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~