On Fri, 12 Aug 2016 18:43:30 +0200, Andrej Kruták wrote:
On Fri, Aug 12, 2016 at 2:30 PM, Takashi Iwai tiwai@suse.de wrote:
On Fri, 12 Aug 2016 14:15:16 +0200,
Also, the blocking read/write control isn't usually done by a semaphore. Then you can handle the interrupt there.
I actually wonder why, semaphores seemed perfect for this... Do you have some hints?
Assume you want to interrupt the user-space app while it's being blocked by the semaphore. With your code, you can't.
You can - down_interruptible() is there for this exact reason.
There is another blocking path: you keep semaphore down after line6_hwdep_read() until line6_hwdep_push_message(). What happens if user-space interrupts during that, and line6_hwdep_push_message() is delayed or stall by some reason?
Actually, I think I don't see what's the "another path" here, could you please elaborate one more bit? I just want to make sure to not reimplement the same race using waitqueue...
What's the point then? line6_hwdep_push_message() could get not scheduled for some while. So until it calls up(), line6_hwdep_read() will block on down_interruptible(), or until signal (in which case user gets -ERESTARTSYS). After up() is called, there are data in buffer...
Well, what happens if user aborts before up() is called in line6_hwdep_push_message()? Now the driver calls close, and it frees the memory. What if, at the very same time, line6_hwdep_push_message() is triggered?
If line6_hwdep_read() returns after interrupt, no problem - the buffer will just continue to be filled and semaphore will be up()'d while there's free buffer space. Or until the device is closed...
Or, what if line6_hwdep_push_message() is triggered twice or more before line6_hwdep_read() is called? It will call up() twice or more. Then at this point, you call line6_hwdep_read() concurrently from two threads... How do they protect against each other?
If I do the same via waitqueue, I will have the same problems, no? Maybe if you could post the steps where you see the race...
In your code, it's not clear that you're protecting from what. A simple lock+wait loop shows it more easily, at least.
Takashi
At the same time, looking at __down_common(), it just does the standard waitqueue stuff (TASK_INTERRUPTIBLE + schedule()) (+counter in down())... So do you have some other race in mind? I'm running in circles, so surely you must :-)
Sorry if I sound like a moron... and thanks for you time!
-- Andrej