Messages in this thread |  | | Date | Wed, 28 Mar 2018 10:19:33 -0400 (EDT) | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH for 4.17 02/21] rseq: Introduce restartable sequences system call (v12) |
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----- 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
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