From: Linus Torvalds <torvalds@linux-foundation.org>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
Linux FS Devel <linux-fsdevel@vger.kernel.org>,
Alexey Dobriyan <adobriyan@gmail.com>,
Alexey Gladkov <legion@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
Oleg Nesterov <oleg@redhat.com>,
Alexey Gladkov <gladkov.alexey@gmail.com>
Subject: Re: [PATCH v2 2/2] proc: Ensure we see the exit of each process tid exactly
Date: Thu, 23 Apr 2020 13:28:18 -0700 [thread overview]
Message-ID: <CAHk-=wgXEJdkgGzZQzBDGk7ijjVdAVXe=G-mkFSVng_Hpwd4tQ@mail.gmail.com> (raw)
In-Reply-To: <87wo66vvnm.fsf_-_@x220.int.ebiederm.org>
On Thu, Apr 23, 2020 at 12:42 PM Eric W. Biederman
<ebiederm@xmission.com> wrote:
>
> +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));
> +}
This function is _very_ hard to read as written.
It really wants a helper function to do the swapping per hlist_head
and hlist_node, I think. And "opid/npid" is very hard to see, and the
naming doesn't make much sense (if it's an "exchange", then why is it
"old/new" - they're symmetric).
At least something like
struct hlist_head *old_pid_hlist = opid->tasks + PIDTYPE_PID;
struct hlist_head *new_pid_hlist = npid->tasks + PIDTYPE_PID;
struct hlist_node *old_pid_node = otask->pid_links + PIDTYPE_PID;
struct hlist_node *new_pid_node = ntask->pid_links + PIDTYPE_PID;
struct hlist_node *old_first_node = old_pid_hlist->first;
struct hlist_node *new_first_node = new_pid_hlist->first;
and then trying to group up the first/pprev/thread_pid/pid accesses
so that you them together, and using a helper function that does the
whole switch, so that you'd have
/* Move new node to old hlist, and update thread_pid/pid fields */
insert_pid_pointers(old_pid_hlist, new_pid_node, new_first_node);
rcu_assign_pointer(ntask->thread_pid, opid);
WRITE_ONCE(ntask->pid, pid_nr(opid));
/* Move old new to new hlist, and update thread_pid/pid fields */
insert_pid_pointers(new_pid_hlist, old_pid_node, old_first_node);
rcu_assign_pointer(otask->thread_pid, npid);
WRITE_ONCE(otask->pid, pid_nr(npid));
or something roughly like that.
(And the above still uses "old/new", which as mentioned sounds wrong
to me. Maybe it should just be "a_xyz" and "b_xyz"? Also note that I
did this in my MUA, so I could have gotten the names and types wrong
etc).
I think that would make it look at least _slightly_ less like random
line noise and easier to follow.
But maybe even a rcu_hlist_swap() helper? We have one for regular
lists. Do we really have to do it all written out, not do it with a
"remove and reinsert" model?
Linus
next prev parent reply other threads:[~2020-04-23 20:28 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-19 14:10 [PATCH v12 0/7] proc: modernize proc to support multiple private instances Alexey Gladkov
2020-04-19 14:10 ` [PATCH v12 1/7] proc: rename struct proc_fs_info to proc_fs_opts Alexey Gladkov
2020-04-19 14:10 ` [PATCH v12 2/7] proc: allow to mount many instances of proc in one pid namespace Alexey Gladkov
2020-04-23 11:28 ` [PATCH v13 " Alexey Gladkov
2020-04-23 12:16 ` Eric W. Biederman
2020-04-23 20:01 ` Alexey Gladkov
2020-04-19 14:10 ` [PATCH v12 3/7] proc: instantiate only pids that we can ptrace on 'hidepid=4' mount option Alexey Gladkov
2020-04-19 14:10 ` [PATCH v12 4/7] proc: add option to mount only a pids subset Alexey Gladkov
2020-04-19 14:10 ` [PATCH v12 5/7] docs: proc: add documentation for "hidepid=4" and "subset=pid" options and new mount behavior Alexey Gladkov
2020-04-19 14:10 ` [PATCH v12 6/7] proc: use human-readable values for hidepid Alexey Gladkov
2020-04-19 14:10 ` [PATCH v12 7/7] proc: use named enums for better readability Alexey Gladkov
[not found] ` <87ftcv1nqe.fsf@x220.int.ebiederm.org>
2020-04-23 17:54 ` [PATCH v2 0/2] proc: Calling proc_flush_task exactly once per task Oleg Nesterov
2020-04-23 19:38 ` Eric W. Biederman
2020-04-23 19:39 ` [PATCH v2 1/2] proc: Use PIDTYPE_TGID in next_tgid Eric W. Biederman
2020-04-24 17:29 ` Oleg Nesterov
2020-04-23 19:39 ` [PATCH v2 2/2] proc: Ensure we see the exit of each process tid exactly Eric W. Biederman
2020-04-23 20:28 ` Linus Torvalds [this message]
2020-04-24 3:33 ` Eric W. Biederman
2020-04-24 18:02 ` Linus Torvalds
2020-04-24 18:46 ` Linus Torvalds
2020-04-24 19:51 ` Eric W. Biederman
2020-04-24 20:10 ` Linus Torvalds
2020-04-24 17:39 ` Oleg Nesterov
2020-04-24 18:10 ` Eric W. Biederman
2020-04-24 20:50 ` [PATCH] proc: Put thread_pid in release_task not proc_flush_pid Eric W. Biederman
[not found] ` <87mu6ymkea.fsf_-_@x220.int.ebiederm.org>
[not found] ` <87blnemj5t.fsf_-_@x220.int.ebiederm.org>
2020-04-26 17:22 ` [PATCH v3 2/6] posix-cpu-timers: Use PIDTYPE_TGID to simplify the logic in lookup_task Oleg Nesterov
2020-04-27 11:51 ` Eric W. Biederman
2020-04-28 18:03 ` Oleg Nesterov
2020-04-27 10:32 ` Thomas Gleixner
2020-04-27 19:46 ` Eric W. Biederman
[not found] ` <875zdmmj4y.fsf_-_@x220.int.ebiederm.org>
2020-04-26 17:40 ` [PATCH v3 3/6] rculist: Add hlist_swap_before_rcu Linus Torvalds
2020-04-27 14:28 ` Eric W. Biederman
2020-04-27 20:27 ` Linus Torvalds
2020-04-28 12:16 ` [PATCH v4 0/2] proc: Ensure we see the exit of each process tid exactly Eric W. Biederman
2020-04-28 12:18 ` [PATCH v4 1/2] rculist: Add hlists_swap_heads_rcu Eric W. Biederman
2020-04-28 12:19 ` [PATCH v4 2/2] proc: Ensure we see the exit of each process tid exactly once Eric W. Biederman
2020-04-28 16:53 ` [PATCH v4 0/2] proc: Ensure we see the exit of each process tid exactly Linus Torvalds
2020-04-28 17:55 ` Eric W. Biederman
2020-04-28 18:55 ` Eric W. Biederman
2020-04-28 19:36 ` Linus Torvalds
2020-04-28 18:05 ` Oleg Nesterov
2020-04-28 18:54 ` Eric W. Biederman
[not found] ` <87h7x6mj6h.fsf_-_@x220.int.ebiederm.org>
2020-04-27 9:43 ` [PATCH v3 1/6] posix-cpu-timers: Always call __get_task_for_clock holding rcu_read_lock Thomas Gleixner
2020-04-27 11:53 ` Eric W. Biederman
[not found] ` <87r1w8ete7.fsf@x220.int.ebiederm.org>
2020-04-27 20:23 ` [PATCH v3] proc: Ensure we see the exit of each process tid exactly Eric W. Biederman
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