Quoting Sonny Rao ([email protected]):
> On Wed, Jun 21, 2006 at 02:09:10PM -0500, Serge E. Hallyn wrote:
> <snip>
> > > Yeah, I proposed a similar patch to Anton, and it would quiet the
> > > warning on powerpc, but that's not the point. It happens that powerpc
> > > doesn't use 0 as a context id, but that may not be true on another
> > > architecture. That's really what I'm concerned about.
> >
> > FWIW, ppc and cris do the NO_CONTEXT check, while others don't
> > even have a arch-specific 'mm->context.id'.
>
> Good point. I probably stated that concern too narrowly. Probably
> what I should say is: What is the pre-condition for calling
> destroy_context() ? Is it that init_new_context() must have
> succeeded? Or is it merely that mm.context has been zeroed
> out?
Right, that may be the right question. If that's the case, then the
problem is really include/linux/sched.h:__mmdrop() which is what's
calling destroy_context(). Separating that out becomes a pretty
big patch affecting at least all mmput() and mmdrop() callers.
> Here's destroy context on sparc64:
>
> void destroy_context(struct mm_struct *mm)
> {
> unsigned long flags, i;
>
> for (i = 0; i < MM_NUM_TSBS; i++)
> tsb_destroy_one(&mm->context.tsb_block[i]);
>
> spin_lock_irqsave(&ctx_alloc_lock, flags);
>
> if (CTX_VALID(mm->context)) {
> unsigned long nr = CTX_NRBITS(mm->context);
> mmu_context_bmap[nr>>6] &= ~(1UL << (nr & 63));
> }
>
> spin_unlock_irqrestore(&ctx_alloc_lock, flags);
> }
>
> It seems to assume that mm->context is valid before doing a check.
>
> Since I don't have a sparc64 box, I can't check to see if this
> actually breaks things or not.
So we can either go through all arch's and make sure destroy_context is
safe for invalid context, or split mmput() and destroy_context()...
The former seems easier, but the latter seems more robust in the face of
future code changes I guess.
-serge
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