Feedback-ID: ia2694551:Fastmail
Date: Tue, 2 Jul 2024 16:55:01 +0200
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Andres Freund <andres@anarazel.de>
Cc: Alexander Lakhin <exclusion@gmail.com>,
	Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>,
 Tom Lane <tgl@sss.pgh.pa.us>, Jeff Davis <pgsql@j-davis.com>,
	pgsql-hackers@lists.postgresql.org,
 Jaime Casanova <jcasanov@systemguards.com.ec>
Subject: Re: LogwrtResult contended spinlock
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On 2024-Jul-02, Andres Freund wrote:

> On 2024-07-01 21:12:25 +0200, Alvaro Herrera wrote:
> > On 2024-Jul-01, Andres Freund wrote:

> > I'm pretty sure the Microsoft docs I linked to are saying it must be
> > aligned.
> 
> I don't think so:
> https://learn.microsoft.com/en-us/windows/win32/api/winnt/nf-winnt-interlockedcompareexchange64
> 
> LONG64 InterlockedCompareExchange64(
>   [in, out] LONG64 volatile *Destination,
>   [in]      LONG64          ExChange,
>   [in]      LONG64          Comperand
> );
> 
> Note that Destination is the only argument passed by reference (and thus the
> caller controls alignment of the in-memory value). ExChange is passed by
> value, so we don't control alignment in any way.

Hmm, you're right, assuming LONG64 is passed by value.  Here
https://learn.microsoft.com/en-us/windows/win32/winprog/windows-data-types
it says that the type is declared as
typedef __int64 LONG64;
and
https://learn.microsoft.com/en-us/cpp/cpp/int8-int16-int32-int64?view=msvc-170
says that __int64 is a normal integer type.  So yes, 'ExChange' is
passed by value and therefore testing it for alignment is useless on
this platform.

> > There are in some of them, but not in pg_atomic_compare_exchange_u64_impl.
> 
> But there's one in pg_atomic_read_u64_impl().  

Sure, but pg_atomic_read_u64 is given 'ptr', not 'currval'.

> But I actually think it's wrong for pg_atomic_monotonic_advance_u64()
> to use _impl(), that's just for the wrapper functions around the
> implementation.  Wheras pg_atomic_monotonic_advance_u64() should just
> use the generic interface.

True.  Well, I can remove the assertion from
pg_atomic_monotonic_advance_u64 and use pg_atomic_compare_exchange_u64
instead.  But that one does this:

static inline bool
pg_atomic_compare_exchange_u64(volatile pg_atomic_uint64 *ptr,
                               uint64 *expected, uint64 newval)
{
#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
    AssertPointerAlignment(ptr, 8);
    AssertPointerAlignment(expected, 8);
#endif
    return pg_atomic_compare_exchange_u64_impl(ptr, expected, newval);
}


AFAICS this is still going to fail, because uint64 *expected comes from
our &currval, which was not aligned before so it'll still be unaligned
now.  The only difference is that the assertion failure will be in
pg_atomic_compare_exchange_u64 instead of in
pg_atomic_monotonic_advance_u64.


Other platforms do have the 'expected' argument as a pointer, so the
assertion there is not completely stupid.  I think we could move the
alignment assertions to appear inside the platform-specific _impl
routines that need it, and refrain from adding it to the MSVC one.


> > > > -	return Max(target_, currval);
> > > > +	return Max(target_, currval.u64);
> > > 
> > > What does the Max() actually achieve here? Shouldn't it be impossible to reach
> > > this with  currval < target_?
> > 
> > When two processes are hitting the cmpxchg concurrently, we need to
> > return the highest value that was written, even if it was the other
> > process that did it.
> 
> Sure. That explains needing to use currval. But not really needing to use
> Max(). If cmpxchg succeeds, we need to return target_, if the loop terminates
> otherwise we need to return currval. No?

Oh, you're suggesting to change the break statement with a return.
Seems reasonable.


> > > And why does target_ end in an underscore?
> > 
> > Heh, you tell me -- I just copied the style of the functions just above.
> 
> IIRC using plain "and" "or" "add" caused conflicts with some headers or such.

Ah, that makes sense.  It should be no problem to remove the underscore.

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/
"La primera ley de las demostraciones en vivo es: no trate de usar el sistema.
Escriba un guión que no toque nada para no causar daños." (Jakob Nielsen)

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From f9df825c8c61f3e4c8597af3cfbccc5d69967872 Mon Sep 17 00:00:00 2001
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: Tue, 2 Jul 2024 13:42:46 +0200
Subject: [PATCH v5] Remove bogus assertion in pg_atomic_monotonic_advance_u64

This code wanted to ensure that the 'exchange' variable passed to
pg_atomic_compare_exchange_u64 has correct alignment, but apparently
platforms don't actually require anything that doesn't come naturally.

While messing with pg_atomic_monotonic_advance_u64: instead of using
Max() to determine the value to return, just use
pg_atomic_compare_exchange_u64()'s return value to decide; also, use
pg_atomic_compare_exchange_u64 instead of the _impl version; also remove
the unnecessary underscore at the end of variable name "target".

Backpatch to 17.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/36796438-a718-cf9b-2071-b2c1b947c1b5@gmail.com
---
 src/include/port/atomics.h                | 17 ++++++-----------
 src/include/port/atomics/arch-ppc.h       |  2 ++
 src/include/port/atomics/arch-x86.h       |  2 ++
 src/include/port/atomics/generic-gcc.h    |  3 +++
 src/include/port/atomics/generic-sunpro.h |  1 +
 5 files changed, 14 insertions(+), 11 deletions(-)

diff --git a/src/include/port/atomics.h b/src/include/port/atomics.h
index c911c6b956..f6fa432d2d 100644
--- a/src/include/port/atomics.h
+++ b/src/include/port/atomics.h
@@ -507,7 +507,6 @@ pg_atomic_compare_exchange_u64(volatile pg_atomic_uint64 *ptr,
 {
 #ifndef PG_HAVE_ATOMIC_U64_SIMULATION
 	AssertPointerAlignment(ptr, 8);
-	AssertPointerAlignment(expected, 8);
 #endif
 	return pg_atomic_compare_exchange_u64_impl(ptr, expected, newval);
 }
@@ -576,7 +575,7 @@ pg_atomic_sub_fetch_u64(volatile pg_atomic_uint64 *ptr, int64 sub_)
  * Full barrier semantics (even when value is unchanged).
  */
 static inline uint64
-pg_atomic_monotonic_advance_u64(volatile pg_atomic_uint64 *ptr, uint64 target_)
+pg_atomic_monotonic_advance_u64(volatile pg_atomic_uint64 *ptr, uint64 target)
 {
 	uint64		currval;
 
@@ -585,23 +584,19 @@ pg_atomic_monotonic_advance_u64(volatile pg_atomic_uint64 *ptr, uint64 target_)
 #endif
 
 	currval = pg_atomic_read_u64_impl(ptr);
-	if (currval >= target_)
+	if (currval >= target)
 	{
 		pg_memory_barrier();
 		return currval;
 	}
 
-#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
-	AssertPointerAlignment(&currval, 8);
-#endif
-
-	while (currval < target_)
+	while (currval < target)
 	{
-		if (pg_atomic_compare_exchange_u64_impl(ptr, &currval, target_))
-			break;
+		if (pg_atomic_compare_exchange_u64(ptr, &currval, target))
+			return target;
 	}
 
-	return Max(target_, currval);
+	return currval;
 }
 
 #undef INSIDE_ATOMICS_H
diff --git a/src/include/port/atomics/arch-ppc.h b/src/include/port/atomics/arch-ppc.h
index 94ba2597fb..edaab7c895 100644
--- a/src/include/port/atomics/arch-ppc.h
+++ b/src/include/port/atomics/arch-ppc.h
@@ -173,6 +173,8 @@ pg_atomic_compare_exchange_u64_impl(volatile pg_atomic_uint64 *ptr,
 	uint32 condition_register;
 	bool ret;
 
+	AssertPointerAlignment(expected, 8);
+
 	/* Like u32, but s/lwarx/ldarx/; s/stwcx/stdcx/; s/cmpw/cmpd/ */
 #ifdef HAVE_I_CONSTRAINT__BUILTIN_CONSTANT_P
 	if (__builtin_constant_p(*expected) &&
diff --git a/src/include/port/atomics/arch-x86.h b/src/include/port/atomics/arch-x86.h
index 3efa79dc3d..2a8eca30fc 100644
--- a/src/include/port/atomics/arch-x86.h
+++ b/src/include/port/atomics/arch-x86.h
@@ -207,6 +207,8 @@ pg_atomic_compare_exchange_u64_impl(volatile pg_atomic_uint64 *ptr,
 {
 	char	ret;
 
+	AssertPointerAlignment(expected, 8);
+
 	/*
 	 * Perform cmpxchg and use the zero flag which it implicitly sets when
 	 * equal to measure the success.
diff --git a/src/include/port/atomics/generic-gcc.h b/src/include/port/atomics/generic-gcc.h
index 9d91370fa8..872d2f02af 100644
--- a/src/include/port/atomics/generic-gcc.h
+++ b/src/include/port/atomics/generic-gcc.h
@@ -240,6 +240,7 @@ static inline bool
 pg_atomic_compare_exchange_u64_impl(volatile pg_atomic_uint64 *ptr,
 									uint64 *expected, uint64 newval)
 {
+	AssertPointerAlignment(expected, 8);
 	return __atomic_compare_exchange_n(&ptr->value, expected, newval, false,
 									   __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);
 }
@@ -253,6 +254,8 @@ pg_atomic_compare_exchange_u64_impl(volatile pg_atomic_uint64 *ptr,
 {
 	bool	ret;
 	uint64	current;
+
+	AssertPointerAlignment(expected, 8);
 	current = __sync_val_compare_and_swap(&ptr->value, *expected, newval);
 	ret = current == *expected;
 	*expected = current;
diff --git a/src/include/port/atomics/generic-sunpro.h b/src/include/port/atomics/generic-sunpro.h
index e060c0868a..840a45e778 100644
--- a/src/include/port/atomics/generic-sunpro.h
+++ b/src/include/port/atomics/generic-sunpro.h
@@ -102,6 +102,7 @@ pg_atomic_compare_exchange_u64_impl(volatile pg_atomic_uint64 *ptr,
 	bool	ret;
 	uint64	current;
 
+	AssertPointerAlignment(expected, 8);
 	current = atomic_cas_64(&ptr->value, *expected, newval);
 	ret = current == *expected;
 	*expected = current;
-- 
2.39.2


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