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ZFS: add emulation of atomic_swap_64 and atomic_load_64

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Some 32-bit platforms do not provide 64-bit atomic operations that ZFS
requires, either in userland or at all.  We emulate those operations for
those platforms using a mutex.  That is not entirely correct and it's
very efficient.  Besides, the loads are plain loads, so torn values are
possible.

Nevertheless, the emulation seems to work for some definition of work.

This change adds atomic_swap_64, which is already used in ZFS code, and
atomic_load_64 that can be used to prevent torn reads.

MFC after:	1 week
This commit is contained in:
avg 2019-10-07 07:54:34 +00:00
parent 87b704d0e3
commit 3458e5d1e6
2 changed files with 25 additions and 0 deletions
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23
sys/cddl/compat/opensolaris/kern/opensolaris_atomic.c
View File

@ -71,6 +71,29 @@ atomic_dec_64(volatile uint64_t *target)
*target -= 1;
mtx_unlock(&atomic_mtx);
}
uint64_t
atomic_swap_64(volatile uint64_t *a, uint64_t value)
{
uint64_t ret;
mtx_lock(&atomic_mtx);
ret = *a;
*a = value;
mtx_unlock(&atomic_mtx);
return (ret);
}
uint64_t
atomic_load_64(volatile uint64_t *a)
{
uint64_t ret;
mtx_lock(&atomic_mtx);
ret = *a;
mtx_unlock(&atomic_mtx);
return (ret);
}
#endif
uint64_t

2
sys/cddl/compat/opensolaris/sys/atomic.h
View File

@ -44,6 +44,8 @@
!defined(ARM_HAVE_ATOMIC64) && !defined(I386_HAVE_ATOMIC64)
extern void atomic_add_64(volatile uint64_t *target, int64_t delta);
extern void atomic_dec_64(volatile uint64_t *target);
extern uint64_t atomic_swap_64(volatile uint64_t *a, uint64_t value);
extern uint64_t atomic_load_64(volatile uint64_t *a);
#endif
#ifndef __sparc64__
extern uint32_t atomic_cas_32(volatile uint32_t *target, uint32_t cmp,