iio:buffer: polling unregistered device stalls user process

Version 2 of this patchset.

Changes since v1 :
* merge both patches
* reword commit log subject
* remove fixes tag

Unregistering a device currently in use may stall userspace process polling for data availability (poll syscall). Here is a typical situation detailed below.

zpa2326 barometer driver is loaded.
<console>
/ # lsmod

Module                  Size  Used by
zpa2326_i2c             2833  0
zpa2326                25360  1 zpa2326_i2c

[...]
</console>

zpa2326 has registered iio device 0 and its own hardware trigger 0. <console>
/ # lsiio
Device 000: zpa2326
Trigger 000: zpa2326-dev0
/ # ls /dev
bus ptmx tty29 tty52
[...]
iio:device0 tty11 tty35 tty59
</console>

Run an everlasting sampling process (iio_generic_buffer) in hardware triggered buffer mode then unload module (zpa2326_i2c) driving the device (iio:device0) under use.
<console>
/ # iio_generic_buffer -a -N 0 -l 2 -c -1 & sleep 1 ; rmmod zpa2326_i2c iio device number being used is 0
iio trigger number being used is 0
No channels are enabled, enabling all channels
Enabling: in_pressure_en
Enabling: in_timestamp_en
Enabling: in_temp_en
/sys/bus/iio/devices/iio:device0 zpa2326-dev0
103.736877 31265.359375 98504890900
103.734001 31369.199219 98568173608
103.739883 31369.199219 98631470327
103.734070 31369.199219 98694748816
103.733070 31369.199219 98758036681
103.736130 31473.039062 98821330691
103.739006 31369.199219 98884620014
103.737068 31473.039062 98947911524
103.732254 31473.039062 99011214754
103.739006 31473.039062 99074498920
103.739006 31473.039062 99137784962
103.734192 31473.039062 99201085431
103.737190 31473.039062 99264364910
103.741066 31473.039062 99327655274
/ #
</console>

After 1 second, iio_generic_buffer output has been muted. A ps shows iio_generic_buffer is still there, in interruptible sleep state. <console>
/ # ps | grep iio_generic_buffer

  123 0         1820 S    iio_generic_buffer -a -N 0 -l 2 -c -1
  128 0         2776 S    grep iio_generic_buffer

/ #
</console>

Strace shows iio_generic_buffer is stalled into poll syscall. Forever... <console>
/ # strace -p 123
Process 123 attached
poll([{fd=3, events=POLLIN}], 1, -1^CProcess 123 detached
<detached ...>
</console>

A sysrq task state trace shows iio_generic_buffer is blocking in... <console>
/ # [ 202.859427] sysrq: SysRq : Show State

[  202.863151]   task                PC stack   pid father
[...]
[  205.667101] iio_generic_buf S 8068a254     0   123     98 0x00000000
[  205.673507] [<8068a254>] (__schedule) from [<8068ac0c>] (schedule+0x64/0x100)
[  205.680664] [<8068ac0c>] (schedule) from [<80690820>] (schedule_hrtimeout_range_clock+0x1d8/0x1e8)
[  205.689641] [<80690820>] (schedule_hrtimeout_range_clock) from [<80690858>] (schedule_hrtimeout_range+0x28/0x30)
[  205.699831] [<80690858>] (schedule_hrtimeout_range) from [<8022b148>] (poll_schedule_timeout+0x84/0xc8)
[  205.709241] [<8022b148>] (poll_schedule_timeout) from [<8022d808>] (do_sys_poll+0x640/0x8c4)
[  205.717697] [<8022d808>] (do_sys_poll) from [<8022db0c>] (do_restart_poll+0x80/0xd4)
[  205.725461] [<8022db0c>] (do_restart_poll) from [<8004f660>] (sys_restart_syscall+0x30/0x34)
[  205.733918] [<8004f660>] (sys_restart_syscall) from [<800151a0>] (ret_fast_syscall+0x0/0x1c)
[  205.742351] Showing busy workqueues and worker pools:
[  205.747406] workqueue events: flags=0x0
[  205.751243]   pwq 4: cpus=2 node=0 flags=0x0 nice=0 active=1/256
[  205.757304]     in-flight: 23:check_carrier
[  205.761551] pool 4: cpus=2 node=0 flags=0x0 nice=0 hung=0s workers=4 idle: 61 77 65

</console>

Killing iio_generic_buffer works.
<console>
/ # kill 123
/ # Caught signal 15
failed to open /sys/bus/iio/devices/iio:device0/buffer/enable
Failed to disable buffer: No such file or directory
Enabling/disabling channels: can't open /sys/bus/iio/devices/iio:device0/scan_elements Failed to disable all channels
[1]+ Done(241) iio_generic_buffer -a -N 0 -l 2 -c -1
/ #
</console>

This situation happens when the device is being unregistered (iio_device_unregister()) while a userspace process is executing the poll syscall.
If device has vanished before running the iio_buffer_fileops poll hook, the latter will keep returning no events and process will be stalled waiting for events that will never come if no timeout has been specified.

This patch fixes this by ensuring iio_buffer_poll() returns POLLERR if device has been previously unregistered. Userspace process will be properly notified something wrong happened so that it may take appropriate action.

Running the same test as above will now show.
<console>
/ # iio_generic_buffer -a -N 0 -l 2 -c -1 & sleep 1; rmmod zpa2326_i2c iio device number being used is 0
iio trigger number being used is 0
No channels are enabled, enabling all channels
Enabling: in_pressure_en
Enabling: in_timestamp_en
Enabling: in_temp_en
/sys/bus/iio/devices/iio:device0 zpa2326-dev0
103.679695 31680.718750 54318428312
103.684631 31680.718750 54381713521
103.682693 31680.718750 54444999458
103.681694 31784.558594 54508288521
103.679878 31784.558594 54571584458
103.680756 31784.558594 54634876698
103.677002 31784.558594 54698161020
103.682755 31784.558594 54761463208
103.681816 31784.558594 54824749979
103.673004 31784.558594 54888037583
103.675819 31784.558594 54951331958
103.679878 31784.558594 55014612270
103.671066 31784.558594 55077905447
103.683693 31784.558594 55141192583
failed to open /sys/bus/iio/devices/iio:device0/buffer/enable
Failed to disable buffer: No such file or directory
Enabling/disabling channels: can't open /sys/bus/iio/devices/iio:device0/scan_elements Failed to disable all channels
[1]+ Done(1) iio_generic_buffer -a -N 0 -l 2 -c -1
</console>

Regards,
Gregor.

Gregor Boirie (1):
iio:buffer: let poll report an error for unregistered devices

 drivers/iio/industrialio-buffer.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

2.1.4