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[POWERPC] Make mmiowb's io_sync preempt safe

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If mmiowb() is always used prior to releasing spinlock as Doc suggests,
then it's safe against preemption; but I'm not convinced that's always
the case.  If preemption occurs between sync and get_paca()->io_sync = 0,
I believe there's no problem.  But in the unlikely event that gcc does
the store relative to another register than r13 (as it did with current),
then there's a small danger of setting another cpu's io_sync to 0, after
it had just set it to 1.  Rewrite ppc64 mmiowb to prevent that.

The remaining io_sync assignments in io.h all get_paca()->io_sync = 1,
which is harmless even if preempted to the wrong cpu (the context switch
itself syncs); and those in spinlock.h are while preemption is disabled.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>
This commit is contained in:
Hugh Dickins 2006-10-31 18:41:51 +00:00 committed by Paul Mackerras
parent 96268889ee
commit 292f86f005
1 changed files with 5 additions and 2 deletions
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include/asm-powerpc/io.h
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@ -163,8 +163,11 @@ extern void _outsl_ns(volatile u32 __iomem *port, const void *buf, long count);
static inline void mmiowb(void)
{
__asm__ __volatile__ ("sync" : : : "memory");
get_paca()->io_sync = 0;
unsigned long tmp;
__asm__ __volatile__("sync; li %0,0; stb %0,%1(13)"
: "=&r" (tmp) : "i" (offsetof(struct paca_struct, io_sync))
: "memory");
}
/*