I am curious though for an example that shows how a reentrant lock could
violate this?
> On Nov 2, 2022, at 2:38 AM, peterGo <go.peter...@gmail.com> wrote:
>
>
> Guanyun,
>
> I tried to think of a simple example.
>
> type Average struct {
> sum float64
> count int64
> mx sync.Mutex
> }
>
> https://go.dev/play/p/4SLCLuqG246
>
> 1. An invariant represents a condition that does not change while the process
> progresses - Niklaus Wirth.
>
> 2. The additional invariant--Average = sum / count--is specific to the data
> structure that the mutex mx guards.
>
> 3. The shared variables sum and count are part of an invariant.
>
> 4. The Add method temporarily violates the invariant by updating sum but
> restores the invariant by updating count.
>
> Peter
>
>> On Wednesday, November 2, 2022 at 12:49:41 AM UTC-4 name....@gmail.com wrote:
>> Hello,
>>
>> This is a passage in book <The Go Programming Language>:
>> --------------------------------------------------------------------------------------------------------
>> There is a good reason Go’s mutexes are not re-entrant.
>>
>> The purpose of a mutex is to ensure that certain invariants of the shared
>> variables are
>> maintained at critical points during program execution.
>>
>> One of the invariants is "no goroutine is accessing the shared variables",
>> but there may be additional invariants specific to the data structures that
>> the mutex guards.
>>
>> When a goroutine acquires a mutex lock, it may assume that the invariants
>> hold. While it holds the lock, it may update the shared variables so that
>> the invariants are temporarily violated.
>>
>> However, when it releases the lock, it must guarantee that order has been
>> restored
>> and the invariants hold once again.
>>
>> Although a re-entrant mutex would ensure that no other goroutines are
>> accessing the shared variables, it cannot protect the additional invariants
>> of those variables.
>>
>> --------------------------------------------------------------------------------------------------------
>>
>> This passage is difficult for me to understand:
>> 1. How to understand invariants "invariants"?
>> 2. What kind of scenarios does “additional invariants” refer to?
>> 3. What is the relationship between "shared variables" and "invariants"?
>> 4. What does "...guarantee that order has been restored..." mean?
>>
>> Thanks,
>> Guanyun
>
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