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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 Message-ID: <1602066458.2029.1522246773927.JavaMail.zimbra@efficios.com> In-Reply-To: <20180328111952.GS4043@hirez.programming.kicks-ass.net> References: <20180327160542.28457-1-mathieu.desnoyers@efficios.com> <20180327160542.28457-3-mathieu.desnoyers@efficios.com> <20180328111952.GS4043@hirez.programming.kicks-ass.net> Subject: Re: [RFC PATCH for 4.17 02/21] rseq: Introduce restartable sequences system call (v12) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Originating-IP: [167.114.142.138] X-Mailer: Zimbra 8.8.7_GA_1964 (ZimbraWebClient - FF52 (Linux)/8.8.7_GA_1964) Thread-Topic: rseq: Introduce restartable sequences system call (v12) Thread-Index: oZa9XxD5Ql08V10mQq0a0I1RlslAQw== 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 Mar 28, 2018, at 7:19 AM, Peter Zijlstra peterz@infradead.org wrote: > On Tue, Mar 27, 2018 at 12:05:23PM -0400, Mathieu Desnoyers wrote: >> +#ifdef CONFIG_RSEQ >> + struct rseq __user *rseq; >> + u32 rseq_len; >> + u32 rseq_sig; >> + /* >> + * RmW on rseq_event_mask must be performed atomically >> + * with respect to preemption. >> + */ >> + unsigned long rseq_event_mask; >> +#endif > >> +static inline void rseq_signal_deliver(struct pt_regs *regs) >> +{ >> + set_bit(RSEQ_EVENT_SIGNAL_BIT, ¤t->rseq_event_mask); >> + rseq_handle_notify_resume(regs); >> +} >> + >> +static inline void rseq_preempt(struct task_struct *t) >> +{ >> + set_bit(RSEQ_EVENT_PREEMPT_BIT, &t->rseq_event_mask); >> + rseq_set_notify_resume(t); >> +} >> + >> +static inline void rseq_migrate(struct task_struct *t) >> +{ >> + set_bit(RSEQ_EVENT_MIGRATE_BIT, &t->rseq_event_mask); >> + rseq_set_notify_resume(t); >> +} > > Given that comment above, do you really need the full atomic set bit? > Isn't __set_bit() sufficient? For each of rseq_signal_deliver, rseq_preempt, and rseq_migrate, we should confirm that their callers guarantee preemption is disabled before we can use __set_bit() in each of those functions. Is that the case ? If so, we should also document the requirement about preemption for each function. AFAIU, rseq_migrate is only invoked from __set_task_cpu, which I *think* always has preemption disabled. rseq_preempt() is called by the scheduler, so this one is fine. On x86, rseq_signal_deliver is called from setup_rt_frame, with preemption enabled. So one approach would be to use __set_bit in both rseq_preempt and rseq_migrate, but keep the atomic set_bit() in rseq_signal_deliver. Thoughts ? Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com