Re: [PATCH] comedi: comedi_test: Prevent timers rescheduling during deletion

On 13/02/2024 18:10, Ian Abbott wrote:

The comedi_test devices have a couple of timers (ai_timer and ao_timer) that can be started to simulate hardware interrupts. Their expiry functions normally reschedule the timer. The driver code calls either del_timer_sync() or del_timer() to delete the timers from the queue, but does not currently prevent the timers from rescheduling themselves so synchronized deletion may be ineffective.

Add a couple of boolean members (one for each timer: ai_timer_enable and ao_timer_enable) to the device private data structure to indicate whether the timers are allowed to reschedule themselves. Set the member to true when adding the timer to the queue, and to false when deleting the timer from the queue in the waveform_ai_cancel() and waveform_ao_cancel() functions.

The del_timer_sync() function is also called from the waveform_detach() function, but the timer enable members will already be set to false when that function is called, so no change is needed there.

Fixes: 403fe7f34e33 ("staging: comedi: comedi_test: fix timer race conditions") Cc: stable@vger.kernel.org # 4.4+ Signed-off-by: Ian Abbott abbotti@mev.co.uk

---

drivers/comedi/drivers/comedi_test.c | 37 +++++++++++++++++++++++++--- 1 file changed, 33 insertions(+), 4 deletions(-)

diff --git a/drivers/comedi/drivers/comedi_test.c b/drivers/comedi/drivers/comedi_test.c index 30ea8b53ebf8..7fefe0de0bcc 100644 --- a/drivers/comedi/drivers/comedi_test.c +++ b/drivers/comedi/drivers/comedi_test.c @@ -87,6 +87,8 @@ struct waveform_private { struct comedi_device *dev; /* parent comedi device */ u64 ao_last_scan_time; /* time of previous AO scan in usec */ unsigned int ao_scan_period; /* AO scan period in usec */

• bool ai_timer_enable:1; /* should AI timer be running? */
• bool ao_timer_enable:1; /* should AO timer be running? */ unsigned short ao_loopbacks[N_CHANS]; };

@@ -232,12 +234,18 @@ static void waveform_ai_timer(struct timer_list *t) if (cmd->stop_src == TRIG_COUNT && async->scans_done >= cmd->stop_arg) { async->events |= COMEDI_CB_EOA; } else {

• unsigned long flags;
  

• if (devpriv->ai_convert_time > now) time_increment = devpriv->ai_convert_time - now; else time_increment = 1;

• mod_timer(&devpriv->ai_timer,
  

• 	  jiffies + usecs_to_jiffies(time_increment));
  

• spin_lock_irqsave(&dev->spinlock, flags);
  

• if (devpriv->ai_timer_enable) {
  

• 	mod_timer(&devpriv->ai_timer,
  

• 		  jiffies + usecs_to_jiffies(time_increment));
  

• }
  

• spin_unlock_irqrestore(&dev->spinlock, flags);
  

Actually, I should have used a plain old spin_lock() in the above (and spin_lock_bh() elsewhere in the code) instead of spin_lock_irqsave(), since there are no hard interrupts involved.

I'll send a v2 patch shortly.