i#7044 sched deadlock: Fix theoretical deadlocks (#7087)

Adds a check for the caller holding the previous input's lock before
acquiring it to retrieve the previous workload. The existing code
acquired the new lock without checking caller_holds_cur_input_lock,
which could hang (it happened not to because the only caller who sets
caller_holds_cur_input_lock to true set the current input to invalid and
set_cur_input() returns prior to this point).

Solves a 2nd theoretical lock issue: disallows the caller holding the input
lock if we might call add_to_ready_queue() (we disallow the lock holding
in dynamic mode in general to avoid this).  Adds an assert.

Issue: #7044

clients/drcachesim/scheduler/scheduler_impl.cpp

@@ -2292,6 +2292,8 @@ scheduler_impl_tmpl_t<RecordType, ReaderType>::set_cur_input(
         // straightforward).
         if (options_.mapping == sched_type_t::MAP_TO_ANY_OUTPUT && prev_input != input &&
             !inputs_[prev_input].at_eof) {
+            // The caller should never hold the input lock for MAP_TO_ANY_OUTPUT.
+            assert(!caller_holds_cur_input_lock);
             add_to_ready_queue(output, &inputs_[prev_input]);
         }
     } else if (options_.schedule_record_ostream != nullptr &&
@@ -2313,7 +2315,12 @@ scheduler_impl_tmpl_t<RecordType, ReaderType>::set_cur_input(
 
     int prev_workload = -1;
     if (outputs_[output].prev_input >= 0 && outputs_[output].prev_input != input) {
-        std::lock_guard<mutex_dbg_owned> lock(*inputs_[outputs_[output].prev_input].lock);
+        // If the caller already holds the lock, do not re-acquire as that will hang.
+        auto scoped_lock =
+            (caller_holds_cur_input_lock && prev_input == outputs_[output].prev_input)
+            ? std::unique_lock<mutex_dbg_owned>()
+            : std::unique_lock<mutex_dbg_owned>(
+                  *inputs_[outputs_[output].prev_input].lock);
         prev_workload = inputs_[outputs_[output].prev_input].workload;
     }
 

clients/drcachesim/scheduler/scheduler_impl.h

@@ -751,7 +751,8 @@ template <typename RecordType, typename ReaderType> class scheduler_impl_tmpl_t
 
     // The caller cannot hold the output or input lock.
     // The caller can hold the output's current input's lock but must pass
-    // true for the 3rd parameter in that case.
+    // true for the 3rd parameter in that case; holding the lock is not
+    // allowed for MAP_TO_ANY_OUTPUT.
     stream_status_t
     set_cur_input(output_ordinal_t output, input_ordinal_t input,
                   bool caller_holds_cur_input_lock = false);