On 09-May-01 Robert Watson wrote:
> On Tue, 8 May 2001, John Baldwin wrote:
>> That's easy enough.  Well, it used to be at least.  You can use 'ps' to
>> find the address of the struct proc (first pointer in the display) and
>> then do 'call psignal(addr, 9)' to send SIGKILL to the process.  Then
>> hit 'c' to continue and voila, the process dies.  I think that may panic
>> now due to proc lock not being held (though the debugger shouldn't need
>> any locks in theory.) Perhaps mtx_assert() should honor db_active and
>> not panic if it is set. 
> I followed everything here fine until you asserted that the debugger
> shouldn't need any locks.  I guess I don't see why that is, at least in
> terms of not corrupting structures.  From a practical perspective, the
> debugger is like any other interupt-driven preemptive code-path: if you
> want to modify a structure, you need to synchronize appropriately to avoid
> corrupting the structure.  This may not be something you really want to do
> in a debugger, so in that sense perhaps you *shouldn't* grab a lock in the
> debugger, but to perform the described action safely, you *should* grab a
> lock so as not to corrupt fields of the proc structure (i.e., if you broke
> into the debugger during a non-atomic flags update).  Violating system
> invariants is something you should be allowed to do in a debugger, but
> this sounded like it was a feature people were looking from to recover
> from unhappy behavior, not to introduce it :-).

I am more worried about the fact that you can deadlock the debugger.  What does
the debugger do if another process hold the proc lock on the process you want
to kill?  Cute, eh?  The debugger is an extra special environment.  Most of the
time you've panic'ed when you are in there (but then the panicstr tests that
skip lock operations save you from that).  Also, in the debugger you know that
no other threads are running.  This is why 'show pcpu' can list spin locks on
other cpu's safely, for example.  I'm not sure if a ddb 'kill' command
shouldn't be better implemented using a 'trylock' and refusing to send the
signal if it can't get the lock so it can avoid doing really bad things.  I
suppose it wouldn't deadlock but would switch to the other task and start
running.  Of course, this would be most disastrous if the current task we just
dropped to the debugger in holds a spin lock. :(


John Baldwin <[EMAIL PROTECTED]> -- http://www.FreeBSD.org/~jhb/
PGP Key: http://www.baldwin.cx/~john/pgpkey.asc
"Power Users Use the Power to Serve!"  -  http://www.FreeBSD.org/

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