Messages in this thread | | | From | Dmitry Vyukov <> | Date | Thu, 10 Jan 2019 13:38:11 +0100 | Subject | Re: seqcount usage in xt_replace_table() |
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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). > > Andrea
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