linux-kernel - Re: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single ipi fallback for smp_call_function

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]

Message-Id: <1234923702.29823.7.camel@vayu>
Date:	Tue, 17 Feb 2009 18:21:42 -0800
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Oleg Nesterov <oleg@...hat.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Steven Rostedt <rostedt@...dmis.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single
	ipi fallback for smp_call_function_many())

On Tue, 2009-02-17 at 03:26 -0800, Nick Piggin wrote:
> --
> Simplify the barriers in generic remote function call interrupt code.
> 
> Firstly, just unconditionally take the lock and check the list in the
> generic_call_function_single_interrupt IPI handler. As we've just taken
> an IPI here, the chances are fairly high that there will be work on the
> list for us, so do the locking unconditionally. This removes the tricky
> lockless list_empty check and dubious barriers. The change looks bigger
> than it is because it is just removing an outer loop.
> 
> Secondly, clarify architecture specific IPI locking rules. Generic code
> has no tools to impose any sane ordering on IPIs if they go outside
> normal cache coherency, ergo the arch code must make them appear to
> obey cache coherency as a "memory operation" to initiate an IPI, and
> a "memory operation" to receive one. This way at least they can be
> reasoned about in generic code, and smp_mb used to provide ordering. 
> 
> The combination of these two changes means that explict barriers can
> be taken out of queue handling for the single case -- shared data is
> explicitly locked, and ipi ordering must conform to that, so no
> barriers needed. An extra barrier is needed in the many handler, so
> as to ensure we load the list element after the IPI is received.
> 
> Does any architecture actually needs barriers? For the initiator I
> could see it, but for the handler I would be surprised. The other
> thing we could do for simplicity is just to require that a full
> barrier is required before generating an IPI, and after receiving an
> IPI. We can't just do that in generic code without auditing
> architectures. There have been subtle hangs here on some archs in
> the past.

x2apic register reads/writes don't have serializing semantics, as
opposed to uncached xapic accesses, which are inherently serializing.

With this patch, we need to fix the corresponding x2apic IPI operations.
I will take a look at it.

thanks,
suresh

--
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/