Messages in this thread | | | Date | Thu, 10 Jan 2019 06:50:29 -0800 | From | "Paul E. McKenney" <> | Subject | Re: seqcount usage in xt_replace_table() |
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On Thu, Jan 10, 2019 at 01:38:11PM +0100, Dmitry Vyukov wrote: > On Thu, Jan 10, 2019 at 1:30 PM Andrea Parri > <andrea.parri@amarulasolutions.com> wrote: > > > > > For seqcounts we currently simply ignore all accesses within the read > > > section (thus the requirement to dynamically track read sections). > > > What does LKMM say about seqlocks? > > > > LKMM does not currently model seqlocks, if that's what you're asking; > > c.f., tools/memory-model/linux-kernel.def for a list of the currently > > supported synchronization primitives. > > > > LKMM has also no notion of "data race", it insists that the code must > > contain no unmarked accesses; we have been discussing such extensions > > since at least Dec'17 (we're not quite there!, as mentioned by Paul). > > How does it call cases that do contain unmarked accesses then? :) > > > My opinion is that ignoring all accesses within a given read section > > _can_ lead to false negatives > > Absolutely. But this is a deliberate decision. > For our tools we consider priority 1: no false positives. Period. > Priority 2: also report some true positives in best effort manner. > > > (in every possible definition of "data > > race" and "read sections" I can think of at the moment ;D): > > > > P0 P1 > > read_seqbegin() x = 1; > > r0 = x; > > read_seqretry() // =0 > > > > ought to be "racy"..., right? (I didn't audit all the callsites for > > read_{seqbegin,seqretry}(), but I wouldn't be surprised to find such > > pattern ;D ... "legacy", as you recalled).
One approach would be to forgive data races in the seqlock read-side critical section only if:
o There was a later matching read_seqretry() that returned true, and
o There were no dereferences of any data-racy load. (Yeah, this one should be good clean fun to model!)
Do people nest read_seqbegin(), and if so, what does that mean? ;-)
Thanx, Paul
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