Re: [RFC PATCH glibc] pthread_setspecific: Provide signal-safety across keys

[Mathieu Desnoyers <mathieu dot desnoyers at efficios dot com>]

----- On Oct 18, 2017, at 1:40 PM, carlos carlos@redhat.com wrote:

> On 10/18/2017 09:21 AM, Mathieu Desnoyers wrote:
>> So let's assume dlopen failure is acceptable if there are too many
>> initial-exec TLS. Would it work today to fix signal handler tracing
>> by changing lttng-ust to use TLS_IE, or would it require changes to
>> glibc ?
> 
> No changes to glibc are required. We already use TLS IE in glibc
> specifically to fix this issue. Also libGL also uses TLS IE in
> the distro for performance reasons, so we get testing there.
> 
> How much TLS data do you have? With libGL it's just a few words,
> and there is lots of spare space in the static TLS space for
> that.

For liburcu-bp, used by lttng-ust, a single pointer:

DEFINE_URCU_TLS(struct rcu_reader *, rcu_reader);

I make sure to do place the bulk of data into mmap'd memory.
So, very little TLS space is required.

Thanks,

Mathieu

> 
> --
> Cheers,
> Carlos.

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com