Date: Sun, 22 Jun 2008 11:14:56 +0200 (CEST)
From: Jan Engelhardt <jengelh@medozas.de>
To: Eric Smith <eric@brouhaha.com>
cc: linux-kernel@vger.kernel.org, Andi Kleen <andi@firstfloor.org>
Subject: Re: Any lightweight way for one thread to force another thread to 
      suspend execution?
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On Sunday 2008-06-22 03:55, Eric Smith wrote:
>Andi wrote:
>> Any such mechanism will need a syscall, and it's unlikely that
>> any syscall will get much cheaper than a kill(SIGSTOP)
>
>But is there a way for the process sending the SIGSTOP to wait until it
>has taken effect?  I need a method to *synchronously* stop another
>thread.  That's why I thought I probably needed something more
>elaborate than SIGSTOP, though I'd like to minimize the number of
>system calls required.

When it is stopped, the process state changes to "T" (in ps and /proc).
Note that debugging a program with gdb or ptrace also puts it in the T state.
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