linux-kernel - Re: [PATCH 3/4] x86,asm: Re-work smp_store

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Tue, 27 Oct 2015 15:01:55 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 3/4] x86,asm: Re-work smp_store_mb()

On Wed, 28 Oct 2015, Linus Torvalds wrote:

>On Wed, Oct 28, 2015 at 4:53 AM, Davidlohr Bueso <dave@...olabs.net> wrote:
>>
>> Note that this might affect callers that could/would rely on the
>> atomicity semantics, but there are no guarantees of that for
>> smp_store_mb() mentioned anywhere, plus most archs use this anyway.
>> Thus we continue to be consistent with the memory-barriers.txt file,
>> and more importantly, maintain the semantics of the smp_ nature.
>
>So I dislike this patch, mostly because it now makes it obvious that
>smp_store_mb() seems to be totally pointless. Every single
>implementation is now apparently WRITE_ONCE+smp_mb(), and there are
>what, five users of it, so why not then open-code it?

So after having gone through pretty much all of smp_store_mb code, this
is a feeling I also share. However I justified its existence (as opposed
to dropping the call, updating all the callers/documenting the barriers
etc.) to at least encapsulate the store+mb logic, which apparently is a
pattern somewhat needed(?). Also, the name is obviously exactly what its
name implies.

But I have no strong preference either way. Now, if we should keep
smp_store_mb(), it should probably be made generic, instead of having
each arch define it.

>
>But more importantly, is the "WRITE_ONCE()" even necessary? If there
>are no atomicity guarantees, then why bother with WRTE_ONCE() either?

Agreed. Hmm, this was introduced by ab3f02fc237 (locking/arch: Add WRITE_ONCE()
to set_mb()), back when atomicity aspects were not clear yet.

>So with this patch, the whole thing becomes pointless, I feel. (Ok, so
>it may have been pointless before too, but at least before this patch
>it generated special code, now it doesn't). So why carry it along at
>all?

Ok, unless others are strongly against it, I'll send a series to drop the
call altogether.

Thanks,
Davidlohr
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/