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Re: Async-signal-safe access to __thread variables from dlopen()ed libraries?
- From: "Carlos O'Donell" <carlos at redhat dot com>
- To: Andrew Hunter <ahh at google dot com>
- Cc: Ian Lance Taylor <iant at google dot com>, Paul Pluzhnikov <ppluzhnikov at google dot com>, Roland McGrath <roland at hack dot frob dot com>, Richard Henderson <rth at twiddle dot net>, GNU C Library <libc-alpha at sourceware dot org>, Alexandre Oliva <aoliva at redhat dot com>
- Date: Tue, 24 Sep 2013 18:00:31 -0400
- Subject: Re: Async-signal-safe access to __thread variables from dlopen()ed libraries?
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On 09/24/2013 05:40 PM, Andrew Hunter wrote:
> On Tue, Sep 24, 2013 at 2:26 PM, Carlos O'Donell <carlos@redhat.com> wrote:
>> On 09/24/2013 04:21 PM, Andrew Hunter wrote:
>>> On Tue, Sep 24, 2013 at 1:15 PM, Carlos O'Donell <carlos@redhat.com> wrote:
>>>> Thanks. I still think we need to look at interrupted initialization
>>>> as another possible problem.
>>>
>>> My patch addresses that (the simplest way possible--masking out signals.)
>>
>> Thanks I saw that. I need to update the design document to cover exactly
>> what the patch is doing in that case.
>>
>> It's certainly a future enhancement to use an atomic operation with a
>> retry, but I don't know if the complexity is worth it. I also don't know
>> if the blocked signals show up at all as increased latency in responding
>> to other signals. Do you have any figures on that?
>>
>
> I don't, sorry. My expectation is that since this cost will be paid
> precisely once per thread per dlopened object with TLS (it's entirely
> off the fast path) that it's unlikely to be a noticable performance
> problem.
>
> FYI, on my recent-ish Intel system, a mask/unmask pair is under 350
> nanoseconds. The block times aren't likely to be much longer; the
> most expensive thing done there is (at most) two small mmaps (if we
> don't override with a better malloc.) mmap'ing a page and populating
> it costs <2 microseconds on my system. I would be very surprised,
> other than in very large TLS arrays or very strange circumstances, if
> tls_get_addr() took more than a millisecond on CPU.
Yeah, I don't disagree, and I think we did our due diligence by
talking it through right now.
Cheers,
Carlos.