Re: Restartable Sequences system call merged into Linux

[Stefan Liebler <stli at linux dot ibm dot com>]

On 07/12/2018 10:10 AM, Szabolcs Nagy wrote:
> On 11/07/18 17:03, Stefan Liebler wrote:
> > On 06/11/2018 09:49 PM, Mathieu Desnoyers wrote:
> > > Hi!
> > >
> > > Good news! The restartable sequences (rseq) system call is now merged 
> > > into the master
> > > branch of the Linux kernel within the 4.18 merge window:
> > >
> > > https://github.com/torvalds/linux/commit/d82991a8688ad128b46db1b42d5d84396487a508 
> > >
> > >
> > > It would be important to discuss how we should proceed to integrate 
> > > the library part
> > > of rseq (see tools/testing/selftests/rseq/rseq*.{ch}) into glibc, or 
> > > if it should
> > > live in a standalone project.
> > >
> > > It should be noted that there can be only one rseq TLS area 
> > > registered per thread,
> > > which can then be used by many libraries and by the executable, so 
> > > this is a
> > > process-wide (per-thread) resource that we need to manage carefully.
> > >
> > > Thoughts ?
> > >
> > > Thanks!
> > >
> > > Mathieu
> > The structs rseq_cs and rseq are aligned to 32 bytes.
> > Please have a look at the bug:
> > "Bug 23403 - Wrong alignment of TLS variables"
> > (https://sourceware.org/bugzilla/show_bug.cgi?id=23403).
> >
> > @Carlos or somebody else:
> > Should this be fixed in the upcoming glibc release?
>
> i think s390 can implement a hack that increases the internal
> pthread_t alignment to 64 or similar.
>
> that effectively fixes 2.28 in practice (huge tls alignment
> should be rare).

Yes, this hack would fix it, but on the other side it hides a real bug.
Therefore I prefer Florian's idea that once it is fixed, it should be 
backported.