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

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Message-ID: <20240104153304.79e607c8@gandalf.local.home>
Date: Thu, 4 Jan 2024 15:33:04 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Linus Torvalds <torvalds@...ux-foundation.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 12:18:06 -0800
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> 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").

OK, I wasn't sure if it was OK to update the ctx->pos for something we
didn't add, so I avoided doing so.

> 
> 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++) {

Well, we don't want to do that and c-- at the same time. But of course, if
we do the shortcut, we can have:

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

which would be OK. And better if we move it before the ei->children list walk.

> 
> 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.

But not really an issue as we just have:

	list_for_each_entry_srcu(ei_child, &ei->children, list,
				 srcu_read_lock_held(&eventfs_srcu)) {

		if (c > 0) {
			c--;
			continue;
		}

		ctx->pos++;

> 
> 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"

Anyway, if I do count ctx->pos++ for every iteration, whether it added
something or not, it appears to work. I'll write up a couple of patches to
handle this.

Thanks,

-- Steve