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Date: Sat, 21 Jun 2008 19:02:28 -0700 (PDT)
Subject: Re: Any lightweight way for one thread to force another thread to 
     suspend execution?
From: "Eric Smith" <eric@brouhaha.com>
To: linux-kernel@vger.kernel.org
Cc: "Bart Van Assche" <bart.vanassche@gmail.com>
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Bart wrote:
> Trying to suspend another thread synchronously is a bad idea because
> this can easily trigger deadlocks. E.g. if you suspend a thread while
> that thread holds a lock on a mutex, your application will deadlock.

I understand that and am willing to take responsibility for ensuring
that my application doesn't get into that situation.  Basically I'm
trying to port something that ran on a strict priority preemptive
kernel (on a single processor) to run on a Linux SMP system, so I
need a higher priority thread to completely preempt a lower priority
thread, even though the lower priority thread might be running on
another processor.

As far as I can tell, pthreads doesn't solve this problem, which is
unfortunately, since in other respects pthreads looks like it is
mostly suitable for use in porting code from other real-time kernels
to Linux.

It appears that some pthreads implementations add a nonstandard
pthread_suspend() that can do this.  What I'm looking for is a way
to efficiently implement pthrad_suspend() on Linux, ideally with
a minimum of system calls.

It seems to me that in principle, if the kernel provided a way to do
this, it should be able to suspend a thread that wasn't currently
executing merely by changing some of that thread's metadata, while
suspending a thread that is executing on another processor would be
more expensive since it would be necessary to force an interrupt
on that processor, etc.

Thanks,
Eric

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