Messages in this thread | | | Date | Sun, 18 Apr 2004 10:40:55 -0400 (EDT) | From | Alan Stern <> | Subject | Re: [linux-usb-devel] [PATCH 1/9] USB usbfs: take a reference to the usb device |
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On Sun, 18 Apr 2004, Duncan Sands wrote:
> > > 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.
There was a lengthy discussion about this a few months ago. On the whole, people felt that using an rwsem wasn't a good idea.
Personally, I think that contention for a single device will be very rare, so we don't need to consider it and can leave things as they are.
Alan Stern
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