From: Al Viro > Sent: 31 August 2020 04:21 > > On Mon, Aug 31, 2020 at 09:43:31AM +0800, Shaokun Zhang wrote: > > > How about this? We try to replace atomic_cmpxchg with atomic_add to improve > > performance. The atomic_add does not check the current f_count value. > > Therefore, the number of online CPUs is reserved to prevent multi-core > > competition. > > No. Really, really - no. Not unless you can guarantee that process on another > CPU won't lose its timeslice, ending up with more than one increment happening on > the same CPU - done by different processes scheduled there, one after another. > > If you have some change of atomic_long_add_unless(), do it there. And get it > past the arm64 folks. get_file_rcu() is nothing special in that respect *AND* > it has to cope with any architecture out there. > > BTW, keep in mind that there's such thing as a KVM - race windows are much > wider there, since a thread representing a guest CPU might lose its timeslice > whenever the host feels like that. At which point you get a single instruction > on a guest CPU taking longer than many thousands of instructions on another > CPU of the same guest. The same thing can happen if a hardware interrupt occurs. Not only the delays for the interrupt itself, but all the softint processing that happens as well. That can take a long time - even milliseconds. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
