Re: [RESEND RFC PATCH 0/3] Provide fast access to thread specific data

From: Mathieu Desnoyers
Date: Fri Sep 10 2021 - 13:33:28 EST

----- On Sep 10, 2021, at 12:37 PM, Florian Weimer fweimer@xxxxxxxxxx wrote:

> * Peter Oskolkov:
>> In short, due to the need to read/write to the userspace from
>> non-sleepable contexts in the kernel it seems that we need to have some
>> form of per task/thread kernel/userspace shared memory that is pinned,
>> similar to what your sys_task_getshared does.
> In glibc, we'd also like to have this for PID and TID. Eventually,
> rt_sigprocmask without kernel roundtrip in most cases would be very nice
> as well. For performance and simplicity in userspace, it would be best
> if the memory region could be at the same offset from the TCB for all
> threads.
> For KTLS, the idea was that the auxiliary vector would contain size and
> alignment of the KTLS. Userspace would reserve that memory, register it
> with the kernel like rseq (or the robust list pointers), and pass its
> address to the vDSO functions that need them. The last part ensures
> that the vDSO functions do not need non-global data to determine the
> offset from the TCB. Registration is still needed for the caches.
> I think previous discussions (in the KTLS and rseq context) did not have
> the pinning constraint.

If this data is per-thread, and read from user-space, why is it relevant
to update this data from non-sleepable kernel context rather than update it as
needed on return-to-userspace ? When returning to userspace, sleeping due to a
page fault is entirely acceptable. This is what we currently do for rseq.

In short, the data could be accessible from the task struct. Flags in the
task struct can let return-to-userspace know that it has outdated ktls
data. So before returning to userspace, the kernel can copy the relevant data
from the task struct to the shared memory area, without requiring any pinning.

What am I missing ?



Mathieu Desnoyers
EfficiOS Inc.