Re: [PATCH v6 1/9] kernel: Support TIF_SYSCALL_INTERCEPT flag

[Peter Zijlstra]

On Fri, Sep 11, 2020 at 04:08:45PM -0400, Gabriel Krisman Bertazi wrote:
> peterz@xxxxxxxxxxxxx writes:
> 
> > On Fri, Sep 04, 2020 at 04:31:39PM -0400, Gabriel Krisman Bertazi wrote:
> >> +static inline void __set_tsk_syscall_intercept(struct task_struct *tsk,
> >> +					   unsigned int type)
> >> +{
> >> +	tsk->syscall_intercept |= type;
> >> +
> >> +	if (tsk->syscall_intercept)
> >> +		set_tsk_thread_flag(tsk, TIF_SYSCALL_INTERCEPT);
> >> +}
> >
> > Did the above want to be:
> >
> > 	unsigned int old = tsk->syscall_intercept;
> > 	tsk->syscall_intercept |= type;
> > 	if (!old)
> > 		set_tsk_thread_flag(tsk, TIF_SYSCALL_INTERCEPT)
> >
> 
> Hi Peter,
> 
> Thanks for the review!
> 
> I'm not sure this change gains us anything.  For now,
> __set_tsk_syscall_intercept cannot be called with !type, so both
> versions behave the same, but my version is safe with that scenario.
> This won't be called frequent enough for the extra calls to
> set_tsk_thread_flag matter.  Am I missing something?

Your version will do set_tsk_thread_flag() for every invocation
(assuming non-zero type). That's sub-optimal.