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Re: [tlaplus] Patterns for detecting data race with TLA+?

Thanks Hillel for the suggestion. Your blog post is also very enlightening, especially on the forbidden state transition checks that I also struggled with[1].

For my problem though, the race conditions that occur in real-life seem difficult to model with TLA directly. For example, if I read/write to a large (multi-word) variable in something like C/C++ without explicit synchronization, it can result in a torn-read. Without synchronization, the compiler may even re-order and/or optimize the statements in unpredictable (undefined) manners. Since in TLA+, the variable values are usually abstract values, it seems difficult to me to model the actual "bad" event.

I suppose what I could do is to create an auxiliary variable that increments and then decrements with each read and write operation, which may require splitting the read/write operations into multiple steps for the aux variable. Then I can use an invariant to assert that the aux variable never increases above 1 to ensure synchronization is properly implement. Not 100% sure if this is the simplest approach as I have yet to test it, but it might be an acceptable work around for this purpose.

Thanks again,

[1] I've worked on an unfinished post detailing my journey on discovering how to write temporaral properties that specifying conditions like A leads to B and remains at B, or A leads to B at least once. Once I get it done, I may ask for more feedback here.

On 2022-05-27 17:45, Hillel Wayne wrote:
Hi Shuhao,

Your spec doesn't /necessarily/ mean the two variables are reading and writing the same variable at the same time. Remember, each label represents an /abstract/ amount of time: for all you know the two actions could be two years apart. It's a good proxy for bad behavior, but it's still an "indirect" property.

I find that "direct properties" scale better. What is the bad state that both processes writing at the same time would /cause/? If you check /that/ as an invariant, race conditions will naturally appear in the error trace. You can then write indirect properties for the specific race conditions you see to make them trigger earlier in the model checking.

(One useful tool for this is Action Properties <https://www.hillelwayne.com/post/action-properties/>, which let you assert properties on how variables change in a step.)


On 5/27/2022 4:13 PM, Shuhao Wu wrote:
Hello all:

I'm currently working on some lock-less algorithms with TLA+ (specifically, with PlusCal). One of the things that needs to be checked is that the algorithm is data-race-free, where no two processes (in PlusCal terms) are reading and writing to the same variable at the same time. Here's a sample spec with a data race on the Global2 variable:

variables Global1 = 0, Global2 = 0;

fair process A = "A"
  variable local;
    local := Global1;
    Global2 := 10;
end process;

fair process B = "B"
  variable local;
    local := Global1;
    local := Global2;
end process;

Checking if a data race occurred in TLC is technically as simple as checking the invariant ~(pc["A"] = "A2" /\ pc["B"] = "B2"). However, I suspect this doesn't scale very well to specifications with more variables and states, as you need to manually enumerate every write (and read) step for the all "global" variables.

Is there a known pattern somewhere to check for this class of problems? My quick googling doesn't show anything. Am I approaching this problem from an incorrect angle?



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