linux-kernel - Re: [for-next][PATCH 2/3] eventfs: Stop using dcache

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Date: Thu, 4 Jan 2024 12:18:06 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, Masami Hiramatsu <mhiramat@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Ajay Kaher <akaher@...are.com>, 
	Al Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>
Subject: Re: [for-next][PATCH 2/3] eventfs: Stop using dcache_readdir() for getdents()

On Thu, 4 Jan 2024 at 12:04, Steven Rostedt <rostedt@...dmis.org> wrote:
>
> Also, I just realized it breaks if we update the 'c--' before the callback. :-/
>
> I have to put this check *after* the callback check.

What? No.

> Reason being, the callback can say "this event doesn't get this file" and
> return 0, which tells eventfs to skip this file.

So yes, there seems to be a bug there, in that ctx->pos is only
updated for successful callbacks (and not for "ignored entry").

But that just means that you should always update 'ctx->pos' as you
'continue' the loop.

The logical place to do that would be in the for-loop itself, which
actually is very natural for the simple case, ie you should just do

        for (i = 0; i < ei->nr_entries; i++, ctx->pos++) {

but in the list_for_each_entry_srcu() case the 'update' part of the
for-loop isn't actually accessible, so it would have to be at the
'continue' point(s).

Which is admittedly a bit annoying.

Looking at that I'm actually surprised that I don't recall that we'd
have hit that issue with our 'for_each_xyz()' loops before.

The update for our "for_each_xyz()" helpers are all hardcoded to just
do the "next iterator" thing, and there's no nice way to take
advantage of the normal for-loop semantics of "do this at the end of
the loop"

            Linus