Re: [musl] questions about __tl_lock

[Rich Felker <dalias@libc.org>]

On Fri, Aug 09, 2024 at 03:21:23AM +0000, guolongqiang wrote:
> Hi, all
>          I have one question about __tl_lock. The current implementation of __tl_lock shown as follow.
> Obviously __thread_list_lock is a private memory, why don't we pass FUTEX_PRIVATE option to __wait?
> 
> ```
> void __tl_lock(void)
> {
> int tid = __pthread_self()->tid;
> int val = __thread_list_lock;
> if (val == tid) {
> tl_lock_count++;
> return;
> }
> while ((val = a_cas(&__thread_list_lock, 0, tid)))
> __wait(&__thread_list_lock, &tl_lock_waiters, val, 0);
> }
> ```
> Thank you to explain.
> 

The thread list wait operation has to use a non-private futex wait
because the wake operation will be performed by the kernel, which
performs a non-private wake because that was the original contract
from before private futex operations existed.

Ideally when private waits were added, the kernel exit code path
should have been updated to do both private and non-private wakes so
that either type of wait would work. But that was overlooked, so even
if it were fixed in the kernel now, we couldn't rely on that.

Rich