linux-kernel - Re: [PATCH v3] do_wait: make PIDTYPE

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Message-ID: <4d9006b4-b65a-6ce0-b367-971f29de1f21@torproject.org>
Date:   Wed, 10 Mar 2021 18:14:44 -0600
From:   Jim Newsome <jnewsome@...project.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Christian Brauner <christian@...uner.io>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] do_wait: make PIDTYPE_PID case O(1) instead of O(n)

On 3/10/21 16:40, Eric W. Biederman wrote:
>> +// Optimization for waiting on PIDTYPE_PID. No need to iterate
through child
>> +// and tracee lists to find the target task.
>
> Minor nit:  C++ style comments look very out of place in this file
>             which uses old school C /* */ comment delimiters for
>             all of it's block comments.

Will do

>> +static int do_wait_pid(struct wait_opts *wo)
>> +{
>> +	struct task_struct *target = pid_task(wo->wo_pid, PIDTYPE_PID);
>                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> This is subtle change in behavior.
> 
> Today on the task->children list we only place thread group leaders.

Shouldn't we allow waiting on clone children if __WALL or __WCLONE is set?

This is already checked later in `eligible_child`, called from
`wait_consider_task`, so I *think* the current form should already do
the right thing. Now I'm confused though how the general path (through
`do_wait_thread`) works if clone children aren't on the task->children
list...?

(In any case it seems this will need another version with at least an
explanatory comment here)

Thanks!
-Jim