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McKenney" , Boqun Feng , Andy Lutomirski , Dave Watson , linux-kernel , linux-api , Paul Turner , Andrew Morton , Russell King , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Andrew Hunter , Andi Kleen , Chris Lameter , Ben Maurer , rostedt , Josh Triplett , Linus Torvalds , Catalin Marinas , Will Deacon , Michael Kerrisk , Alexander Viro Subject: Re: [RFC PATCH for 4.17 02/21] rseq: Introduce restartable sequences system call (v12) Message-ID: <20180328174935.GK4082@hirez.programming.kicks-ass.net> References: <20180327160542.28457-1-mathieu.desnoyers@efficios.com> <20180327160542.28457-3-mathieu.desnoyers@efficios.com> <20180328125004.GV4043@hirez.programming.kicks-ass.net> <1523662633.2105.1522248474778.JavaMail.zimbra@efficios.com> <20180328145946.GH4082@hirez.programming.kicks-ass.net> <265889560.1.1522250045589.JavaMail.zimbra@efficios.com> <20180328152814.GI4082@hirez.programming.kicks-ass.net> <533214853.56.1522251426819.JavaMail.zimbra@efficios.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <533214853.56.1522251426819.JavaMail.zimbra@efficios.com> User-Agent: Mutt/1.9.3 (2018-01-21) Sender: linux-api-owner@vger.kernel.org X-Mailing-List: linux-api@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-Mailing-List: linux-kernel@vger.kernel.org List-ID: On Wed, Mar 28, 2018 at 11:37:06AM -0400, Mathieu Desnoyers wrote: > ----- On Mar 28, 2018, at 11:28 AM, Peter Zijlstra peterz@infradead.org wrote: > > > On Wed, Mar 28, 2018 at 11:14:05AM -0400, Mathieu Desnoyers wrote: > > > >> > If at all possible I would make it SIGSEGV when issueing SYSCALL()s from > >> > within an RSEQ. > >> > >> What's the goal there ? rseq critical sections can technically do system calls > >> if they wish. Why prevent this ? > > > > This all started as a way to do 'small' _fast_ per-cpu ops, System calls > > do NOT fit in that pattern. If you're willing to do a system calls the > > cost of atomics is not a problem. > > I'm not arguing that a typical rseq would do a system call. I'm merely > pointing out that if we start putting arbitrary limitations like "SIGSEGV > when a fork or system call is encountered on top of rseq", this will cause > pain in user-space. I don't think disallowing system calls is arbitrary. And I think that is something we really want to enforce, because it's batshit insane to allow. And if we allow now, people _will_ use it and we can't ever take it away again.