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Re: [PATCH -tip v3&10 07/18] x86: Add text_poke_smp for SMP cross modifying code
- From: Masami Hiramatsu <mhiramat at redhat dot com>
- To: Mathieu Desnoyers <mathieu dot desnoyers at efficios dot com>
- Cc: Ingo Molnar <mingo at elte dot hu>, Frederic Weisbecker <fweisbec at gmail dot com>, Ananth N Mavinakayanahalli <ananth at in dot ibm dot com>, lkml <linux-kernel at vger dot kernel dot org>, systemtap <systemtap at sources dot redhat dot com>, DLE <dle-develop at lists dot sourceforge dot net>, Jim Keniston <jkenisto at us dot ibm dot com>, Srikar Dronamraju <srikar at linux dot vnet dot ibm dot com>, Christoph Hellwig <hch at infradead dot org>, Steven Rostedt <rostedt at goodmis dot org>, "H. Peter Anvin" <hpa at zytor dot com>, Anders Kaseorg <andersk at ksplice dot com>, Tim Abbott <tabbott at ksplice dot com>, Andi Kleen <andi at firstfloor dot org>, Jason Baron <jbaron at redhat dot com>
- Date: Thu, 25 Feb 2010 22:53:16 -0500
- Subject: Re: [PATCH -tip v3&10 07/18] x86: Add text_poke_smp for SMP cross modifying code
- References: <20100225133342.6725.26971.stgit@localhost6.localdomain6> <20100225133438.6725.80273.stgit@localhost6.localdomain6> <20100225153305.GC12635@Krystal>
Mathieu Desnoyers wrote:
> * Masami Hiramatsu (mhiramat@redhat.com) wrote:
[...]
>> +
>> +/*
>> + * Cross-modifying kernel text with stop_machine().
>> + * This code originally comes from immediate value.
>> + */
>> +static atomic_t stop_machine_first;
>> +static int wrote_text;
>> +
>> +struct text_poke_params {
>> + void *addr;
>> + const void *opcode;
>> + size_t len;
>> +};
>> +
>> +static int __kprobes stop_machine_text_poke(void *data)
>> +{
>> + struct text_poke_params *tpp = data;
>> +
>> + if (atomic_dec_and_test(&stop_machine_first)) {
>> + text_poke(tpp->addr, tpp->opcode, tpp->len);
>> + smp_wmb(); /* Make sure other cpus see that this has run */
>> + wrote_text = 1;
>> + } else {
>> + while (!wrote_text)
>> + smp_rmb();
>> + sync_core();
>
> Hrm, there is a problem in there. The last loop, when wrote_text becomes
> true, does not perform any smp_mb(), so you end up in a situation where
> cpus in the "else" branch may never issue any memory barrier. I'd rather
> do:
Hmm, so how about this? :)
---
} else {
do {
smp_rmb();
while (!wrote_text);
sync_core();
}
---
>
> +static volatile int wrote_text;
>
> ...
>
> +static int __kprobes stop_machine_text_poke(void *data)
> +{
> + struct text_poke_params *tpp = data;
> +
> + if (atomic_dec_and_test(&stop_machine_first)) {
> + text_poke(tpp->addr, tpp->opcode, tpp->len);
> + smp_wmb(); /* order text_poke stores before store to wrote_text */
> + wrote_text = 1;
> + } else {
> + while (!wrote_text)
> + cpu_relax();
> + smp_mb(); /* order wrote_text load before following execution */
> + }
>
> If you don't like the "volatile int" definition of wrote_text, then we
> should probably use the ACCESS_ONCE() macro instead.
hm, yeah, volatile will be required.
Thank you,
--
Masami Hiramatsu
e-mail: mhiramat@redhat.com