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Date:   Thu, 24 Sep 2020 13:24:41 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Gabriel Krisman Bertazi <krisman@collabora.com>
Cc:     luto@kernel.org, tglx@linutronix.de, keescook@chromium.org,
        x86@kernel.org, linux-kernel@vger.kernel.org,
        linux-api@vger.kernel.org, willy@infradead.org,
        linux-kselftest@vger.kernel.org, shuah@kernel.org,
        kernel@collabora.com
Subject: Re: [PATCH v6 1/9] kernel: Support TIF_SYSCALL_INTERCEPT flag
Message-ID: <20200924112441.GF2628@hirez.programming.kicks-ass.net>
References: <20200904203147.2908430-1-krisman@collabora.com>
 <20200904203147.2908430-2-krisman@collabora.com>
 <20200911093221.GD1362448@hirez.programming.kicks-ass.net>
 <878sdgnkj6.fsf@collabora.com>
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On Fri, Sep 11, 2020 at 04:08:45PM -0400, Gabriel Krisman Bertazi wrote:
> peterz@infradead.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.