Messages in this thread | | | Date | Thu, 8 Dec 2011 00:34:50 +0400 | From | Cyrill Gorcunov <> | Subject | Re: [PATCH] fs, proc: Introduce the /proc/<pid>/children entry v2 |
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On Wed, Dec 07, 2011 at 11:19:16PM +0400, Cyrill Gorcunov wrote: ... > > > > > > This looks "obviously wrong". > > > > > > We can not trust ->children->next after rcu_read_unlock(). Another > > > rcu_read_lock() can't help. > > > > > > Once again, I can be easily wrong, need to read the patch first. > > > > > > > Wait, Oleg, I might be wrong as well, but it's now as > > > > children_seq_open > > get_proc_task (so ref to task increased) > > > > the children_seq_start/children_seq_stop works > > in iteration and every new iteration seq_list_next > > walks over the whole children list from the list > > head under rcu lock, so even if task is removed > > or added the link should exsist until rcu is unlocked > > and sync'ed no? > > > > On the other hands some if (task) tests are redundant > and might be dropped since we have a reference to a > task until seq-file is not released. I'll update it > and shrink a patch some more. >
So while adding new task to a list is not a problem (the reader will simply not notice it) removing task from a list is a bit tricky, but as far as I see switch_task_namespaces (from exit_notify) uses synchronize_rcu as well as release_task calls for call_rcu for put_task_struct) so I think we will not get any wrong dereference in
static int children_seq_show(struct seq_file *seq, void *v) { struct task_struct *task = container_of(v, struct task_struct, sibling); return seq_printf(seq, " %lu", (unsigned long)pid_vnr(task_pid(task))); }
while we're in rcu reader section. I might be wrong of course, so please verify this claim.
Cyrill
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