mem-ruby: fix atomic deadlock with WB GPU L2 caches

By default the GPU VIPER coherence protocol uses a WT L2 cache.
However it has support for using WB caches (although this is not
tested currently).  When using a WB L2 cache for the GPU, this
results in deadlocks with atomics.

Specifically, when an atomic reaches the L2 and the line is
currently in M or W, the line must be written back before the atomic
can be performed.  However, the current support has two issues:

a) it never performs the atomic operation -- while VIPER current
assumes all atomics are system scope atomics and thus cannot be
performed at the L2 and this transition requires the dirty line be
written back before performing the atomic, the transition never
performs the atomic nor does the response path handle it.
b) putting the atomic action right after the write back is not
safe because we need to ensure the requests are ordered when they
reach memory -- thus we have to wait until the write back is
acknowledged before it's safe to send/perform the atomic.

To fix this, this change modifies the transition in question to
put the atomic on the stalled requests buffer, which the WBAck will
check when it returns to the L2 (and thus perform the atomic, which
will result in the atomic being sent on to the directory).

This fix has been tested and verified with both the per-checkin and
nightly GPU Ruby Random tester tests (with a WB L2 cache).

Change-Id: I9a43fd985dc71297521f4b05c47288d92c314ac7
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/68978
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Matthew Poremba <matthew.poremba@amd.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/mem/ruby/protocol/GPU_VIPER-TCC.sm

@@ -816,10 +816,14 @@ transition(I, Atomic, A) {TagArrayRead} {
   }
 
   transition({M, W}, Atomic, WI) {TagArrayRead} {
-    p_profileHit;
     t_allocateTBE;
     wb_writeBack;
-    p_popRequestQueue;
+    // after writing back the current line, we need to wait for it to be done
+    // before we try to perform the atomic
+    // by putting the stalled requests in a buffer, we reduce resource contention
+    // since they won't try again every cycle and will instead only try again once
+    // woken up
+    st_stallAndWaitRequest;
   }
 
   transition(I, WrVicBlk) {TagArrayRead} {