[Qemu-devel] [PATCH] linux-user: always start with parallel_cpus set to true

[Laurent Vivier]

We always need real atomics, as we can have shared memory between
processes.

A good test case is the example from futex(2), futex_demo.c:

the use case is

    mmap(...);
    fork();

    Parent and Child:

    while(...)
        __sync_bool_compare_and_swap(...)
        ...
        futex(...)

In this case we need real atomics in __sync_bool_compare_and_swap(),
but as parallel_cpus is set to 0, we don't have.

We also revert "b67cb68 linux-user: enable parallel code generation on clone"
as parallel_cpus in unconditionally set now.

Of course, this doesn't fix atomics that are emulated using
cpu_loop_exit_atomic() as we can't stop virtual CPUs from another processes.

Signed-off-by: Laurent Vivier <address@hidden>
---
 linux-user/syscall.c | 8 --------
 translate-all.c      | 4 ++++
 2 files changed, 4 insertions(+), 8 deletions(-)

diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 7b77503..db697c0 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -6164,14 +6164,6 @@ static int do_fork(CPUArchState *env, unsigned int 
flags, abi_ulong newsp,
         sigfillset(&sigmask);
         sigprocmask(SIG_BLOCK, &sigmask, &info.sigmask);
 
-        /* If this is our first additional thread, we need to ensure we
-         * generate code for parallel execution and flush old translations.
-         */
-        if (!parallel_cpus) {
-            parallel_cpus = true;
-            tb_flush(cpu);
-        }
-
         ret = pthread_create(&info.thread, &attr, clone_func, &info);
         /* TODO: Free new CPU state if thread creation failed.  */
 
diff --git a/translate-all.c b/translate-all.c
index 3dd9214..0b0bb09 100644
--- a/translate-all.c
+++ b/translate-all.c
@@ -142,7 +142,11 @@ static void *l1_map[V_L1_MAX_SIZE];
 
 /* code generation context */
 TCGContext tcg_ctx;
+#ifdef CONFIG_USER_ONLY
+bool parallel_cpus = true;
+#else
 bool parallel_cpus;
+#endif
 
 /* translation block context */
 #ifdef CONFIG_USER_ONLY
-- 
2.7.4