Eric Smith writes:
> Andi wrote about my inquire regarding a way to synchronously stop
> another thread:
> > Any such mechanism will need a syscall, and it's unlikely that
> > any syscall will get much cheaper than a kill(SIGSTOP)
>
> Unfortunately, as Bart pointed out, this stops ALL the threads
> in a process (required for POSIX conformance). However, it appears that
> I may be able to do it by creating my threads using clone() with appropriate
> parameters rather than pthread_create().
>
> The next question, which I haven't yet been able to answer for myself
> from staring at the kernel sources, is whether sending SIGSTOP is
> synchronous. In other words, when the kill() returns, is the target
> thread already stopped? Even on an SMP system where the thread may have
> been executing concurrently on another processor?
>
> If not, the next question will be whether there's a more efficient way
> to wait for a process to enter the "T" state than busy-polling a file
> in /proc.
SIGSTOP is unreliable since some other process may wake the
target up with a SIGCONT.
man ptrace(), see the PTRACE_ATTACH request.
(Presumably it will DTRT if you give it a tid not just a pid,
but I haven't tested that.)
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