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Message-ID: <18528.43356.997214.585578@harpo.it.uu.se>
Date: Tue, 24 Jun 2008 09:59:24 +0200
From: Mikael Pettersson <mikpe@it.uu.se>
To: "Eric Smith" <eric@brouhaha.com>
Cc: "Andi Kleen" <andi@firstfloor.org>, linux-kernel@vger.kernel.org
Subject: Re: Any lightweight way for one thread to force another thread to  
     suspend execution?
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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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