On Mon, Aug 05, 2019 at 12:20:45PM +0800, Jason Wang wrote: > > On 2019/8/2 下午8:46, Jason Gunthorpe wrote: > > On Fri, Aug 02, 2019 at 05:40:07PM +0800, Jason Wang wrote: > > > > This must be a proper barrier, like a spinlock, mutex, or > > > > synchronize_rcu. > > > > > > I start with synchronize_rcu() but both you and Michael raise some > > > concern. > > I've also idly wondered if calling synchronize_rcu() under the various > > mm locks is a deadlock situation. > > > Maybe, that's why I suggest to use vhost_work_flush() which is much > lightweight can can achieve the same function. It can guarantee all previous > work has been processed after vhost_work_flush() return. If things are already running in a work, then yes, you can piggyback on the existing spinlocks inside the workqueue and be Ok However, if that work is doing any copy_from_user, then the flush becomes dependent on swap and it won't work again... > > > 1) spinlock: add lots of overhead on datapath, this leads 0 performance > > > improvement. > > I think the topic here is correctness not performance improvement> > But the whole series is to speed up vhost. So? Starting with a whole bunch of crazy, possibly broken, locking and claiming a performance win is not reasonable. > Spinlock is correct but make the whole series meaningless consider it won't > bring any performance improvement. You can't invent a faster spinlock by opencoding some wild scheme. There is nothing special about the usage here, it needs a blocking lock, plain and simple. Jason
