Andi Kleen wrote:
>> I could just force all of the seqbegins to hit the slowpath by hacking
>> the code and see what happens (aside from slowing down, of course ;)
>>
>
> Only if you don't believe it will really crash? I think it's pretty
> clear even without trying it.
>
Well, I guess it was just to prove to myself that I broke something
because I dont understand how the vsyscall interface works. But given
your expertise here, I have no problem with just taking your word for it.
>
>> Question: Which seqlock_t does userspace use? I assume it uses
>> seqlock_t and not raw_seqlock_t.
>>
>
>
>> But the only reason that I ask is that
>> I converted raw_seqlock_t to use the new style as well to be consistent,
>>
>
> There's no raw_seqlock_t anywhere in mainline?
>
Yeah, understood. There is both in -rt and I was just saying that we
technically only need the seqlock_t fix in -rt. So if raw_seqlock_t
could be left pristine and solve this problem, that is an acceptable
compromise to me.
> Anyways the variable is declared (in mainline) in asm-x86/vgtod.h
>
>
>> even though it is not strictly necessary for the same reasons. So if
>> perchance userspace uses the raw variant, I could solve this issue by
>> only re-working the seqlock_t variant. Kind of a long shot, but figured
>> I would mention it :)
>>
>
> I guess you could define a new seqlock_t which is explicitely user space
> safe. That might avoid such issues in the future. But then
> that would likely require some code duplication and be ugly.
>
> On the other hand whatever problem you fixing in the kernel
> (to be honest it's still unclear to me what the problem is)
> needs to be likely fixed for the userland lock too.
>
Yeah, it would possibly be a problem in both cases.
The problem I am addressing only exists in -rt since it has seqlock_t
and raw_seqlock_t (with the former using preemptible spinlock_t's).
Since the underlying seqlock_t->spinlock_t is preemptible, you can see
that one thread that does:
{
write_seqlock();
/* asl */
write_sequnlock();
}
while other high-prio threads do
do { read_seqbegin(); /* asl */; } while (read_seqretry());
The readers could preempt the writer mid critical section and enter a
live-locked loop.
raw_seqlock_t (which is equivalent to a mainline seqlock_t) do not have
this problem because the spinlock acquisition inside the write_seqlock
disables preemption.
HTH
-Greg
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