Re: VM in 2.4.7-pre hurts...

[Andrea Arcangeli]

On Mon, Jul 09, 2001 at 09:20:23PM -0700, Linus Torvalds wrote:
> In contrast, the version in pre4 doesn't depend on any memory ordering
> between BH_Locked at all - it really only depends on a memory barrier
> before the final atomic_dec() that releases the buffer, as it ends up
> being sufficient for try_to_free_buffers() to just worry about the buffer
> count when it comes to IO completion. The b_flags BUSY bits don't matter
> wrt the IO completion at all - they end up being used only for "idle"
> buffers (which in turn are totally synchronized by the LRU and hash
> spinlocks, so that is the "obviously correct" case)
> 
> I personally think it's a hard thing to depend on memory ordering,

Sometime memory ordering pays off by avoiding locks, but this isn't the
case ;).

> especially if there are two independent fields. Which is why I really
> don't think that the pre4 fix is "overkill".

It certainly isn't overkill in respect of doing get_bh in an implicitly
sychronized points where we submit the I/O (that was my second argument
and that was plain wrong).

My first arguments about "overkill" were for async I/O and kiobufs, where
the race cannot trigger. Mainly for the kiobufs I/O I'm still not very
convinced.

> Oh, it does really need a
> 
> 	smp_mb_before_atomic_dec();
> 
> as part of the "put_bh()". On x86, this obviously is a no-op. And we
> actually need that one in general - not just for IO completion - as long
> as we consider the "atomic_dec(&bh->b_flags)" to "release" the buffer.
> 
> Andrea?

yes, agreed.

Andrea