Messages in this thread | ![/](/images/icornerl.gif) | | Date | Wed, 25 Jan 2023 17:53:30 -0800 | From | "Paul E. McKenney" <> | Subject | Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test) |
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On Wed, Jan 25, 2023 at 08:45:44PM -0500, Alan Stern wrote: > On Wed, Jan 25, 2023 at 03:33:08PM -0800, Paul E. McKenney wrote: > > Ah, and returning to the earlier question as to whether srcu_read_unlock() > > can use release semantics instead of smp_mb(), at the very least, this > > portion of the synchronize_srcu() function's header comment must change: > > > > On systems with more than one CPU, when synchronize_srcu() > > returns, each CPU is guaranteed to have executed a full > > memory barrier since the end of its last corresponding SRCU > > read-side critical section whose beginning preceded the call > > to synchronize_srcu(). > > Yes, that would not be true. But on the other hand, it would be true > that each CPU is guaranteed to have executed a release memory barrier > since the end of its last corresponding SRCU read-side critical section > whose beginning preceded the call to synchronize_srcu(), _and_ the CPU > executing synchronize_srcu() is guaranteed to have executed a full > memory barrier after seeing the values from all those release stores. > This is not quite the same thing but it ought to be just as good.
Here is hoping!
> > I don't know of any SRCU code that relies on this, but it would be good to > > check. There used to (and might still) be RCU code relying on this, which > > is why this sentence was added to the header comment in the first place. > > If there is code relying on that guarantee, it ought to work just as > well by relying on the modified guarantee.
Again, here is hoping!
> Of course, there might be code relying on a guarantee that > srcu_read_unlock() executes a full memory barrier. This guarantee would > certainly no longer hold. But as I understand it, this guarantee was > never promised by the SRCU subsystem.
That indented sentence was copied from the synchronize_srcu() function's header comment, which might be interpreted by some as a promise by the SRCU subsystem.
Thanx, Paul
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