cpu: Fix LLSC atomic CPU wakeup

Writes to locked memory addresses (LLSC) did not wake up the locking
CPU. This can lead to deadlocks on multi-core runs. In AtomicSimpleCPU,
recvAtomicSnoop was checking if the incoming packet was an invalidation
(isInvalidate) and only then handled a locked snoop. But, writes are
seen instead of invalidates when running without caches (fast-forward
configurations). As as simple fix, now handleLockedSnoop is also called
even if the incoming snoop packet are from writes.

src/cpu/minor/lsq.cc

@@ -1617,7 +1617,9 @@ LSQ::recvTimingSnoopReq(PacketPtr pkt)
      * this action on snoops. */
 
     /* THREAD */
-    TheISA::handleLockedSnoop(cpu.getContext(0), pkt, cacheBlockMask);
+    if (pkt->isInvalidate() || pkt->isWrite()) {
+        TheISA::handleLockedSnoop(cpu.getContext(0), pkt, cacheBlockMask);
+    }
 }
 
 }

src/cpu/o3/lsq_unit_impl.hh

@@ -438,10 +438,8 @@ LSQUnit<Impl>::checkSnoop(PacketPtr pkt)
     int load_idx = loadHead;
     DPRINTF(LSQUnit, "Got snoop for address %#x\n", pkt->getAddr());
 
-    // Unlock the cpu-local monitor when the CPU sees a snoop to a locked
-    // address. The CPU can speculatively execute a LL operation after a pending
-    // SC operation in the pipeline and that can make the cache monitor the CPU
-    // is connected to valid while it really shouldn't be.
+    // Only Invalidate packet calls checkSnoop
+    assert(pkt->isInvalidate());
     for (int x = 0; x < cpu->numContexts(); x++) {
         ThreadContext *tc = cpu->getContext(x);
         bool no_squash = cpu->thread[x]->noSquashFromTC;

src/cpu/simple/atomic.cc

@@ -292,7 +292,10 @@ AtomicSimpleCPU::AtomicCPUDPort::recvAtomicSnoop(PacketPtr pkt)
     }
 
     // if snoop invalidates, release any associated locks
-    if (pkt->isInvalidate()) {
+    // When run without caches, Invalidation packets will not be received
+    // hence we must check if the incoming packets are writes and wakeup
+    // the processor accordingly
+    if (pkt->isInvalidate() || pkt->isWrite()) {
         DPRINTF(SimpleCPU, "received invalidation for addr:%#x\n",
                 pkt->getAddr());
         for (auto &t_info : cpu->threadInfo) {

src/cpu/simple/timing.cc

@@ -876,8 +876,14 @@ TimingSimpleCPU::DcachePort::recvTimingSnoopReq(PacketPtr pkt)
         }
     }
 
-    for (auto &t_info : cpu->threadInfo) {
-        TheISA::handleLockedSnoop(t_info->thread, pkt, cacheBlockMask);
+    // Making it uniform across all CPUs:
+    // The CPUs need to be woken up only on an invalidation packet (when using caches)
+    // or on an incoming write packet (when not using caches)
+    // It is not necessary to wake up the processor on all incoming packets
+    if (pkt->isInvalidate() || pkt->isWrite()) {
+        for (auto &t_info : cpu->threadInfo) {
+            TheISA::handleLockedSnoop(t_info->thread, pkt, cacheBlockMask);
+        }
     }
 }