_wakeup+0xf/0xc0
[<ffffffffa000c97d>] ? i2c_transfer+0x9d/0xc0 [i2c_core]
[<ffffffffa000ca2b>] ? i2c_master_recv+0x3b/0x50 [i2c_core]
[<ffffffffa07220ef>] ? si2168_cmd_execute+0x9f/0xd0 [si2168]
[<ffffffffa07221a7>] ? si2168_read_status+0x87/0x170 [si2168]
[<ffffffffa073d6a4>] ? dvb_frontend_ioctl_legacy.isra.9+0x94/0xb50 [dvb_core]
[<ffffffff810baf3a>] ? hrtimer_try_to_cancel+0x3a/0xd0
[<ffffffff810bafea>] ? hrtimer_cancel+0x1a/0x30
[<ffffffff810ca35d>] ? futex_wait+0x17d/0x230
[<ffffffff810ba890>] ? hrtimer_get_res+0x40/0x40
[<ffffffffa073e260>] ? dvb_frontend_ioctl+0x100/0xe90 [dvb_core]
[<ffffffffa0734970>] ? dvb_usercopy+0xa0/0x130 [dvb_core]
[<ffffffffa073e160>] ? dvb_frontend_ioctl_legacy.isra.9+0xb50/0xb50 [dvb_core]
[<ffffffff810cc355>] ? do_futex+0x105/0xa60
[<ffffffff81197561>] ? cp_new_stat+0x111/0x130
[<ffffffffa0734a1e>] ? dvb_generic_ioctl+0x1e/0x40 [dvb_core]
[<ffffffff811a4a47>] ? do_vfs_ioctl+0x2e7/0x4f0
[<ffffffff810ccd19>] ? SyS_futex+0x69/0x150
[<ffffffff811a4cc9>] ? SyS_ioctl+0x79/0x90
[<ffffffff814e7029>] ? system_call_fastpath+0x12/0x17

I believe what has happened is the thread in the tuner has

locked its own i2c_mutex in si2157_cmd_execute
locked the parent bus_lock rt_mutex in i2c_transfer
tried to lock the si2168 i2c_mutex in si2168_select

The demod thread has locked its own i2c_mutex in si2168_cmd_execute
tried to lock the adapter bus_lock rt_mutex in i2c_transfer

Before I try to work out how to solve this it would be helpful to have
a confirmation that I have understood the relationship between the various
mutexes in use here. In particular I haven't completely followed the
way parent I2C adapters are defined but it does seem logical that there
should be one lock for the main bus and the bus on the far side of the
gate.

My current thought is that si2168_select doesn't need to take the
si2168 i2c_mutex as the demod can't do anything with the bus anyway
until the bus_lock is freed

Thanks

Adam Baker