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Commitac883ac

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Fix shm_toc.c to always return buffer-aligned memory.
Previously, if you passed a non-aligned size to shm_toc_create(), thememory returned by shm_toc_allocate() would be similarly non-aligned.This was exposed by commit3cda10f, which allocated structs containinga pg_atomic_uint64 field with shm_toc_allocate(). On systems withMAXIMUM_ALIGNOF = 4, such structs still need to be 8-bytes aligned, butthe memory returned by shm_toc_allocate() was only 4-bytes aligned.It's quite bogus that we abuse BUFFERALIGN to align the structs forpg_atomic_uint64. It doesn't really have anything to do with buffers. Butthat's a separate issue.This ought to fix the buildfarm failures on 32-bit x86 systems.Discussion:https://www.postgresql.org/message-id/7e0a73a5-0df9-1859-b8ae-9acf122dc38d@iki.fi
1 parent4395f7d commitac883ac

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+20
-5
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2 files changed

+20
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lines changed

‎src/backend/storage/ipc/shm_toc.c

Lines changed: 19 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -44,7 +44,12 @@ shm_toc_create(uint64 magic, void *address, Size nbytes)
4444
Assert(nbytes> offsetof(shm_toc,toc_entry));
4545
toc->toc_magic=magic;
4646
SpinLockInit(&toc->toc_mutex);
47-
toc->toc_total_bytes=nbytes;
47+
48+
/*
49+
* The alignment code in shm_toc_allocate() assumes that the starting
50+
* value is buffer-aligned.
51+
*/
52+
toc->toc_total_bytes=BUFFERALIGN_DOWN(nbytes);
4853
toc->toc_allocated_bytes=0;
4954
toc->toc_nentry=0;
5055

@@ -88,7 +93,12 @@ shm_toc_allocate(shm_toc *toc, Size nbytes)
8893
Sizenentry;
8994
Sizetoc_bytes;
9095

91-
/* Make sure request is well-aligned. */
96+
/*
97+
* Make sure request is well-aligned. XXX: MAXALIGN is not enough,
98+
* because atomic ops might need a wider alignment. We don't have a
99+
* proper definition for the minimum to make atomic ops safe, but
100+
* BUFFERALIGN ought to be enough.
101+
*/
92102
nbytes=BUFFERALIGN(nbytes);
93103

94104
SpinLockAcquire(&toc->toc_mutex);
@@ -252,7 +262,11 @@ shm_toc_lookup(shm_toc *toc, uint64 key, bool noError)
252262
Size
253263
shm_toc_estimate(shm_toc_estimator*e)
254264
{
255-
returnadd_size(offsetof(shm_toc,toc_entry),
256-
add_size(mul_size(e->number_of_keys,sizeof(shm_toc_entry)),
257-
e->space_for_chunks));
265+
Sizesz;
266+
267+
sz= offsetof(shm_toc,toc_entry);
268+
sz+=add_size(sz,mul_size(e->number_of_keys,sizeof(shm_toc_entry)));
269+
sz+=add_size(sz,e->space_for_chunks);
270+
271+
returnBUFFERALIGN(sz);
258272
}

‎src/include/c.h

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -598,6 +598,7 @@ typedef NameData *Name;
598598
#defineLONGALIGN_DOWN(LEN)TYPEALIGN_DOWN(ALIGNOF_LONG, (LEN))
599599
#defineDOUBLEALIGN_DOWN(LEN)TYPEALIGN_DOWN(ALIGNOF_DOUBLE, (LEN))
600600
#defineMAXALIGN_DOWN(LEN)TYPEALIGN_DOWN(MAXIMUM_ALIGNOF, (LEN))
601+
#defineBUFFERALIGN_DOWN(LEN)TYPEALIGN_DOWN(ALIGNOF_BUFFER, (LEN))
601602

602603
/*
603604
* The above macros will not work with types wider than uintptr_t, like with

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