On Wed, 29 Mar 2006, Pete Zaitcev wrote: > Alan, > > I looked at uhci_scan_schedule today, and it seems to me that the code > which prevents re-entry is completely superfluous. I do not see a code > path which would loop back through usb_giveback_urb and end in > uhci_scan_schedule again. It is only called from ->stop, suspend, > and an interrupt. The access from other CPU is bracketed with uhci->lock.
It's also called from uhci_hub_status_data. But the entry points you listed are already bad enough. uhci->lock is dropped during usb_giveback_urb, so access is not protected during a giveback. Hence either a stop or a suspend (not to mention hub_status_data) on another CPU could re-enter uhci_scan_schedule. > What do you think about dropping this code? Not a good idea. Don't forget that the interaction works the other way also. If uhci_scan_schedule is running because it was called from stop, suspend, or hub_status_data, then it could be re-entered on another CPU because of an interrupt. > I only noticed because Fujitsu submitted a patch for RHEL which backports > this stuff, and they fumbled with spinlocks. But on their platform it's > an instant death. Apparently, this mechanism is never excercised. > It must be a carryover from some old implementation. No, this is stuff I added relatively recently (i.e., within the last couple of years) as part of my reorganization of the driver. Those re-entry paths are unlikely, admittedly, but since they exist the code is needed. Alan Stern ------------------------------------------------------- This SF.Net email is sponsored by xPML, a groundbreaking scripting language that extends applications into web and mobile media. Attend the live webcast and join the prime developer group breaking into this new coding territory! http://sel.as-us.falkag.net/sel?cmd=lnk&kid=110944&bid=241720&dat=121642 _______________________________________________ linux-usb-devel@lists.sourceforge.net To unsubscribe, use the last form field at: https://lists.sourceforge.net/lists/listinfo/linux-usb-devel
