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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