linux-kernel - Re: [PATCH v2 2/2] proc: Ensure we see the exit of each process tid exactly

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Message-ID: <87h7x8sqjj.fsf@x220.int.ebiederm.org>
Date:   Fri, 24 Apr 2020 13:10:40 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Alexey Gladkov <legion@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexey Gladkov <gladkov.alexey@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 2/2] proc: Ensure we see the exit of each process tid exactly

Oleg Nesterov <oleg@...hat.com> writes:

> On 04/23, Eric W. Biederman wrote:
>>
>> When the thread group leader changes during exec and the old leaders
>> thread is reaped proc_flush_pid
>
> This is off-topic, but let me mention this before I forget...
>
> Note that proc_flush_pid() does nothing if CONFIG_PROC_FS=n, this mean
> that in this case release_task() leaks thread_pid.

Good catch.  Wow.  I seem to be introducing almost as many bugs as I am
fixing right now.  Ouch.

>> +void exchange_tids(struct task_struct *ntask, struct task_struct *otask)
>> +{
>> +	/* pid_links[PIDTYPE_PID].next is always NULL */
>> +	struct pid *npid = READ_ONCE(ntask->thread_pid);
>> +	struct pid *opid = READ_ONCE(otask->thread_pid);
>> +
>> +	rcu_assign_pointer(opid->tasks[PIDTYPE_PID].first, &ntask->pid_links[PIDTYPE_PID]);
>> +	rcu_assign_pointer(npid->tasks[PIDTYPE_PID].first, &otask->pid_links[PIDTYPE_PID]);
>> +	rcu_assign_pointer(ntask->thread_pid, opid);
>> +	rcu_assign_pointer(otask->thread_pid, npid);
>> +	WRITE_ONCE(ntask->pid_links[PIDTYPE_PID].pprev, &opid->tasks[PIDTYPE_PID].first);
>> +	WRITE_ONCE(otask->pid_links[PIDTYPE_PID].pprev, &npid->tasks[PIDTYPE_PID].first);
>> +	WRITE_ONCE(ntask->pid, pid_nr(opid));
>> +	WRITE_ONCE(otask->pid, pid_nr(npid));
>> +}
>
> Oh, at first glance this breaks posix-cpu-timers.c:lookup_task(), the last
> user of has_group_leader_pid().
>
> I think that we should change lookup_task() to return "struct *pid", this
> should simplify the code... Note that none of its callers needs task_struct.
>
> And, instead of thread_group_leader/has_group_leader_pid checks we should
> use pid_has_task(TGID).

Somehow I thought we could get away without fiddling with that right
now, but on second glance I can see the races.

I played with this earlier and I agree returning a struct pid *
is desirable.  I will see if I can track down the patches I was
playing with as that definitely needs to get fixed first.

> After that, this patch should kill has_group_leader_pid().
>
> What do you think?

I agree completely.  has_group_leader_pid is the same as
thread_group_leader after this so should be removed.  Especially as it
won't have any users.

There are several other potential cleanups as well.  Such as not
using a hlist for PIDTYPE_PID.  Which would allow us to run the hlists
through struct signal_struct instead.  I think that would clean things
up but that touches so many things it may just be pointless code churn.

Just for mentioning I am thinking we should rename PIDTYPE_PID to
PIDTYPE_TID just to create a distance in peoples minds between
the kernel concepts and the user space concepts.

Eric