linux-kernel - Re: [RFC PATCH v11 for 4.15 01/24] Restartable sequences system call

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Date:   Tue, 14 Nov 2017 20:49:54 +0000
From:   Ben Maurer <bmaurer@...com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>, Andrew Hunter <ahh@...gle.com>,
        Andi Kleen <andi@...stfloor.org>, Chris Lameter <cl@...ux.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC PATCH v11 for 4.15 01/24] Restartable sequences system call

(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.