[PATCH] malloc: Ensure that ptmalloc_init runs only once

[Florian Weimer]

* Siddhesh Poyarekar:

> On 6/17/21 8:01 PM, Florian Weimer via Libc-alpha wrote:
>> * Siddhesh Poyarekar via Libc-alpha:
>> 
>>> It is possible that multiple threads simultaneously enter
>>> ptmalloc_init and succeed the < 0 check.  Make the comparison and
>>> setting of __malloc_initialized atomic so that only one of them goes
>>> through.  Additionally, if a thread sees that another thread is
>>> running the initialization (i.e. __malloc_initialized == 0) then wait
>>> till it is done.
>> No, this cannot happen because pthread_create calls malloc before
>> creating the new thread.
>
> Yes but I wonder if we should rely on that.  If we decide to rely on
> this semantic then we implicitly specify thread creation through
> methods other than pthread_create that happen to not call malloc as
> unsupported.

There is no way to allocate a TCB for such foreign threads, and malloc
depends on the TCB, so this is not something that can work for
completely different reasons.

THanks,
Florian