Author: kib
Date: Thu Oct 5 12:16:45 2017
New Revision: 324307
URL: https://svnweb.freebsd.org/changeset/base/324307
Log:
Complete r323772 on amd64.
Compilers are allowed to combine plain reads into group operations,
e.g. 64bit element copies of one array into another can be
legitimately optimized back to a memcpy() call, which r323772 tried to
prevent.
Qualify accesses to LDT descriptors with volatile dereference to
ensure that each write indeed occurs. After that, our usual claim of
native-size aligned writes being atomic applies.
This is equivalent to atomic_store(memory_order_relaxed) C11 accesses,
but our machine/atomic.h does not provide corresponding primitive.
Noted and reviewed by: bde
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Modified:
head/sys/amd64/amd64/sys_machdep.c
Modified: head/sys/amd64/amd64/sys_machdep.c
==============================================================================
--- head/sys/amd64/amd64/sys_machdep.c Thu Oct 5 12:07:38 2017
(r324306)
+++ head/sys/amd64/amd64/sys_machdep.c Thu Oct 5 12:16:45 2017
(r324307)
@@ -616,7 +616,7 @@ amd64_set_ldt(struct thread *td, struct i386_ldt_args
return (EINVAL);
mtx_lock(&dt_lock);
for (i = uap->start; i < largest_ld; i++)
- ((uint64_t *)(pldt->ldt_base))[i] = 0;
+ ((volatile uint64_t *)(pldt->ldt_base))[i] = 0;
mtx_unlock(&dt_lock);
return (0);
}
@@ -734,15 +734,15 @@ amd64_set_ldt_data(struct thread *td, int start, int n
{
struct mdproc *mdp;
struct proc_ldt *pldt;
- uint64_t *dst, *src;
+ volatile uint64_t *dst, *src;
int i;
mtx_assert(&dt_lock, MA_OWNED);
mdp = &td->td_proc->p_md;
pldt = mdp->md_ldt;
- dst = (uint64_t *)(pldt->ldt_base);
- src = (uint64_t *)descs;
+ dst = (volatile uint64_t *)(pldt->ldt_base);
+ src = (volatile uint64_t *)descs;
for (i = 0; i < num; i++)
dst[start + i] = src[i];
return (0);
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