linux-kernel - Re: [PATCH 1/2] lockref: speculatively spin waiting for the lock to be released

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Date: Thu, 13 Jun 2024 11:56:16 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: Christian Brauner <brauner@...nel.org>, viro@...iv.linux.org.uk, jack@...e.cz, 
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/2] lockref: speculatively spin waiting for the lock to
 be released

On Thu, 13 Jun 2024 at 11:48, Mateusz Guzik <mjguzik@...il.com> wrote:
>
> perhaps lockdep in your config?

Yes, happily it was just lockdep, and the fact that my regular tree
doesn't have debug info, so I looked at my allmodconfig tree.

I didn't *think* anything in the dentry struct should care about
debugging, but clearly that sequence number thing did.

But with that fixed, it's still the case that "we used to know about
this", but what you actually fixed is the case of larger names than 8
bytes.

You did mention the name clashing in your commit message, but I think
that should be the important part in the code comments too: make them
about "these fields are hot and pretty much read-only", "these fields
don't matter" and "this field is hot and written, and shouldn't be
near the read-only ones".

The exact byte counts may change, the basic notion doesn't.

(Of course, putting it at the *end* of the structure then possibly
causes cache conflicts with the next one - we don't force the dentries
be cacheline aligned even if we've tried to make them generally work
that way)

             Linus