Re: [RFC PATCH v6 1/5] Thread-local ABI system call: cache CPU number of running thread

On Tue, Apr 05, 2016 at 06:02:25PM +0200, Florian Weimer wrote:

On 04/04/2016 10:48 PM, Mathieu Desnoyers wrote:

quoted

Moreover, the feature set that the application knows about, glibc
knows about, and the kernel knows about are three different things.
My intent here is to have glibc stay out of the way as much as possible,
since this is really an interface between various applications/libraries
and the kernel.

Surely glibc can allocate the space based on what is advertised as
needed by the kernel?  Why would it limit itself to what is supported by
the kernel headers it is compiled against if the actual size can be
queried from the kernel?

I guess the question is; can we do thread local variable arrays like:

  __thread uint32_t[x]; /* with x being a runtime constant */

Because then we can do:

  __thread struct thread_local_abi tla;

where sizeof(struct thread_local_abi) is a runtime variable.

Without that we cannot have this thread-local-abi structure be part of
the immediately addressable TLS space. That is, we then need a pointer
like:

  __thread struct thread_local_abi *tla;

and every usage will need the extra pointer deref.

Because ideally this structure would be part of the initial (glibc) TCB
with fixed offset etc.