RE: [patch 1/21] x86, bts: fix race when bts tracer is removed

[Metzger, Markus T]

>-----Original Message-----
>From: Oleg Nesterov [mailto:oleg@xxxxxxxxxx]
>Sent: Wednesday, April 01, 2009 1:48 AM
>To: Metzger, Markus T
>Cc: linux-kernel@xxxxxxxxxxxxxxx; mingo@xxxxxxx; tglx@xxxxxxxxxxxxx; hpa@xxxxxxxxx;
>markus.t.metzger@xxxxxxxxx; roland@xxxxxxxxxx; eranian@xxxxxxxxxxxxxx; Villacis, Juan;
>ak@xxxxxxxxxxxxxxxxxx
>Subject: Re: [patch 1/21] x86, bts: fix race when bts tracer is removed
>
>On 03/31, Markus Metzger wrote:
>>
>> Read the tracer once during a context switch.
>> ...
>> @@ -1044,36 +1051,39 @@ void ds_switch_to(struct task_struct *pr
>>  {
>>  	struct ds_context *prev_ctx = prev->thread.ds_ctx;
>>  	struct ds_context *next_ctx = next->thread.ds_ctx;
>> +	unsigned long debugctlmsr = next->thread.debugctlmsr;
>>
>>  	if (prev_ctx) {
>> +		struct bts_tracer *tracer = prev_ctx->bts_master;
>> +
>>  		update_debugctlmsr(0);
>>
>> -		if (prev_ctx->bts_master &&
>> -		    (prev_ctx->bts_master->trace.ds.flags & BTS_TIMESTAMPS)) {
>> +		if (tracer && (tracer->flags & BTS_TIMESTAMPS)) {
>
>In theory, we need barrier() after reading ->bts_master.
>
>(actually, I did see the bug reports when the compiler read the pointer
> twice with the code like above).

I guess the same is true for prev_ctx, next_ctx, and debugctlmsr, then.

Ingo,
would it be OK to resend this one patch with the barrier()s added?


>Off-topic, but afaics modulo bts_task_departs/bts_task_arrives we
>have the identical code for prev_ctx/next_ctx, perhaps it makes
>sense to make a helper which calls bts_write().

Agreed.

>To clarify, even _if_ I am right and _if_ you agree, we can do this
>later, I am not suggesting to change this patch right now.

Agreed, as well.


thanks and regards,
markus.
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