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Fix shm_toc.c to always return buffer-aligned memory.

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Previously, if you passed a non-aligned size to shm_toc_create(), the
memory returned by shm_toc_allocate() would be similarly non-aligned.
This was exposed by commit 3cda10f41b, which allocated structs containing
a pg_atomic_uint64 field with shm_toc_allocate(). On systems with
MAXIMUM_ALIGNOF = 4, such structs still need to be 8-bytes aligned, but
the memory returned by shm_toc_allocate() was only 4-bytes aligned.

It's quite bogus that we abuse BUFFERALIGN to align the structs for
pg_atomic_uint64. It doesn't really have anything to do with buffers. But
that'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
This commit is contained in:
Heikki Linnakangas 2017-08-16 21:52:38 +03:00
parent 4395f7daf3
commit ac883ac453
2 changed files with 20 additions and 5 deletions
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24
src/backend/storage/ipc/shm_toc.c
View File

@ -44,7 +44,12 @@ shm_toc_create(uint64 magic, void *address, Size nbytes)
Assert(nbytes > offsetof(shm_toc, toc_entry));
toc->toc_magic = magic;
SpinLockInit(&toc->toc_mutex);
toc->toc_total_bytes = nbytes;
/*
* The alignment code in shm_toc_allocate() assumes that the starting
* value is buffer-aligned.
*/
toc->toc_total_bytes = BUFFERALIGN_DOWN(nbytes);
toc->toc_allocated_bytes = 0;
toc->toc_nentry = 0;
@ -88,7 +93,12 @@ shm_toc_allocate(shm_toc *toc, Size nbytes)
Size nentry;
Size toc_bytes;
/* Make sure request is well-aligned. */
/*
* Make sure request is well-aligned. XXX: MAXALIGN is not enough,
* because atomic ops might need a wider alignment. We don't have a
* proper definition for the minimum to make atomic ops safe, but
* BUFFERALIGN ought to be enough.
*/
nbytes = BUFFERALIGN(nbytes);
SpinLockAcquire(&toc->toc_mutex);
@ -252,7 +262,11 @@ shm_toc_lookup(shm_toc *toc, uint64 key, bool noError)
Size
shm_toc_estimate(shm_toc_estimator *e)
{
return add_size(offsetof(shm_toc, toc_entry),
add_size(mul_size(e->number_of_keys, sizeof(shm_toc_entry)),
e->space_for_chunks));
Size sz;
sz = offsetof(shm_toc, toc_entry);
sz += add_size(sz, mul_size(e->number_of_keys, sizeof(shm_toc_entry)));
sz += add_size(sz, e->space_for_chunks);
return BUFFERALIGN(sz);
}

1
src/include/c.h
View File

@ -598,6 +598,7 @@ typedef NameData *Name;
#define LONGALIGN_DOWN(LEN) TYPEALIGN_DOWN(ALIGNOF_LONG, (LEN))
#define DOUBLEALIGN_DOWN(LEN) TYPEALIGN_DOWN(ALIGNOF_DOUBLE, (LEN))
#define MAXALIGN_DOWN(LEN) TYPEALIGN_DOWN(MAXIMUM_ALIGNOF, (LEN))
#define BUFFERALIGN_DOWN(LEN) TYPEALIGN_DOWN(ALIGNOF_BUFFER, (LEN))
/*
* The above macros will not work with types wider than uintptr_t, like with