On 05/11, Andrew Morton wrote: > > On Fri, 11 May 2007 00:36:25 +0200 > "Rafael J. Wysocki" <[EMAIL PROTECTED]> wrote: > > > static inline int is_user_space(struct task_struct *p) > > { > > - return p->mm && !(p->flags & PF_BORROWED_MM); > > + int ret; > > + > > + task_lock(p); > > + ret = p->mm && !(p->flags & PF_BORROWED_MM); > > + task_unlock(p); > > + return ret; > > } > > The whole function is racy, isn't it? I mean, the condition which it is > testing can go from true->false or false->true at any instant after this > function returns its now-wrong value. > > iow, callers of this function need to to something to prevent the expression > `p->mm && !(p->flags & PF_BORROWED_MM);' from changing value _anyway_. In > which case the new locking is not needed?
freeze_processes() first freezes user-space tasks only, then kernel threads. Without task_lock() we can miss PF_BORROWED_MM and count the kernel thread (which is doing use_mm()) as a user space process. This means it will be frozen prematurely. true->false means daemonize() or do_exit(), seems harmless. false->true means exec from kernel space. That is why FREEZER_KERNEL_THREADS in fact means all tasks, not only kernel threads. Oleg. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/