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From: "Satyam Sharma" <satyam.sharma@gmail.com>
To: "Heiko Carstens" <heiko.carstens@de.ibm.com>
Cc: "Jan Glauber" <jan.glauber@de.ibm.com>,
	"David Miller" <davem@davemloft.net>,
	akpm@osdl.org, mingo@elte.hu, ak@suse.de, schwidefsky@de.ibm.com,
	linux-kernel@vger.kernel.org, "Alan Cox" <alan@redhat.com>,
	"Avi Kivity" <avi@qumranet.com>
Subject: Re: [patch] i386/x86_64: smp_call_function locking inconsistency
Date: Thu, 7 Jun 2007 22:48:10 +0530	[thread overview]
Message-ID: <a781481a0706071018t26e75c84jba5314dde3fc6019@mail.gmail.com> (raw)
In-Reply-To: <a781481a0706070954l636d752am82ef7ca9703788aa@mail.gmail.com>

> On 6/7/07, Heiko Carstens <heiko.carstens@de.ibm.com> wrote:
> [...]
> > Avi Kivity has already a patch which introduces an on_cpu() function which
> > looks quite like on_each_cpu(). That way you don't have to open code this
> > stuff over and over again:
> >
> > preempt_disable();
> > if (cpu == smp_processor_id())
> >         func();
> > else
> >         smp_call_function_single(...);
> > preempt_enable();
> >
> > There are already quite a few of these around.
>
> Indeed -- this was doubly problematic because the un-safeness
> was because of smp_processor_id() as well as the (eventual)
> access of cpu_online_map (via smp_call_function() ->
> num_online_cpus()) ... thanks for letting me know about this.

Oh wait, the on_one_cpu() patch proposes on UP:

+static inline int on_one_cpu(int cpu, void (*func)(void *info), void *info,
+			     int retry, int wait)
+{

/* this needs a if (cpu == 0) check here, IMO */

+	local_irq_disable();
+	func(info);
+	local_irq_enable();
+	return 0;

/* else WARN and return -EINVAL; */

+}

which is broken without the suggested additions, IMHO
(this is what got me into this in the first place). There
_is_ a difference between on_each_cpu() and the
smp_call_function* semantics (as discussed on the other
thread -- gargh! my mistake for opening this discussion up
on so many threads), and in its current form on_one_cpu()
has quite confused semantics, trying to mix the two. I guess
on_one_cpu() would be better off simply being just an
atomic wrapper over smp_processor_id() and
smp_call_function_single() (which is the *real* issue that
needs solving in the first place), and do it well.

Satyam

  reply	other threads:[~2007-06-07 17:19 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2007-02-08 20:32 Heiko Carstens
2007-02-08 20:43 ` David Miller
2007-02-09  8:42   ` Heiko Carstens
2007-02-09 12:57     ` Jan Glauber
2007-06-07 14:07       ` Satyam Sharma
2007-06-07 16:27         ` Heiko Carstens
2007-06-07 16:54           ` Satyam Sharma
2007-06-07 17:18             ` Satyam Sharma [this message]
2007-06-07 17:22               ` Avi Kivity
2007-06-07 17:33                 ` Satyam Sharma
2007-06-10  7:38                   ` Avi Kivity
2007-06-08 19:43             ` Andi Kleen
2007-06-08 19:42         ` Andi Kleen
2007-02-09  7:40 ` Andi Kleen

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