linux-kernel - Re: [PATCH v6 1/9] kernel: Support TIF_SYSCALL

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Message-ID: <20200924112441.GF2628@hirez.programming.kicks-ass.net>
Date:   Thu, 24 Sep 2020 13:24:41 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Gabriel Krisman Bertazi <krisman@...labora.com>
Cc:     luto@...nel.org, tglx@...utronix.de, keescook@...omium.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, willy@...radead.org,
        linux-kselftest@...r.kernel.org, shuah@...nel.org,
        kernel@...labora.com
Subject: Re: [PATCH v6 1/9] kernel: Support TIF_SYSCALL_INTERCEPT flag

On Fri, Sep 11, 2020 at 04:08:45PM -0400, Gabriel Krisman Bertazi wrote:
> peterz@...radead.org 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.