On Tue, Feb 04, 2025 at 10:44:45AM -0500, Joel Fernandes wrote: > On Thu, Jan 30, 2025 at 12:56 AM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > By "where we were initially", one might reasonably guess that you meant > > that the initial value and the target are one and the same. But I suspect > > that you are thinking of a third value, the value of the underlying > > grace-period sequence number at the time of the call to rcu_seq_snap(). > > But you might be thinking of something else entirely. > > > > > Now we move rcu_seq_done_exact. It has the exact same behavior except > > > that instead of ULONG_MAX/2, the above situations are shrunk to about > > > 10 counts from the target. So if target is 28, then the initial > > > sequence should have been at least 18 to avoid false-positive, but > > > again it is possible and I certain see in the code that it cannot be > > > used this way.. (at least so far).. So all we are left with is the > > > false-negative band of ~2.5 GPs.. > > > > Here, I have the same questions. As you no doubt guessed. > > > > > About "what are the consequences of failing to get this right". I > > > believe false-positive is unacceptable unless it is maybe debug code - > > > that can break functionality in code, too short GPs and all that. > > > However false-negative is OK as long as the usecase can retry later > > > and can afford to wait. Did I get that much correct? > > > > Maybe? > > > > Please look at this on a per-use-case basis. > > Sure. FWIW, I started a world-editable document here. I am planning to > work on this a bit more before our meeting this week. If others > knowledgeable like Frederic and others could make edits/comments, > that'd be welcomed. But I basically summarized this whole thread here: > https://docs.google.com/document/d/1sFNuGH4U6DFCE8sunbdWMjFhJhb1aG4goOJAnS6c6gQ/edit?usp=sharing > > My thought is (AFAICS), this patch is still valid and > rcu_seq_done_exact is a potentially better replacement to > rcu_seq_done. But famous last words... Let's start with the use case where the sequence number is being used by rcu_barrier(). What happens? Thanx, Paul
