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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: <20180328145946.GH4082@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1523662633.2105.1522248474778.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 10:47:54AM -0400, Mathieu Desnoyers wrote: > ----- On Mar 28, 2018, at 8:50 AM, Peter Zijlstra peterz@infradead.org wrote: > > > On Tue, Mar 27, 2018 at 12:05:23PM -0400, Mathieu Desnoyers wrote: > >> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > >> index fb5fc458547f..66b070444a7e 100644 > >> --- a/kernel/sched/sched.h > >> +++ b/kernel/sched/sched.h > >> @@ -1249,6 +1249,7 @@ static inline void __set_task_cpu(struct task_struct *p, > >> unsigned int cpu) > >> #endif > >> p->wake_cpu = cpu; > >> #endif > >> + rseq_migrate(p); > >> } > > > > I think you want that in set_task_cpu(), right next to nr_migrations++. > > This would miss the __set_task_cpu() call from sched_fork() and wake_up_new_task(). Correct; but since those are _new_ tasks they _SHOULD_ not have an active RSEQ to begin with. > Those cases are not accounted as explicit "migrations", but it does change the CPU > of the current task. So if for some weird reason userspace wants to fork() while in > a rseq critical section, we want to trigger a rseq restart. If at all possible I would make it SIGSEGV when issueing SYSCALL()s from within an RSEQ. > An alternative to this would be to call rseq_migrate() in rseq_fork(). > > Thoughts ? Yes, don't try and support that at all. It's _insane_.