accel: use store_release/load_acquire for cross-thread exit_request

Reads and writes cpu->exit_request do not use a load-acquire/store-release
pair right now, but this means that cpu_exit() may not write cpu->exit_request
after any flags that are read by the vCPU thread.

Probably everything is protected one way or the other by the BQL, because
cpu->exit_request leads to the slow path, where the CPU thread often takes
the BQL (for example, to go to sleep by waiting on the BQL-protected
cpu->halt_cond); but it's not clear, so use load-acquire/store-release
consistently.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 5 insertions, 2 deletions

diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
index 508d2d2d9e..f838535d11 100644
--- a/accel/tcg/cpu-exec.c
+++ b/accel/tcg/cpu-exec.c
@@ -851,8 +851,11 @@ static inline bool cpu_handle_interrupt(CPUState *cpu,
     }
 #endif /* !CONFIG_USER_ONLY */
 
-    /* Finally, check if we need to exit to the main loop.  */
-    if (unlikely(qatomic_read(&cpu->exit_request)) || icount_exit_request(cpu)) {
+    /*
+     * Finally, check if we need to exit to the main loop.
+     * The corresponding store-release is in cpu_exit.
+     */
+    if (unlikely(qatomic_load_acquire(&cpu->exit_request)) || icount_exit_request(cpu)) {
         qatomic_set(&cpu->exit_request, 0);
         if (cpu->exception_index == -1) {
             cpu->exception_index = EXCP_INTERRUPT;