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Re: For review: nptl(7) man page
- From: Nicholas Miell <nmiell at gmail dot com>
- To: Torvald Riegel <triegel at redhat dot com>
- Cc: Roland McGrath <roland at hack dot frob dot com>, Rich Felker <dalias at libc dot org>, "Michael Kerrisk (man-pages)" <mtk dot manpages at gmail dot com>, "linux-man at vger dot kernel dot org" <linux-man at vger dot kernel dot org>, "libc-alpha at sourceware dot org" <libc-alpha at sourceware dot org>, Carlos O'Donell <carlos at redhat dot com>
- Date: Wed, 5 Aug 2015 11:14:32 -0700
- Subject: Re: For review: nptl(7) man page
- Authentication-results: sourceware.org; auth=none
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> On Aug 5, 2015, at 2:36 AM, Torvald Riegel <firstname.lastname@example.org> wrote:
>> On Tue, 2015-08-04 at 11:50 -0700, Nicholas Miell wrote:
>>> On Aug 4, 2015, at 8:06 AM, Roland McGrath <email@example.com> wrote:
>>> I think we are asserting that they are exactly that by dint of the confstr
>>> results for _CS_POSIX_V7_ILP32_OFF32_CFLAGS et al. So the question is what
>>> POSIX actually does or doesn't say about process-shared synchronization
>>> objects being shared between processes running programs built in different
>>> POSIX compilation environments.
>>> The other relevant question is whether 32/64 sharing of each particular
>>> pshared object has in fact worked reliably under glibc in the past. Since
>>> we haven't been clear and explicit about the subject before AFAIK, then if
>>> in fact it worked before then people might well have inferred that we made
>>> such an ABI guarantee. (I hope not, since if so we just broke it.)
>> The relevant questions aren't what's the least useful behavior that POSIX lets us get away with or can we leave it broken because it never worked;'the questions are what do the other operating systems do and what do the users want.
> Those are relevant questions, but they are not the only relevant ones.
> Some users would also like to share data structures between processes
> using different glibc builds (eg, different versions), and we won't
> promise that this works for obvioius reasons.
The reasonable scenario is what we have right now -- 32 and 64-bit versions of glibc built from the exact same git commit. And I'd exclude static versions of glibc even if they're the same version.
> The semaphore example shows that there can be a disadvantage to
> guaranteeing 32/64b interoperability (specifically, the 64b code is more
> efficient). For mutex, I *currently* don't see a reason why we couldn't
> get away with just doing 32b stuff for the pshared case, but there's no
> guarantee that I can foresee all future needs either.
> Thus, if we would decide to guarantee 32/64b interoperability, we'd need
> to have at least strong use cases for that and a decent amount of
> confidence that making such a guarantee is unlikely to constrain the
> implementation in the future.
Well, POSIX semaphores are supposed to be a replacement for System V semaphores (and this extends to the rest of the POSIX IPC primitives); right now they aren't.