linux-kernel - Re: [PATCH] futex: Support smaller futexes of one byte or two byte size.

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Message-ID: <20191206173705.GE2871@hirez.programming.kicks-ass.net>
Date:   Fri, 6 Dec 2019 18:37:05 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Malte Skarupke <malteskarupke@....de>
Cc:     tglx@...utronix.de, mingo@...hat.com, dvhart@...radead.org,
        linux-kernel@...r.kernel.org, malteskarupke@...tmail.fm
Subject: Re: [PATCH] futex: Support smaller futexes of one byte or two byte
 size.

On Fri, Dec 06, 2019 at 04:31:29PM +0100, Peter Zijlstra wrote:
> > +		case FUTEX_WAKE:
> > +		case FUTEX_REQUEUE:
> > +			/*
> > +			 * these instructions work with sized mutexes, but you
> > +			 * don't need to pass the size. we could silently
> > +			 * ignore the size argument, but the code won't verify
> > +			 * that the correct size is used, so it's preferable
> > +			 * to make that clear to the caller.
> > +			 *
> > +			 * for requeue the meaning would also be ambiguous: do
> > +			 * both of them have to be the same size or not? they
> > +			 * don't, and that's clearer when you just don't pass
> > +			 * a size argument.
> > +			 */
> > +			return -EINVAL;
> 
> Took me a while to figure out this relies on FUTEX_NONE to avoid the
> alignment tests.

And thikning more on that, I really _realy_ hate this.

You're basically saying WAKE is size-less, but that means we must
consider what it means to have a u32 WAIT on @ptr, and a u8 WAKE on
@ptr+1. If the wake really is size-less that should match.

I'd be much happier with requiring strict sizing. Because conversely,
what happens if you have a u32-WAIT at @ptr paired with a u8-WAKE at
@ptr? If we demand strict size we can say that should not match. This
does however mean we should include the size in the hash-match function.

Your Changelog did not consider these implications at all.