Re: [PATCH 3/3] exec: Transform exec_update_mutex into a rw_semaphore

Bernd Edlinger
2020-12-04T16:09:06Z

Hi Eric,

I think I remembered from a previous discussion about this topic,
that it was unclear if the rw_semaphores are working the same
in RT-Linux. Will this fix work in RT as well?

On 12/3/20 9:12 PM, Eric W. Biederman wrote:
> --- a/kernel/kcmp.c
> +++ b/kernel/kcmp.c
> @@ -70,25 +70,25 @@ get_file_raw_ptr(struct task_struct *task, unsigned int idx)
> return file;
> }
>
> -static void kcmp_unlock(struct mutex *m1, struct mutex *m2)
> +static void kcmp_unlock(struct rw_semaphore *l1, struct rw_semaphore *l2)
> {
> - if (likely(m2 != m1))
> - mutex_unlock(m2);
> - mutex_unlock(m1);
> + if (likely(l2 != l1))

is this still necessary ?

> + up_read(l2);
> + up_read(l1);
> }
>
> -static int kcmp_lock(struct mutex *m1, struct mutex *m2)
> +static int kcmp_lock(struct rw_semaphore *l1, struct rw_semaphore *l2)
> {
> int err;
>
> - if (m2 > m1)
> - swap(m1, m2);
> + if (l2 > l1)
> + swap(l1, l2);

and this is probably also no longer necessary?

>
> - err = mutex_lock_killable(m1);
> - if (!err && likely(m1 != m2)) {
> - err = mutex_lock_killable_nested(m2, SINGLE_DEPTH_NESTING);
> + err = down_read_killable(l1);
> + if (!err && likely(l1 != l2)) {

and this can now be unconditionally, right?

> + err = down_read_killable_nested(l2, SINGLE_DEPTH_NESTING);
> if (err)
> - mutex_unlock(m1);
> + up_read(l1);
> }
>
> return err;
> @@ -156,8 +156,8 @@ SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
> /*
> * One should have enough rights to inspect task details.
> */
> - ret = kcmp_lock(&task1->signal->exec_update_mutex,
> - &task2->signal->exec_update_mutex);
> + ret = kcmp_lock(&task1->signal->exec_update_lock,
> + &task2->signal->exec_update_lock);
> if (ret)
> goto err;
> if (!ptrace_may_access(task1, PTRACE_MODE_READ_REALCREDS) ||
> @@ -212,8 +212,8 @@ SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
> }
>
> err_unlock:
> - kcmp_unlock(&task1->signal->exec_update_mutex,
> - &task2->signal->exec_update_mutex);
> + kcmp_unlock(&task1->signal->exec_update_lock,
> + &task2->signal->exec_update_lock);
> err:
> put_task_struct(task1);
> put_task_struct(task2);

Thanks
Bernd.