Re: For review: nptl(7) man page

[Rich Felker <dalias at libc dot org>]

On Wed, Aug 05, 2015 at 11:14:32AM -0700, Nicholas Miell wrote:
> 
> 
> > On Aug 5, 2015, at 2:36 AM, Torvald Riegel <triegel@redhat.com> wrote:
> > 
> >> On Tue, 2015-08-04 at 11:50 -0700, Nicholas Miell wrote:
> >>> On Aug 4, 2015, at 8:06 AM, Roland McGrath <roland@hack.frob.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.

Why would you exclude static? IMO two static-linked executables using
the same version of glibc should be compatible with each other even if
you don't want to guarantee they'll be compatible with the
same-version shared glibc.

> > 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.

Only for some usage cases. As far as I can tell, POSIX semaphores are
not intended to be required to be implemented as a kernel resource.
They don't have permissions enforcement/safety against malicious
processes, backout on async process termination, etc.

Rich