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