Re: New USB stack on FreeBSD w/Linux emulation layer coming

On Sat, 19 May 2007, David Brownell wrote:

> > Absolutely right, I'm being an idiot here. I think I looked at root
> > hub code in the rush to the FreedomHEC preparations. We do not have
> > local_irq_save in the giveback routine. So, when Alan wrote "USB
> > callbacks cannot be interrupted", he meant normal no-reentrancy
> > guarantees, and not that the interrupts are disabled.

No, I really meant that interrupts are disabled.  It's possible that I
was wrong...

> And by "normal no-reentrancy" you mean the normal guarantee that
> each IRQ handler won't be re-entered while it's running.
> 
> I observe that we don't register PCI IRQ handlers with IRQF_DISABLED,
> so it's possible that some *other* IRQ happens while it's running;
> but most of the non-PCI irq handlers are registered with that flag set.
> 
> That seems potentially buglike to me; it's legit for any IRQ handler
> to submit an URB.  Which would then cause them to try grabbing the
> spinlock held by the HCD's IRQ handler, if that "other" IRQ handler
> happened to fire when the HCD's IRQ handler was running.
> 
> So maybe we should always register with IRQF_DISABLED, unattractive
> though that could be in terms of latencies ... or, somehow try to
> define a rule whereby URBs can't be submitted in certain IRQ contexts.

Or better still, always acquire the HCD's spinlock with
spin_lock_irqsave().  That's what uhci-hcd does, even though ehci-hcd
and ohci-hcd don't.

Of course, if we don't use IRQF_DISABLED then we would have to verify 
that none of the callbacks rely on interrupts being disabled.

Alan Stern


-------------------------------------------------------------------------
This SF.net email is sponsored by DB2 Express
Download DB2 Express C - the FREE version of DB2 express and take
control of your XML. No limits. Just data. Click to get it now.
http://sourceforge.net/powerbar/db2/
_______________________________________________
[email protected]
To unsubscribe, use the last form field at:
https://lists.sourceforge.net/lists/listinfo/linux-usb-devel