Re: [patch] fixed both processes in D state and the /proc/ oopses [Re: [patch] Fixed the race that was oopsing Linux-2.2.0]

[Andrea Arcangeli <andrea@e-mind.com>]

On Thu, 28 Jan 1999, Linus Torvalds wrote:

> Incorrect, see my previous email. It may not be strictly necessary right
> now due to us probably holding the kernel lock everywhere, but it is
> conceptually necessary, and it is _not_ an argument for a spinlock.

If you remove the kernel lock around do_exit() you _need_ my mm_lock
spinlock. You need it to make atomic the decreasing of mm->count and
current->mm = &init_mm. If the two instructions are not atomic you have
_no_ way to know if you can mmget() at any time the mm of a process.

I repeat in another way (just trying to avoid English mistakes): 
decreasing mm->count has to go in sync with updating current->mm,
otherwise you don't know if you can access the mm of a process (and so
also touching mm->count) because the mm of such process could be just been
deallocated (the process could be a zombie).

As far I can see the mm->count will _never_ have 1 reason to be atomic_t
even removing the function lock_kernel() from
/usr/src/linux/include/asm/smplock.h . 

Using an int for mm->count (isntead of atomic_t) is far from be something
of magic. Doing that change it means that I don't expect work by magic,
but I know that it will work fine. It's instead magic (out of the human
reason) how the kernel works now, if you think that we _need_ atomic_t
instead of int. 

It's hard for me to be quiet even if you asked me to go away because I
think you are plain wrong telling that we need mm->count atomic_t for the
future. 

Andrea Arcangeli

PS. You have not hurted me asking me to go away, because I _always_ do the
_best_ I _can_, and if my best is plain wrong I can't change this reality. 
What I care is to do the best I can. Maybe I should really go away and do
something in my sparetime where it's not requested to be clever... I hope
it's not the case though and that you wasn't serious.

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