Re: [Qemu-devel] [PATCH v2 10/45] translate-all: guarantee that tb_hash only holds valid TBs

[Richard Henderson]

On 07/17/2017 02:27 PM, Emilio G. Cota wrote:
> On Mon, Jul 17, 2017 at 12:55:03 -1000, Richard Henderson wrote:
> > On 07/16/2017 10:03 AM, Emilio G. Cota wrote:
> > > @@ -1073,13 +1073,17 @@ void tb_phys_invalidate(TranslationBlock *tb, 
> > > tb_page_addr_t page_addr)
> > >       assert_tb_locked();
> > > -    atomic_set(&tb->invalid, true);
> > > -
> > >       /* remove the TB from the hash list */
> > >       phys_pc = tb->page_addr[0] + (tb->pc & ~TARGET_PAGE_MASK);
> > >       h = tb_hash_func(phys_pc, tb->pc, tb->flags, tb->trace_vcpu_dstate);
> > >       qht_remove(&tcg_ctx.tb_ctx.htable, tb, h);
> > > +    /*
> > > +     * Mark the TB as invalid *after* it's been removed from tb_hash, which
> > > +     * eliminates the need to check this bit on lookups.
> > > +     */
> > > +    tb->invalid = true;
> >
> > I believe you need atomic_store_release here.  Previously we were relying on
> > the lock acquisition in qht_remove to provide the required memory barrier.
> >
> > We definitely need to make sure this reaches memory before we zap the TB in
> > the CPU_FOREACH loop.
>
> After this patch tb->invalid is only read/set with tb_lock held, so no need for
> atomics while accessing it.

I think there's a path by which we do get stale data.
For threads A and B,

  (A) Lookup succeeds for TB in hash without tb_lock
       (B) Removes TB from hash
       (B) Sets tb->invalid
       (B) Clears FORALL_CPU jmp_cache
  (A) Store TB into local jmp_cache

... and since we never check for invalid again, there's nothing to evict TB 
from the jmp_cache again.

Here's a plan that will make me happy:

(1) Drop this patch, leaving the set of tb->invalid (or CF_INVALID) in place.
(2) Include CF_INVALID in the mask of bits compared in tb_lookup__cpu_state.
    (a) At this point in the patch set that's just

        (tb->flags & CF_INVALID) == 0

    (b) Later in the patch series when CF_PARALLEL is introduced
        (and CF_HASH_MASK, lets call it, instead of the cf_mask
        function you have now), this becomes

        (tb->flags & (CF_HASH_MASK | CF_INVALID)) == cf_mask

So that we continue to check CF_INVALID each time we lookup a TB, but now we 
get it for free as a part of the other flags check.


r~