linux-kernel - Re: [PATCH 0/9 RFC] Make wake_up

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Message-ID: <CAHk-=whxS9qM36w5jmf-F32LSC=+m3opufAdgfOBCoTDaS1_Ag@mail.gmail.com>
Date: Tue, 20 Aug 2024 14:58:05 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: NeilBrown <neilb@...e.de>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 0/9 RFC] Make wake_up_{bit,var} less fragile

On Tue, 20 Aug 2024 at 14:47, NeilBrown <neilb@...e.de> wrote:
>
> I can definitely get behind the idea has having a few more helpers and
> using them more widely.  But unless we get rid of wake_up_bit(), people
> will still use and some will use it wrongly.

I do not believe this is a valid argument.

"We have interfaces that somebody can use wrongly" is a fact of life,
not an argument.

The whole "wake_up_bit()" is a very special thing, and dammit, if
people don't know the rules, then they shouldn't be using it.

Anybody using that interface *ALREADY* has to have some model of
atomicity for the actual bit they are changing. And yes, they can get
that wrong too.

The only way to actually make it a simple interface is to do the bit
operation and the wakeup together. Which is why I think that
interfaces like clear_bit_and_wake() or set_bit_and_wake() are fine,
because at that point you actually have a valid rule for the whole
operation.

But wake_up_bit() on its own ALREADY depends on the user doing the
right thing for the bit itself. Putting a memory barrier in it will
only *HIDE* incompetence, it won't be fixing it.

So no. Don't add interfaces that hide the problem.

                  Linus