[linux-dvb] Re: [PATCH] support for multiple feeds per pid

[Oliver Endriss <o.endriss@gmx.de>]

Emard wrote:
> Johannes Stezenbach wrote:
> > I think that spin_lock_irq() is "stronger" than spin_lock_bh().
> > On SMP the critical region will be protected by the spin lock,
> > on UP you can't get interrupted when irqs are disabled. But
> > it is of course more efficient to not to lock irqs when not
> > necessary, that's the whole point of using tasklets.

Ack. We should not even think about these optimizations before the 
driver is stable. Using stronger locks does not hurt.

> It is stronger, but from some people machine lockup
> reports (machine hosed, ICMP PING still working) I think that this
> lock happens during execution of hardware irq, and it's (sometimes)
> released before finish of tasklet execution (after the hardware
> irq has finished), while it should be actually keep locked after
> the hardware irq, during the tasklet running and then released.

Locks should never be held longer than necessary. Taking a lock in the 
interrupt handler and releasing it in the tasklet might lead to 
deadlocks. If you take a lock in the interrupt handler you have to 
release in in the interrupt handler. If you take a lock in the tasklet 
you have to release it in the tasklet.

spin_lock_irq() / spin_unlock_irq():
Routines called from HW interrupts should never use spin_lock_irq() / 
spin_unlock_irq() because spin_unlock_irq() enables interrupts. 
Obviously this is not correct if (and only if) spin_lock_irq() was 
called with interrupts disabled. In this case spin_lock_irqsave() / 
spin_unlock_irqrestore() should be used which will restore interrupts 
to the previous state. (I have not seen any issues related to this in 
the driver.)

> So we can replace them in dvb_demux.c with spin_lock_bh() and test
> it a bit if it still works after the change :-)?

If you are using spin_lock_bh(), interrupt handlers cannot access the 
protected data anymore, i.e. you must not call these routines from an 
interrupt handler.

Oliver



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