On Sat, Aug 22, 2009 at 03:09:52PM +0200, Oleg Nesterov wrote:
>
> OK, cgroups-add-ability-to-move-all-threads-in-a-process-to-a-new-cgroup-atomically.patch
> does
>
> threadgroup_sighand = threadgroup_fork_lock(leader);
>
> rcu_read_lock();
> list_for_each_entry_rcu(tsk, &leader->thread_group, thread_group)
>
> and this is equally wrong afais. Hmm, and other similar
> list_for_each_entry_rcu's doesn't look right. This code can do something
> like
>
> rcu_read_lock();
> if (!tsk->sighand) // tsk is leader or not, doesn't matter
> fail;
> list_for_each_rcu(tsk->thread_group) {}
> rcu_read_unlock();
>
> though.
>
>
>
> And why do we need sighand->threadgroup_fork_lock ? I gueess, to protect
> against clone(CLONE_THREAD).
>
> But. threadgroup_fork_lock() has another problem. Even if the process
> P is singlethreaded, I can't see how ->threadgroup_fork_lock work.
>
> threadgroup_fork_lock() bumps P->sighand->count. If P exec, it will
> notice sighand->count != 1 and switch to another ->sighand.
>
> Oleg.
So how about this: Each time we take tasklist_lock to iterate over
thread_group (once in getting the sighand, once to move all the tasks),
check if we raced with exec. The two problems are 1) group_leader
changes - we'll need to find the new leader's task_struct anyway - means
we can't iterate over thread_group, and 2) sighand changes after we take
the old one, means we've taken a useless lock.
I put together draft revisions of the old patches that check for racing
with exec in both cases, and if so, returning EAGAIN. I have the wrapper
function cgroup_procs_write loop around the return value, but it could
also possibly give EAGAIN back to userspace. Hopefully the code is safe
and sane this time :)
-- bblum
---
Documentation/cgroups/cgroups.txt | 7
include/linux/cgroup.h | 14 -
include/linux/init_task.h | 9
include/linux/sched.h | 15 +
kernel/cgroup.c | 519 ++++++++++++++++++++++++++++++++++----
kernel/fork.c | 9
6 files changed, 524 insertions(+), 49 deletions(-)
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