Messages in this thread | | | Date | Mon, 2 Oct 2017 17:03:46 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH v2 2/2] pid: Remove pidhash |
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On 09/30, Gargi Sharma wrote: > > On Wed, Sep 27, 2017 at 9:58 PM, Oleg Nesterov <oleg@redhat.com> wrote: > > If I was not clear... > > > > in short, after this patch the very first idr_alloc_cyclic() is already > > wrong. Because, once again, the new not-fully-initialized pid can be found > > by find_pid_ns(). > > If the PIDNS_ADDING check fails, I jump to the flag that performs > this > while (++i <= ns->level) > idr_remove(&ns->idr, (pid->numbers + i)->nr); > So when find_pid_ns() is called, it will not find this pid.
You misunderstood.
OK, to simplify lets forget about namespaces, locking, everything. So after this patch alloc_pid() roughly does:
pid = kmem_cache_alloc();
nr = idr_alloc_cyclic(idr, pid); // lets suppose it returns 1234
/* WINDOW */
for (type = 0; type < PIDTYPE_MAX; ++type) INIT_HLIST_HEAD(&pid->tasks[type]);
now suppose that in that WINDOW above another CPU does, just for example, sys_tkill(1234, SIG) which implies find_task_by_vpid(1234) which does pid_task(find_pid_ns(1234)).
find_pid_ns() returns the non-initialized pid above, because with this patch it is idr_find() and this pid was already added by idr_alloc_cyclic().
Then pid_task(pid) returns garbage because pid->tasks[] was not initialised yet.
And of course we have the same problems with pid->count/numbers/etc.
See?
> > perhaps you should chane the previous patch to do > > idr_alloc_cyclic(ptr = NULL) and use idr_replace() in this patch after > > the PIDNS_HASH_ADDING check. > > I'm not sure if I understand this. Do we want to do this to make sure > the pid namespace is > initialized before the first process enters into > the namespace? If yes,
No,
> how does idr_alloc_cyclic(ptr = NULL) help?
In this case find_pid_ns/idr_find will return NULL until we do idr_replace(idr, pid) when this pid is fully initialized.
Oleg.
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