Re: [PATCH v7 07/11] proc: flush task dcache entries from all procfs instances

[Linus Torvalds]

On Sat, Jan 25, 2020 at 5:06 AM Alexey Gladkov <gladkov.alexey@xxxxxxxxx> wrote:
>
> This allows to flush dcache entries of a task on multiple procfs mounts
> per pid namespace.

>From a quick read-through, this is the only one I really react negatively to.

The locking looks odd. It only seems to protect the new proc_mounts
list, but then it's a whole big rwsem, and it's taken over all of
proc_flush_task_mnt(), and the locking is exported to all over as a
result of that - including the dummy functions for "there is no proc"
case.

And proc_flush_task_mnt() itself should need no locking over any of
it, so it's all just for the silly looping over the list.

So

 (a) this looks fishy and feels wrong - I get a very strong feeling
that the locking is wrong to begin with, and could/should have been
done differently

 (b) all the locking should have been internal to /proc, and those
wrappers shouldn't exist in a common header file (and certainly not
for the non-proc case).

Yes, (a) is just a feeling, and I don't have any great suggestions.
Maybe make it an RCU list and use a spinlock for updating it?

But (b) is pretty much a non-starter in this form. Those wrappers
shouldn't be in a globally exported core header file. No way.

               Linus