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Subject: Re: [RFC PATCH v11 for 4.15 01/24] Restartable sequences system call
Thread-Topic: [RFC PATCH v11 for 4.15 01/24] Restartable sequences system call
Thread-Index: AQHTXYPhmIqd6HY5PUe1T5SV8sIkp6MUVpZlgAAB7cA=
Date:   Tue, 14 Nov 2017 20:49:54 +0000
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(apologies for the duplicate email, the previous one bounced as it was acci=
dentally using HTML formatting)

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

> +static int rseq_need_restart(struct task_struct *t, uint32_t cs_flags)
> +{
> +       bool need_restart =3D false;
> +       uint32_t flags;
> +
> +       /* Get thread flags. */
> +       if (__get_user(flags, &t->rseq->flags))
> +               return -EFAULT;
> +
> +       /* Take into account critical section flags. */
> +       flags |=3D 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 =3D true;
> +       else if (t->rseq_preempt
> +                       && !(flags & RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT))
> +               need_restart =3D true;
> +       else if (t->rseq_signal
> +                       && !(flags & RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL))
> +               need_restart =3D true;

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

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