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Backpatch-through: 13
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Previously we had a fallback implementation that made a harmless system
call, based on the assumption that system calls must contain a memory
barrier. That shouldn't be reached on any current system, and it seems
highly likely that we can easily find out how to request explicit memory
barriers, if we've already had to find out how to do atomics on a
hypothetical new system.
Removed comments and a function name referred to a spinlock used for
fallback memory barriers, but that changed in 1b468a13, which left some
misleading words behind in a few places.
Reviewed-by: Heikki Linnakangas <[email protected]>
Suggested-by: Andres Freund <[email protected]>
Discussion: https://postgr.es/m/721bf39a-ed8a-44b0-8b8e-be3bd81db748%40technowledgy.de
Discussion: https://postgr.es/m/3351991.1697728588%40sss.pgh.pa.us
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Previously we had a fallback implementation of pg_compiler_barrier()
that called an empty function across a translation unit boundary so the
compiler couldn't see what it did. That shouldn't be needed on any
current systems, and might not even work with a link time optimizer.
Since we now require compiler-specific knowledge of how to implement
atomics, we should also know how to implement compiler barriers on a
hypothetical new system.
Reviewed-by: Heikki Linnakangas <[email protected]>
Suggested-by: Andres Freund <[email protected]>
Discussion: https://postgr.es/m/721bf39a-ed8a-44b0-8b8e-be3bd81db748%40technowledgy.de
Discussion: https://postgr.es/m/3351991.1697728588%40sss.pgh.pa.us
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Modern versions of all relevant architectures and tool chains have
atomics support. Since edadeb07, there is no remaining reason to carry
code that simulates atomic flags and uint32 imperfectly with spinlocks.
64 bit atomics are still emulated with spinlocks, if needed, for now.
Any modern compiler capable of implementing C11 <stdatomic.h> must have
the underlying operations we need, though we don't require C11 yet. We
detect certain compilers and architectures, so hypothetical new systems
might need adjustments here.
Reviewed-by: Heikki Linnakangas <[email protected]>
Reviewed-by: Tom Lane <[email protected]> (concept, not the patch)
Reviewed-by: Andres Freund <[email protected]> (concept, not the patch)
Discussion: https://postgr.es/m/3351991.1697728588%40sss.pgh.pa.us
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A later change will require atomic support, so it wouldn't make sense
for a hypothetical new system not to be able to implement spinlocks.
Reviewed-by: Heikki Linnakangas <[email protected]>
Reviewed-by: Tom Lane <[email protected]> (concept, not the patch)
Reviewed-by: Andres Freund <[email protected]> (concept, not the patch)
Discussion: https://postgr.es/m/3351991.1697728588%40sss.pgh.pa.us
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This old CPU architecture hasn't been produced in decades, and
whatever instances might still survive are surely too underpowered
for anyone to consider running Postgres on in production. We'd
nonetheless continued to carry code support for it (largely at my
insistence), because its unique implementation of spinlocks seemed
like a good edge case for our spinlock infrastructure. However,
our last buildfarm animal of this type was retired last year, and
it seems quite unlikely that another will emerge. Without the ability
to run tests, the argument that this is useful test code fails to
hold water. Furthermore, carrying code support for an untestable
architecture has costs not to be ignored. So, remove HPPA-specific
code, in the same vein as commits 718aa43a4 and 92d70b77e.
Discussion: https://postgr.es/m/[email protected]
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Reported-by: Michael Paquier
Discussion: https://postgr.es/m/[email protected]
Backpatch-through: 12
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Backpatch-through: 11
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HP-UX hardware is no longer produced, build farm coverage recently
ended, and there are no known active maintainers targeting this OS.
Since there is a major rewrite of the build system in the pipeline for
PostgreSQL 16, and that requires development, testing and maintainance
for each OS and tool chain, it seems like a good time to drop support
for:
* HP-UX, the operating system.
* HP aCC, the HP-UX native compiler.
Reviewed-by: Tom Lane <[email protected]>
Reviewed-by: Andres Freund <[email protected]>
Reviewed-by: Peter Eisentraut <[email protected]>
Discussion: https://postgr.es/m/1415825.1656893299%40sss.pgh.pa.us
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Backpatch-through: 10
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Backpatch-through: 9.5
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Backpatch-through: update all files in master, backpatch legal files through 9.4
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Backpatch-through: certain files through 9.4
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The atomics fallback implementation for pg_atomic_flag was broken,
returning the inverted value from pg_atomic_test_set_flag(). This was
unnoticed because a) atomic flags were unused until recently b) the
test code wasn't run when the fallback implementation was in
use (because it didn't allow to test for some edge cases).
Fix the bug, and improve the fallback so it has the same behaviour as
the non-fallback implementation in the problematic edge cases. That
breaks ABI compatibility in the back branches when fallbacks are in
use, but given they were broken until now...
Author: Andres Freund
Reported-by: Daniel Gustafsson
Discussion:
https://postgr.es/m/[email protected]
https://postgr.es/m/[email protected]
Backpatch: 9.5-, where the atomics abstraction was introduced.
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Backpatch-through: certain files through 9.3
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When adding atomics back in b64d92f1a, I added 64bit support as
optional; there wasn't yet a direct user in sight. That turned out to
be a bit short-sighted, it'd already have been useful a number of times.
Add a fallback implementation of 64bit atomics, just like the one we
have for 32bit atomics.
Additionally optimize reads/writes to 64bit on a number of platforms
where aligned writes of that size are atomic. This can now be tested
with PG_HAVE_8BYTE_SINGLE_COPY_ATOMICITY.
Author: Andres Freund
Reviewed-By: Amit Kapila
Discussion: https://postgr.es/m/[email protected]
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I somehow had assumed that in the spinlock (in turn possibly using
semaphores) based fallback atomics implementation 32 bit writes could be
done without a lock. As far as the write goes that's correct, since
postgres supports only platforms with single-copy atomicity for aligned
32bit writes. But writing without holding the spinlock breaks
read-modify-write operations like pg_atomic_compare_exchange_u32(),
since they'll potentially "miss" a concurrent write, which can't happen
in actual hardware implementations.
In 9.6+ when using the fallback atomics implementation this could lead
to buffer header locks not being properly marked as released, and
potentially some related state corruption. I don't see a related danger
in 9.5 (earliest release with the API), because pg_atomic_write_u32()
wasn't used in a concurrent manner there.
The state variable of local buffers, before this change, were
manipulated using pg_atomic_write_u32(), to avoid unnecessary
synchronization overhead. As that'd not be the case anymore, introduce
and use pg_atomic_unlocked_write_u32(), which does not correctly
interact with RMW operations.
This bug only caused issues when postgres is compiled on platforms
without atomics support (i.e. no common new platform), or when compiled
with --disable-atomics, which explains why this wasn't noticed in
testing.
Reported-By: Tom Lane
Discussion: <[email protected]>
Backpatch: 9.5-, where the atomic operations API was introduced.
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Backpatch certain files through 9.1
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So far we have worked around the fact that some very old compilers do
not support 'inline' functions by only using inline functions
conditionally (or not at all). Since such compilers are very rare by
now, we have decided to rely on inline functions from 9.6 onwards.
To avoid breaking these old compilers inline is defined away when not
supported. That'll cause "function x defined but not used" type of
warnings, but since nobody develops on such compilers anymore that's
ok.
This change in policy will allow us to more easily employ inline
functions.
I chose to remove code previously conditional on PG_USE_INLINE as it
seemed confusing to have code dependent on a define that's always
defined.
Blacklisting of compilers, like in c53f73879f, now has to be done
differently. A platform template can define PG_FORCE_DISABLE_INLINE to
force inline to be defined empty.
Discussion: [email protected]
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This is because there are many __asm__ blocks there that pgindent messes
up. Also configure pgindent to skip that directory in the future.
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Plus some tiny wordsmithing of not-quite-typos.
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If the compiler/arch combination does not provide compiler barriers,
provide a fallback. That fallback simply consists out of a function
call into a externally defined function. That should guarantee
compiler barrierer semantics except for compilers that do inter
translation unit/global optimization - those better provide an actual
compiler barrier.
Hopefully this fixes Tom's report of linker failures due to
pg_compiler_barrier_impl not being provided.
I'm not backpatching this commit as it builds on the new atomics
infrastructure. If we decide an equivalent fix needs to be
backpatched, I'll do so in a separate commit.
Discussion: [email protected]
Per report from Tom Lane.
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Backpatch certain files through 9.0
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Several upcoming performance/scalability improvements require atomic
operations. This new API avoids the need to splatter compiler and
architecture dependent code over all the locations employing atomic
ops.
For several of the potential usages it'd be problematic to maintain
both, a atomics using implementation and one using spinlocks or
similar. In all likelihood one of the implementations would not get
tested regularly under concurrency. To avoid that scenario the new API
provides a automatic fallback of atomic operations to spinlocks. All
properties of atomic operations are maintained. This fallback -
obviously - isn't as fast as just using atomic ops, but it's not bad
either. For one of the future users the atomics ontop spinlocks
implementation was actually slightly faster than the old purely
spinlock using implementation. That's important because it reduces the
fear of regressing older platforms when improving the scalability for
new ones.
The API, loosely modeled after the C11 atomics support, currently
provides 'atomic flags' and 32 bit unsigned integers. If the platform
efficiently supports atomic 64 bit unsigned integers those are also
provided.
To implement atomics support for a platform/architecture/compiler for
a type of atomics 32bit compare and exchange needs to be
implemented. If available and more efficient native support for flags,
32 bit atomic addition, and corresponding 64 bit operations may also
be provided. Additional useful atomic operations are implemented
generically ontop of these.
The implementation for various versions of gcc, msvc and sun studio have
been tested. Additional existing stub implementations for
* Intel icc
* HUPX acc
* IBM xlc
are included but have never been tested. These will likely require
fixes based on buildfarm and user feedback.
As atomic operations also require barriers for some operations the
existing barrier support has been moved into the atomics code.
Author: Andres Freund with contributions from Oskari Saarenmaa
Reviewed-By: Amit Kapila, Robert Haas, Heikki Linnakangas and Álvaro Herrera
Discussion: CA+TgmoYBW+ux5-8Ja=Mcyuy8=VXAnVRHp3Kess6Pn3DMXAPAEA@mail.gmail.com,
[email protected],
[email protected]
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