[Qemu-devel] [PATCH] linux-user: fix memory leaks with NPTL emulation

[Nathan Froyd]

Running programs that create large numbers of threads, such as this
snippet from libstdc++'s pthread7-rope.cc:

  const int max_thread_count = 4;
  const int max_loop_count = 10000;
  ...
  for (int j = 0; j < max_loop_count; j++)
    {
      ...
      for (int i = 0; i < max_thread_count; i++)
        pthread_create (&tid[i], NULL, thread_main, 0);

      for (int i = 0; i < max_thread_count; i++)
        pthread_join (tid[i], NULL);
    }

in user-mode emulation will quickly run out of memory.  This is caused
by a failure to free memory in do_syscall prior to thread exit:

          /* TODO: Free CPU state.  */
          pthread_exit(NULL);

The first step in fixing this is to make all TaskStates used by QEMU
dynamically allocated.  The TaskState used by the initial thread was
not, as it was allocated on main's stack.  So fix that, free the
cpu_env, free the TaskState, and we're home free, right?

Not exactly.  When we create a thread, we do:

        ts = qemu_mallocz(sizeof(TaskState) + NEW_STACK_SIZE);
        ...
        new_stack = ts->stack;
        ...
        ret = pthread_attr_setstack(&attr, new_stack, NEW_STACK_SIZE);

If we blindly free the TaskState, then, we yank the current (host)
thread's stack out from underneath it while it still has things to do,
like calling pthread_exit.  That causes problems, as you might expect.

The solution adopted here is to let the C library allocate the thread's
stack (so the C library can properly clean it up at pthread_exit) and
provide a hint that we want NEW_STACK_SIZE bytes of stack.

With those two changes, we're done, right?  Well, almost.  You see,
we're creating all these host threads and their parent threads never
bother to check that their children are finished.  There's no good place
for the parent threads to do so.  Therefore, we need to create the
threads in a detached state so the parent thread doesn't have to call
pthread_join on the child to release the child's resources; the child
does so automatically.

With those three major changes, we can comfortably run programs like the
above without exhausting memory.  We do need to delete 'stack' from the
TaskState structure.

Signed-off-by: Nathan Froyd <address@hidden>
---
 linux-user/main.c    |    4 ++--
 linux-user/qemu.h    |    2 --
 linux-user/syscall.c |   11 +++++++----
 3 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/linux-user/main.c b/linux-user/main.c
index dbba8be..7d41d4a 100644
--- a/linux-user/main.c
+++ b/linux-user/main.c
@@ -2711,7 +2711,7 @@ int main(int argc, char **argv, char **envp)
     struct target_pt_regs regs1, *regs = &regs1;
     struct image_info info1, *info = &info1;
     struct linux_binprm bprm;
-    TaskState ts1, *ts = &ts1;
+    TaskState *ts;
     CPUState *env;
     int optind;
     const char *r;
@@ -3038,7 +3038,7 @@ int main(int argc, char **argv, char **envp)
     }
     target_argv[target_argc] = NULL;
 
-    memset(ts, 0, sizeof(TaskState));
+    ts = qemu_mallocz (sizeof(TaskState));
     init_task_state(ts);
     /* build Task State */
     ts->info = info;
diff --git a/linux-user/qemu.h b/linux-user/qemu.h
index 708021e..00c6549 100644
--- a/linux-user/qemu.h
+++ b/linux-user/qemu.h
@@ -126,8 +126,6 @@ typedef struct TaskState {
     struct sigqueue sigqueue_table[MAX_SIGQUEUE_SIZE]; /* siginfo queue */
     struct sigqueue *first_free; /* first free siginfo queue entry */
     int signal_pending; /* non zero if a signal may be pending */
-
-    uint8_t stack[0];
 } __attribute__((aligned(16))) TaskState;
 
 extern char *exec_path;
diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index d44f512..5761106 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -3601,9 +3601,8 @@ static int do_fork(CPUState *env, unsigned int flags, 
abi_ulong newsp,
         new_thread_info info;
         pthread_attr_t attr;
 #endif
-        ts = qemu_mallocz(sizeof(TaskState) + NEW_STACK_SIZE);
+        ts = qemu_mallocz(sizeof(TaskState));
         init_task_state(ts);
-        new_stack = ts->stack;
         /* we create a new CPU instance. */
         new_env = cpu_copy(env);
 #if defined(TARGET_I386) || defined(TARGET_SPARC) || defined(TARGET_PPC)
@@ -3639,7 +3638,8 @@ static int do_fork(CPUState *env, unsigned int flags, 
abi_ulong newsp,
             info.parent_tidptr = parent_tidptr;
 
         ret = pthread_attr_init(&attr);
-        ret = pthread_attr_setstack(&attr, new_stack, NEW_STACK_SIZE);
+        ret = pthread_attr_setstacksize(&attr, NEW_STACK_SIZE);
+        ret = pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
         /* It is not safe to deliver signals until the child has finished
            initializing, so temporarily block all signals.  */
         sigfillset(&sigmask);
@@ -3667,6 +3667,7 @@ static int do_fork(CPUState *env, unsigned int flags, 
abi_ulong newsp,
         if (flags & CLONE_NPTL_FLAGS2)
             return -EINVAL;
         /* This is probably going to die very quickly, but do it anyway.  */
+        new_stack = qemu_mallocz (NEW_STACK_SIZE);
 #ifdef __ia64__
         ret = __clone2(clone_func, new_stack, NEW_STACK_SIZE, flags, new_env);
 #else
@@ -4240,7 +4241,9 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
               sys_futex(g2h(ts->child_tidptr), FUTEX_WAKE, INT_MAX,
                         NULL, NULL, 0);
           }
-          /* TODO: Free CPU state.  */
+          thread_env = NULL;
+          qemu_free(cpu_env);
+          qemu_free(ts);
           pthread_exit(NULL);
       }
 #endif
-- 
1.6.3.2