(11/22/2013 12:54 PM), Oleg Nesterov wrote: > fs_struct->in_exec == T means that this ->fs is used by a single > process (thread group), and one of the treads does do_execve(). > > To avoid the mt-exec races this code has the following complications: > > 1. check_unsafe_exec() returns -EBUSY if ->in_exec was > already set by another thread. > > 2. do_execve_common() records "clear_in_exec" to ensure > that the error path can only clear ->in_exec if it was > set by current. > > However, after 9b1bf12d5d51 "signals: move cred_guard_mutex from > task_struct to signal_struct" we do not need these complications: > > 1. We can't race with our sub-thread, this is called under > per-process ->cred_guard_mutex. And we can't race with > another CLONE_FS task, we already checked that this fs > is not shared. > > We can remove the dead -EAGAIN logic. > > 2. "out_unmark:" in do_execve_common() is either called > under ->cred_guard_mutex, or after de_thread() which > kills other threads, so we can't race with sub-thread > which could set ->in_exec. And if ->fs is shared with > another process ->in_exec should be false anyway. > > We can clear in_exec unconditionally. > > This also means that check_unsafe_exec() can be void. > > Signed-off-by: Oleg Nesterov <o...@redhat.com>
I have found no problem in this patch. However, I have a very basic question. Why do we need to keep fs->in_exec? If I understand correctly, it is needed for retrieving fork() and exec() race in the same process. If it is correct, can't we move it it to signal->in_exec? It seems to match signal->cred_guard_mutex and _I_ can easily understand what the code want. In the other words, currently we have no protection against making new thread when p->fs is shared w/ another process and I have no idea why multi process sharing influence multi thread behavior. I am not expert in this area and I may overlook something. Please correct me if I am silly. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
