icount: Take iothread lock when running QEMU timers

The function icount_prepare_for_run() is called with the iothread
unlocked, but it can call icount_notify_aio_contexts() which will
run qemu timer handlers. Those are supposed to be run only with
the iothread lock held, so take the lock while we do that.

Since icount mode runs everything on a single thread anyway,
not holding the lock is likely mostly not going to introduce
races, but it can cause us to trip over assertions that we
do hold the lock, such as the one reported in issue 1130.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1130
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Pavel Dovgalyuk <Pavel.Dovgalyuk@ispras.ru>
Message-id: 20220801164527.3134765-1-peter.maydell@linaro.org

accel/tcg/tcg-accel-ops-icount.c

@@ -109,7 +109,13 @@ void icount_prepare_for_run(CPUState *cpu)
     replay_mutex_lock();
 
     if (cpu->icount_budget == 0) {
+        /*
+         * We're called without the iothread lock, so must take it while
+         * we're calling timer handlers.
+         */
+        qemu_mutex_lock_iothread();
         icount_notify_aio_contexts();
+        qemu_mutex_unlock_iothread();
     }
 }