Re: [linux-usb-devel] [PATCH 1/9] USB usbfs: take a reference to the usb device

[Oliver Neukum]

> > > gives correctness, but at the cost of a probable performance hit.  In
> > > later steps we can (1) turn dev->serialize into a rwsem
> >
> > Rwsems are _slower_ in the normal case of no contention.
>
> Right, but remember that dev->serialize is per device, not per interface. 
> So if two programs grab different interfaces of the same device using
> usbfs, or if multiple threads in the same program beat on the same
> interface, then they could lose time fighting for dev->serialize when in
> fact they could run in parallel.  Personally I doubt it matters much, but
> since most of usbfs only requires read access to the data structures
> protected by dev->serialize, it seems logical to use a rwsem.

Multiple interfaces are uncommon. Devices with several interfaces bound
to usbfs are uncommoner. Concurrent use is still uncommoner. You are
slowing the common case.

> > > (2) push the acquisition of dev->serialize down to the lower levels as
> > > they are fixed up.
> >
> > Why?
>
> Efficiency.  The main reason is that the copy to/from user calls are inside
> the locked region :) As for the other places where the lock could be
> dropped, I guess measurement is required to see if it gains anything.

OK. I see. But IMHO usbfs is not written for speed anyway, so don't
worry too much.

	Regards
		Oliver

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