Re: [9fans] Privalloc(2) and rfork(RFPROC|RFMEM) (was: a pair nec bugs)

[Giacomo Tesio <giacomo@tesio.it>]

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... and given getpid(2) implementation, a pid based table could be quite
expensive, since MyStruct is accessed very very often.

2015-09-05 16:03 GMT+02:00 Giacomo Tesio <giacomo@tesio.it>:

> 2011-05-20 3:30 GMT+02:00 erik quanstrom <quanstro@quanstro.net>:
>
>> one note is that while i'm aware of privalloc(2), i didn't use it.  the
>> implementation doesn't appear correct for shared-memory procs.
>> i think there are two issues
>> - locking is unnecessary.  the only preemptable unit of execution is
>> a process and each process is guarenteed to have its own instance
>> of _privates and _nprivates.
>> - for shared-memory procs, we will run out of privates because
>> the static privinit will be falsely shared.  privinit should be replaced
>> by using a private entry.
>>
>
> In a set of processes that share memory, I need a single per-process
> location to store the address of structure (which is in turn allocated in
> the global memory space).
>
> Privalloc(2) seemed what I need, but seem that I can't use it properly.
>
> In the parent process I do:
>
> 1) initialialize a global variable p = (MyStruct **)privalloc();
> 2) allocate a MyStruct for every process I'm going to spawn
> 3) spawn processes with rfork(RFMEM|RFPROC)
>
> The spawn() function that call rfork takes a MyStruct* as argument and
> assign it to *p in the new process.
> For extra safety I added an assert(*p == nil) just after rfork, and after
> a few (apparently?) successful spawns, the assert fails.
>
> What I need is a sort of thread-local storage for the MyStruct*, so that
> each child process can find it's own dedicated MyStruct.
> I know that could get this with an hashtable based on the pid, but I'd
> prefer to avoid the book keeping if possible.
>
>
> Giacomo
>

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