Messages in this thread | | | Date | Fri, 20 Jan 2023 09:58:04 -0800 | From | "Paul E. McKenney" <> | Subject | Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test) |
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On Fri, Jan 20, 2023 at 11:01:03AM -0500, Alan Stern wrote: > On Wed, Jan 18, 2023 at 04:02:14PM -0800, Paul E. McKenney wrote: > > There are pairs of per-CPU counters. One pair (->srcu_lock_count[]) > > counts the number of srcu_down_read() operations that took place on > > that CPU and another pair (->srcu_unlock_count[]) counts the number > > of srcu_down_read() operations that took place on that CPU. There is > > an ->srcu_idx that selects which of the ->srcu_lock_count[] elements > > should be incremented by srcu_down_read(). Of course, srcu_down_read() > > returns the value of ->srcu_idx that it used so that the matching > > srcu_up_read() will use that same index when incrementing its CPU's > > ->srcu_unlock_count[]. > > > > Grace periods go something like this: > > > > 1. Sum up the ->srcu_unlock_count[!ssp->srcu_idx] counters. > > > > 2. smp_mb(). > > > > 3. Sum up the ->srcu_unlock_count[!ssp->srcu_idx] counters. > > Presumably you meant to write "lock" here, not "unlock".
You are quite right, and apologies for my confusion.
> > 4. If the sums are not equal, retry from #1. > > > > 5. smp_mb(). > > > > 6. WRITE_ONCE(ssp->srcu_idx, !ssp->srcu_idx); > > > > 7. smp_mb(). > > > > 8. Same loop as #1-4. > > > > So similar to r/w semaphores, but with two separate distributed counts. > > This means that the number of readers need not go to zero at any given > > point in time, consistent with the need to wait only on old readers. > > Reasoning from first principles, I deduce the following: > > You didn't describe exactly how srcu_down_read() and srcu_up_read() > work. Evidently the unlock increment in srcu_up_read() should have > release semantics, to prevent accesses from escaping out the end of the > critical section. But the lock increment in srcu_down_read() has to be > stronger than an acquire; to prevent accesses in the critical section > escaping out the start, the increment has to be followed by smp_mb().
You got it! There is some work going on to see if srcu_read_lock()'s smp_mb() can be weakened to pure release, but we will see.
> The smp_mb() fences in steps 5 and 7 appear to be completely > unnecessary.
For correctness, agreed. Their purpose is instead forward progress. One can argue that step 5 is redundant due to control dependency, but control dependencies are fragile, and as you say below, this code is nowhere near a fastpath.
> Provided an smp_mb() is added at the very start and end of the grace > period, the memory barrier in step 2 and its copy in step 8 can be > demoted to smp_rmb().
This might need to be smp_mb() to allow srcu_read_unlock() to be demoted to release ordering. Work in progress.
> These changes would be small optimizations at best, and you may consider > them unimportant in view of the fact that grace periods often last quite > a long time.
Agreed, keeping it simple and obvious is important on this code, which is nowhere near a fastpath. The case of srcu_read_unlock() is another thing altogether.
Thanx, Paul
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