Date:	Sat, 21 Nov 2009 10:32:53 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Masami Hiramatsu <mhiramat@...hat.com>
Cc:	Jason Baron <jbaron@...hat.com>, mingo@...e.hu,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, hpa@...or.com, tglx@...utronix.de,
	rostedt@...dmis.org, andi@...stfloor.org, roland@...hat.com,
	rth@...hat.com
Subject: Re: [RFC PATCH 2/6] jump label v3 - x86: Introduce generic jump
	patching without stop_machine

* Masami Hiramatsu (mhiramat@...hat.com) wrote:
> Mathieu Desnoyers wrote:
> [...]
>>>>> +	if (unlikely(len<= 1))
>>>>> +		return text_poke(addr, opcode, len);
>>>>> +
>>>>> +	/* Preparing */
>>>>> +	patch_fixup_addr = fixup;
>>>>> +	wmb();
>>>>
>>>> hrm, missing comment ?
>>>
>>> Ah, it's a barrier between patch_fixup_addr and patch_fixup_from.
>>> int3 trap handler checks patch_fixup_from first and refers patch_fixup_addr.
>>
>> When a smp_wmb() is probably enough, and the matching smp_rmb() is
>> missing in the int3 handler.
>
> OK, thank you.
>
>> But why to you care about the order of these two ? I agree that an
>> unrelated int3 handler (from kprobes ?) could be running concurrently at
>> that point, but it clearly cannot be called for this specific address
>> until the int3 is written by text_poke.
>
> Ah, it's my fault. I fixed that a month ago, and forgot to push it...
> Actually, we don't need to care the order of those two. Instead,
> we have to update the patch_fixup_* before int3 embedding.
>
>>
>> What I am pretty much certain is missing would be a smp_wmb()...
>
> Agreed.
>
>>
>>>
>>>>
>>>>> +	patch_fixup_from = (u8 *)addr + int3_size; /* IP address after int3 */
>>
>> ..right here, between where you write to the data used by the int3
>> handler and where you write the actual breakpoint. On the read-side,
>> this might be a problem with architectures like alpha needing
>> smp_read_barrier_depends(), but not for Intel. However, in a spirit to
>> make this code solid, what I did in the immed. val. is:
>>
>>
>>                  target_after_int3 = insn + BREAKPOINT_INS_LEN;
>>                  /* register_die_notifier has memory barriers */
>>                  register_die_notifier(&imv_notify);
>>                  /* The breakpoint will single-step the bypass */
>>                  text_poke((void *)insn,
>>                          ((unsigned char[]){BREAKPOINT_INSTRUCTION}), 1);
>
> Hmm, it strongly depends on arch. Is smp_wmb() right after setting
> patch_fixup_from enough on x86?

What else do you have in mind ? wmb() ? Or adding a
smp_read_barrier_depends() at the beginnig of the handler ?

Clearly, smp_read_barrier_depends() is a no-op on x86, but it might be
good to add it just for code clarity (it helps commenting which ordering
has to be done on the read-side).


>
>> And I unregister the die notifier at the end after having reached
>> quiescent state. At least we know that the die notifier chain read-side
>> has the proper memory barriers, which is not the case for the breakpoint
>> instruction itself.
>>
>>
>>>>> +
>>>>> +	/* Cap by an int3 */
>>>>> +	text_poke(addr,&int3_insn, int3_size);
>>>>> +	sync_core_all();
>>>>> +
>>>>> +	/* Replace tail bytes */
>>>>> +	text_poke((char *)addr + int3_size, (const char *)opcode + int3_size,
>>>>> +		  len - int3_size);
>>>>> +	sync_core_all();
>>>>> +
>>>>> +	/* Replace int3 with head byte */
>>>>> +	text_poke(addr, opcode, int3_size);
>>>>> +	sync_core_all();
>>>>> +
>>>>> +	/* Cleanup */
>>>>> +	patch_fixup_from = NULL;
>>>>> +	wmb();
>>>>
>>>> missing comment here too.
>>>>
>>>>> +	return addr;
>>>>
>>>> Little quiz question:
>>>>
>>>> When patch_fixup_from is set to NULL, what ensures that the int3
>>>> handlers have completed their execution ?
>>>>
>>>> I think it's probably OK, because the int3 is an interrupt gate, which
>>>> therefore disables interrupts as soon as it runs, and executes the
>>>> notifier while irqs are off. When we run sync_core_all() after replacing
>>>> the int3 by the new 1st byte, we only return when all other cores have
>>>> executed an interrupt, which implies that all int3 handlers previously
>>>> running should have ended. Is it right ? It looks to me as if this 3rd
>>>> sync_core_all() is only needed because of that. Probably that adding a
>>>> comment would be good.
>>>
>>> Thanks, it's a good point and that's more what I've thought.
>>> As you said, it is probably safe. Even if it's not safe,
>>> we can add some int3 fixup handler (with lowest priority)
>>> which set regs->ip-1 if there is no int3 anymore, for safety.
>>
>> Well, just ensuring that the we reaches a "disabled IRQ code quiescent
>> state" should be enough. Another way would be to use
>> synchronize_sched(), but it might take longer. Actively poking the other
>> CPUs with IPIs seems quicker. So I would be tempted to leave your code
>> as is in this respect, but to add a comment.
>
> Agreed. synchronize_sched() waits too long for this purpose.
> OK, I'll add a comment for that "waiting for disabled IRQ code
> quiescent state" :-) Thanks for the good advice!
>
>>>> Another thing: I've recently noticed that the following locking seems to
>>>> hang the system with doing stress-testing concurrently with cpu
>>>> hotplug/hotunplug:
>>>>
>>>> mutex_lock(&text_mutex);
>>>>    on_each_cpu(something, NULL, 1);
>>>>
>>>> The hang seems to be caused by the fact that alternative.c has:
>>>>
>>>> within cpu hotplug (cpu hotplug lock held)
>>>>    mutex_lock(&text_mutex);
>>>>
>>>> It might also be caused by the interaction with the stop_machine()
>>>> performed within the cpu hotplug lock. I did not find the root cause of
>>>> the problem, but this probably calls for lockdep improvements.
>>>
>>> Hmm, would you mean it will happen even if we use stop_machine()
>>> under text_mutex locking?
>>> It seems that bigger problem of cpu-hotplug and on_each_cpu() etc.
>>
>> Yes, but, again.. this calls for more testing. Hopefully it's not
>> something else in my own code I haven't seen. For not I can just say
>> that I've been noticing hangs involving cpu hotplug and text mutex, and
>> taking the cpu hotplug mutex around text mutex (in my immediate values
>> code) fixed the problem.
>
> Hmm, I guess that we'd better merge those two mutexes since
> text modification always requires disabling cpu-hotplug...

Maybe.. although it's not clear to me that CPU hotplug is required to be
disabled around on_each_cpu calls.

Mathieu

>
> Thank you,
>
>
> -- 
> Masami Hiramatsu
>
> Software Engineer
> Hitachi Computer Products (America), Inc.
> Software Solutions Division
>
> e-mail: mhiramat@...hat.com
>

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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