Messages in this thread | | | From | Oliver Neukum <> | Subject | Re: [linux-usb-devel] [PATCH 1/9] USB usbfs: take a reference to the usb device | Date | Sun, 18 Apr 2004 15:57:58 +0200 |
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> > > 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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