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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- 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