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Date:   Tue, 14 Nov 2017 21:03:24 +0000 (UTC)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Ben Maurer <bmaurer@fb.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
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        Dave Watson <davejwatson@fb.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-api <linux-api@vger.kernel.org>,
        Paul Turner <pjt@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Russell King <linux@arm.linux.org.uk>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
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        Andi Kleen <andi@firstfloor.org>, Chris Lameter <cl@linux.com>,
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        Josh Triplett <josh@joshtriplett.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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        Alexander Viro <viro@zeniv.linux.org.uk>
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Subject: Re: [RFC PATCH v11 for 4.15 01/24] Restartable sequences system
 call
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Thread-Topic: Restartable sequences system call
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----- On Nov 14, 2017, at 3:49 PM, Ben Maurer bmaurer@fb.com wrote:

> (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

Yes, more precisely, it runs on return to user-space, after every context
switch going back to a registered rseq thread.

> 
>> +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 we detect this situation, the rseq_need_restart caller will end up
sending a SIGSEGV signal to user-space. Note that the two nested if()
checks are only executing in the unlikely case where the NO_RESTART_ON_SIGNAL
flag is set.

> 
> 
>> +       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.

Given that those are not requests impacting the ABI presented to user-space,
I'm tempted to keep these optimizations for the following 4.16 merge window.
Is that ok with you ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

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