On Fri, 2018-08-17 at 20:15 +0200, Lukas Wunner wrote: > > The two steps (enumeration and driver attachment) are protected by > pci_lock_rescan_remove(). Initially, when a pci_dev is newly allocated > with kzalloc(), is_added is 0. The transition from 0 to 1 happens while > under pci_lock_rescan_remove(). When that lock is released, is_added is > always 1, barring a device_attach() error, in which case it will remain > at 0. > > AFAICS, there is no second mutex needed to ensure synchronization of > is_added, the existing mutex should be sufficient and the only problem > are RMW races when accessing adjacent flags. Those were correctly addressed > by Hari's patch. Benjamin seems to be alleging that concurrency issues > exist beyond the RMW races, I fail to see them, sorry.
Im saying that fixing those issues using atomic bitops is a generally unsafe practice even if it happens to work in a specific case. In this one, I argue that the root problem was simply that is_added was set in the wrong place. The "fix" from Hari touches 9 files, adds horrible relative includes to an architecture and generally bloats things while a single 3 liner would have been sufficient. The current use of is_added is asymetric (it's cleared during device_attach but set during detach) which is bogus and the entire race stems from the fact that it is set after device_attach returns. So setting it early is, imho, the right fix, is much simpler, and fixes the odd imbalance we have to begin with. Ben.
