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SubjectRe: [RFC PATCH v11 for 4.15 01/24] Restartable sequences system call
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(apologies for the duplicate email, the previous one bounced as it was accidentally using HTML formatting)

If I understand correctly this is run on every context switch so we probably want to make it really fast

> +static int rseq_need_restart(struct task_struct *t, uint32_t cs_flags)
> +{
> + bool need_restart = false;
> + uint32_t flags;
> +
> + /* Get thread flags. */
> + if (__get_user(flags, &t->rseq->flags))
> + return -EFAULT;
> +
> + /* Take into account critical section flags. */
> + flags |= cs_flags;
> +
> + /*
> + * Restart on signal can only be inhibited when restart on
> + * preempt and restart on migrate are inhibited too. Otherwise,
> + * a preempted signal handler could fail to restart the prior
> + * execution context on sigreturn.
> + */
> + if (flags & RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL) {
> + if (!(flags & RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE))
> + return -EINVAL;
> + if (!(flags & RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT))
> + return -EINVAL;
> + }

How does this error even get to userspace? Is it worth doing this switch on every execution?


> + if (t->rseq_migrate
> + && !(flags & RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE))
> + need_restart = true;
> + else if (t->rseq_preempt
> + && !(flags & RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT))
> + need_restart = true;
> + else if (t->rseq_signal
> + && !(flags & RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL))
> + need_restart = true;

This could potentially be sped up by having the rseq_* fields in t use a single bitmask with the same bit offsets as RSEQ_CS_FLAG_NO_* then using bit operations to check the appropriate overlap.
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