Re: [PATCH v2 14/14] RFC elf: Fix slow tls access after dlopen [BZ #19924]

[Carlos O'Donell <carlos@redhat.com>]

On Tue, Apr 13, 2021 at 09:21:58AM +0100, Szabolcs Nagy via Libc-alpha wrote:
> In short: __tls_get_addr checks the global generation counter,
> _dl_update_slotinfo updates up to the generation of the accessed
> module. If the global generation is newer than geneneration of the
> module then __tls_get_addr keeps hitting the slow path that updates
> the dtv.
> 
> Possible approaches i can see:
> 
> 1. update to global generation instead of module,
> 2. check the module generation in the fast path.
> 
> This patch is 1.: it needs additional sync (load acquire) so the
> slotinfo list is up to date with the observed global generation.
> 
> Approach 2. would require walking the slotinfo list at all times.
> I don't know how to make that fast with many modules.
> 
> Note: in the x86_64 version of dl-tls.c the generation is only loaded
> once, since relaxed mo is not faster than acquire mo load.
> 
> I have not benchmarked this yet.

Szabolcs,

Could you please repost this as a distinct patch? I'd like to track
this as a possible solution that we can discuss and improve. It's the
only patch that we haven't applied from this series.

Cheers,
Carlos.