accel/tcg/cpu-exec: Fix precise single-stepping after interrupt

In some cases, cpu->exit_request can be false after handling the
interrupt, leading to another TB being executed instead of returning
to the main loop.

Fix this by returning true unconditionally when in single-step mode.

Fixes: ba3c35d ("tcg/cpu-exec: precise single-stepping after an interrupt")
Signed-off-by: Luc Michel <lmichel@kalray.eu>
Message-Id: <20220214132656.11397-1-lmichel@kalray.eu>
[rth: Unlock iothread mutex; simplify indentation]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

accel/tcg/cpu-exec.c

@@ -799,8 +799,12 @@ static inline bool cpu_handle_interrupt(CPUState *cpu,
                  * raised when single-stepping so that GDB doesn't miss the
                  * next instruction.
                  */
-                cpu->exception_index =
-                    (cpu->singlestep_enabled ? EXCP_DEBUG : -1);
+                if (unlikely(cpu->singlestep_enabled)) {
+                    cpu->exception_index = EXCP_DEBUG;
+                    qemu_mutex_unlock_iothread();
+                    return true;
+                }
+                cpu->exception_index = -1;
                 *last_tb = NULL;
             }
             /* The target hook may have updated the 'cpu->interrupt_request';