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RE: [DotGNU]Safely implementing Thread.Abort

From: Thong (Tum) Nguyen
Subject: RE: [DotGNU]Safely implementing Thread.Abort
Date: Thu, 20 May 2004 18:20:03 +1200

> -----Original Message-----
> From: Russell Stuart [mailto:address@hidden
> Sent: Thursday, 20 May 2004 2:18 p.m.
> To: Thong (Tum) Nguyen
> Cc: DotGnu-Develop
> Subject: Re: [DotGNU]Safely implementing Thread.Abort
> On Thu, 2004-05-20 at 07:36, Rhys Weatherley wrote:
> > Outputting abort check instructions on every backward
> > branch is OK with me, as long as you only do it in
> > programs that use Thread.Abort.  This can be detected
> > by the loader (any assembly with a MemberRef to
> > Thread.Abort is a candidate).
> This relies on knowing whether any code calls
> Thread.Abort().  There are two issues with that.  First
> of all, how are you going to know?  Scan every assembly
> referenced when you first start up?  Some of these
> assemblies might use Thread.Abort(), and yet it may
> never be called.
> Secondly, what about programs that do a
> System.Reflection.Assembly.Load? You might discover
> the loaded assembly might calls Thread.Abort() after
> you have generated some code.  A variation on the theme
> is Remoting.  A remote procedure may call Thread.Abort().
> I see how you could know that.
> On Thu, 2004-05-20 at 08:47, Rhys Weatherley wrote:
> > For now, implement the check on backwards jump as-is.
> This will always work, but you have generate equivalent
> code for the JIT.  Will you be able to bring yourself
> to do that?  It won't be just an instruction or two.
> The "tst" op-code will overwrite the CPU's condition
> flags, which will have to be restored if you are
> doing a conditional backward jump.  In any decently
> optimised code most of them are conditional.
> On Thu, 2004-05-20 at 08:18, Rhys Weatherley wrote:
> > How exactly are you going to do that?  The CVM instruction
> > pointer is probably going to be in a register (esi on x86).
> > You cannot guarantee that the CVM instruction pointer is in
> > "thread->pc" when an abort occurs.
> This is true.  In fact even in a Unix Signal you will have
> no idea of the "true" value of thread->pc - because you don't
> in general know what register it is in.
> Thus my original suggestion was modify every backward jump
> in the current method.
> That won't catch every case - the thread being aborted may
> be in the middle of constructing/destroying a frame.  The
> only way I can see around that is the Abort() method hanging
> around until it sees the aborted thread has processed the
> abort, or until its the stack frame changes - in which case
> it has another go at stopping it.
> On Thu, 2004-05-20 at 07:36, Rhys Weatherley wrote:
> > You cannot use any approach that requires the method code
> > to be modified after it is first generated.  It isn't thread safe:
> I am not convinced of that.  Firstly, no matter what you do,
> you have set a thread->aborted flag, and you have to force
> the thread to see it, ie you have to force it into the
> cache of the CPU it is executing on.  I think all operating
> systems that support multiple CPU's provide a means of doing
> that, as tum has noted.  Forcing the CPU to see modifications
> you have made to its instruction stream is essentially the
> same problem.
> Secondly, on all multi CPU architectures I know of writes to
> a word are atomic.  By this I mean a if a single instruction
> writes some bytes that do not cross a word boundary, no CPU
> will see some bytes written and some not.  Either they will
> see the entire write, or none of it.  Here "word" means
> the width of the narrowest memory bus - typically the one
> to main memory.  With DIMM's this is 64 bits.
> With CVM, where we have control over the op-codes, life is
> very simple.  All you need to do is have two versions of the
> "backward jump" opcode.  The one normally generated doesn't
> check for Abort()'s.  When Abort()'s is executed, you
> replace it with one that does. This can't possibly be
> non-thread safe.
> With the JIT life gets harder, because you can't typically
> just modify one byte.  You have to insert some sort of
> "call" instruction.  It could be a "call", or it could
> generate a software interrupt (like the ia32 int3).
> Whatever you do, providing you can make the modification
> with just one write to a word aligned value, you can
> get away with it.  This means the JIT would have to word
> align the backward jumps and returns.
> I don't really think its a question of whether its possible
> to implement Thread.Abort() with zero overhead to programs
> that don't use it - it is.  With CVM its not even that hard
> to do.  With the JIT - well it sounds like an awful lot of
> effort to me, but I can't think of a simpler solution ATM.
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