Re: unlock_buffer() and clear_bit()

[Nick Piggin <nickpiggin@yahoo.com.au>]

Nick Piggin wrote:

> smp_mb__after_clear_bit() is supposed to, when run directly after
> a clear_bit operation, provide the equivalent of an smp_mb().

Actually I guess I'm wrong here: it appears that it really should
order before, and after the clear_bit, respectively (looking at
its usage in unlock_page.

So ia64's smp_mb__before_clear_bit needs to be a full barrier, but
__after_clear_bit can be a release. I think?

By the way unlock_page is issuing extra barriers:
Documentation/atomic_ops.txt defines test_and_clear_bit operations
to provide full memory barriers before and after them, so no need
for smp_mb__before/after there.

...ia64 gets these test_and_x_bit operations wrong as well...

--
SUSE Labs, Novell Inc.
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