Re: syscall_regfunc() && TIF_SYSCALL_TRACEPOINT

From: Oleg Nesterov
Date: Sat Mar 31 2012 - 16:45:12 EST

On 03/30, Steven Rostedt wrote:
> On Fri, 2012-03-30 at 22:15 +0200, Oleg Nesterov wrote:
> > But I don't really understand why do you think that "clear" is more
> > important.
> They are both important. But as I tend to consider performance when
> tracing is off as critical, I'm more concerned about that. But both must
> be fixed, because not reporting traces can confuse a developer.

Ah, got it, thanks.

I was going to send the simple patch we discussed, but suddenly I
realized that I have another question.

Why do we want to filter out the kernel threads in syscall_regfunc?

>From cc3b13c1 "tracing: Don't trace kernel thread syscalls"

then it has no effect to trace the kernel thread calls
to syscalls in that path.
Setting the TIF_SYSCALL_TRACEPOINT flag is then useless for these.

OK, but then it doesn't hurt? Or is there another reason why
TIF_SYSCALL_TRACEPOINT is not desirable on kthread?

The problem is ____call_usermodehelper() which execs the user-space
task. This clears PF_KTHREAD (sets ->mm), but obviously if
sys_tracepoint_refcount != 0 this is too late.

So what do you think we should do,

- keep this check

- remove it

- remove it in a separate patch

- add the "sync with sys_tracepoint_refcount" hook
before kernel_execve()



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