Re: [RFC][PATCH] irq: remove IRQF_DISABLED

Linus Torvalds
2009-03-02T18:30:25Z

On Mon, 2 Mar 2009, Bartlomiej Zolnierkiewicz wrote:
> >
> > Could we make just the IDE driver itself enable interrupts? Sure. But that
>
> Actually it has been doing it for years (some host drivers don't do this by
> default and still need "hdparm -u" or equivalent but I was planning to change
> it for 2.6.30).

The IDE layer has the option to enable irq's during the transfer itself,
yes. But it actually works the reverse way from what you think: the irq
layer will enable interrupts, and the IDE layer will then _not_ disable
them during the transfer if you use "hdparm -u".

Look at ide_intr: it generally gets called with interrupts _enabled_
(because it doesn't use IRQF_DISABLED) and then it does:

spin_lock_irqsave(&hwif->lock, flags);
..
spin_unlock(&hwif->lock);
..
if (drive->dev_flags & IDE_DFLAG_UNMASK)
local_irq_enable_in_hardirq();
...
spin_lock_irq(&hwif->lock);
...
spin_unlock_irqrestore(&hwif->lock, flags);

where the magic thing is how it enables irqs again if the "irq unmask"
flag is set.

The point I'm making is that

- as far as the generic irq layer is concerned, IDE might as well have
interrupts enabled all the time (and disabling them is a local issue,
more to do with locking and with timing-induced hardware _bugs_ rather
than anything else)

- .. and more importantly, that is AS IT MUST BE. Because quite frankly,
if the irq handler enables interrupts (like IDE does), the generic IRQ
layer really _must_ know about it, because it may depend on
non-reentrancy of that interrupt.

(Small detail: the current irq layer actually does have that
"IRQ_INPROGRESS" flag to handle re-entrancy issues regardless of anything
else, so I guess we technically are robust in this regard. But that's
partly an SMP issue, and conceptually it's still really really important
information to know whether interrupts can nest. In practice, it does
affect things like stack usage too, for example, so even with
IRQ_INPROGRESS, there really is a difference between IRQF_DISABLED and not
having it on).

Linus