linux-kernel - Re: [PATCH 3/5] fs: sys

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Date: Sun, 23 Jun 2024 18:21:36 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Jens Axboe <axboe@...nel.dk>, brauner@...nel.org, viro@...iv.linux.org.uk, 
	Bernd Schubert <bernd.schubert@...tmail.fm>, linux-mm@...ck.org, Josef Bacik <josef@...icpanda.com>
Subject: Re: [PATCH 3/5] fs: sys_ringbuffer

On Mon, Jun 24, 2024 at 12:13:36AM +0200, Thomas Gleixner wrote:
> Kent!
> 
> On Sun, Jun 02 2024 at 20:33, Kent Overstreet wrote:
> > +/*
> > + * ringbuffer_ptrs - head and tail pointers for a ringbuffer, mappped to
> > + * userspace:
> > + */
> > +struct ringbuffer_ptrs {
> 
> The naming is confusing. ringbuffer_ctrl or something like that would be
> more clear because it's more than just the pointers, which are in fact
> positions. You have size, mask ... too, no?

I like that name, yeah.

> > +	/*
> > +	 * We use u32s because this type is shared between the kernel and
> > +	 * userspace - ulong/size_t won't work here, we might be 32bit userland
> > +	 * and 64 bit kernel, and u64 would be preferable (reduced probability
> > +	 * of ABA) but not all architectures can atomically read/write to a u64;
> > +	 * we need to avoid torn reads/writes.
> 
> union rbmagic {
> 	u64	__val64;
>         struct {
>                 // TOOTIRED: Add big/little endian voodoo
> 	        u32	__val32;
>                 u32	__unused;
>         };
> };
> 
> Plus a bunch of accessors which depend on BITS_PER_LONG, no?

Not sure I follow?

I know biendian machines exist, but I've never heard of both big and
little endian being used at the same time. Nor why we'd care about
BITS_PER_LONG? This just uses fixed size integer types.