linux-kernel - Re: [PATCH 1/3] Fix: tracepoint: static call: compare data on transition from 2->1 callees

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Message-ID: <1270289445.7118.1628186250992.JavaMail.zimbra@efficios.com>
Date:   Thu, 5 Aug 2021 13:57:30 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     rostedt <rostedt@...dmis.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        paulmck <paulmck@...nel.org>,
        Stefan Metzmacher <metze@...ba.org>,
        stable <stable@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] Fix: tracepoint: static call: compare data on
 transition from 2->1 callees

----- On Aug 5, 2021, at 1:07 PM, rostedt rostedt@...dmis.org wrote:

> On Thu,  5 Aug 2021 09:27:15 -0400
> Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
> 
>> On transition from 2->1 callees, we should be comparing .data rather
>> than .func, because the same callback can be registered twice with
>> different data, and what we care about here is that the data of array
>> element 0 is unchanged to skip rcu sync.
>> 
>> Link:
>> https://lore.kernel.org/io-uring/4ebea8f0-58c9-e571-fd30-0ce4f6f09c70@samba.org/
> 
> FYI, You only need to show one Fixes.
> 
>> Fixes: d25e37d89dd2 ("tracepoint: Optimize using static_call()")
> 
> The above is fixed by the one below. Which means all the stable kernels
> that have the above, will also have the below, and thus the above is
> just redundant.
> 
>> Fixes: 547305a64632 ("tracepoint: Fix out of sync data passing by static
>> caller")
> 
> The above is what the patch actually fixes.
> 
>> Fixes: 352384d5c84e ("tracepoints: Update static_call before tp_funcs when
>> adding a tracepoint")
> 
> How does this patch fix the above? Perhaps the above did not go enough
> to fix the issue, but it's unrelated.

OK

> 
> I'll remove the first and last Fixes tag.

OK

> 
>> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
>> Cc: Steven Rostedt <rostedt@...dmis.org>
>> Cc: Ingo Molnar <mingo@...hat.com>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: "Paul E. McKenney" <paulmck@...nel.org>
>> Cc: Stefan Metzmacher <metze@...ba.org>
>> Cc: <stable@...r.kernel.org> # 5.10+
> 
> The "# 5.10+" is now obsolete, and not needed. The Fixes tag is used to
> determine where this gets backported to.
> 
> Other than that. This patch looks good.

Great, thanks!

Mathieu

> 
> -- Steve
> 
>> ---
>>  kernel/tracepoint.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
>> index fc32821f8240..133b6454b287 100644
>> --- a/kernel/tracepoint.c
>> +++ b/kernel/tracepoint.c
>> @@ -338,7 +338,7 @@ static int tracepoint_remove_func(struct tracepoint *tp,
>>  	} else {
>>  		rcu_assign_pointer(tp->funcs, tp_funcs);
>>  		tracepoint_update_call(tp, tp_funcs,
>> -				       tp_funcs[0].func != old[0].func);
>> +				       tp_funcs[0].data != old[0].data);
>>  	}
>>  	release_probes(old);
> >  	return 0;

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com