From: Peter Zijlstra <peterz@infradead.org>
To: Malte Skarupke <malteskarupke@web.de>
Cc: tglx@linutronix.de, mingo@redhat.com, dvhart@infradead.org,
linux-kernel@vger.kernel.org, malteskarupke@fastmail.fm
Subject: Re: [PATCH] futex: Support smaller futexes of one byte or two byte size.
Date: Fri, 6 Dec 2019 18:37:05 +0100 [thread overview]
Message-ID: <20191206173705.GE2871@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20191206153129.GI2844@hirez.programming.kicks-ass.net>
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.
next prev parent reply other threads:[~2019-12-06 17:37 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-04 23:52 [PATCH] futex: Support smaller futexes of one byte or two byte size Malte Skarupke
2019-12-06 15:31 ` Peter Zijlstra
2019-12-06 17:37 ` Peter Zijlstra [this message]
2019-12-08 22:18 ` Malte Skarupke
2019-12-11 13:44 ` Peter Zijlstra
2019-12-11 19:48 ` Malte Skarupke
2019-12-20 19:08 ` Malte Skarupke
2019-12-20 19:09 ` Malte Skarupke
2019-12-20 19:09 ` Malte Skarupke
2019-12-21 15:56 ` Malte Skarupke
2019-12-21 15:56 ` Malte Skarupke
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20191206173705.GE2871@hirez.programming.kicks-ass.net \
--to=peterz@infradead.org \
--cc=dvhart@infradead.org \
--cc=linux-kernel@vger.kernel.org \
--cc=malteskarupke@fastmail.fm \
--cc=malteskarupke@web.de \
--cc=mingo@redhat.com \
--cc=tglx@linutronix.de \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).