Messages in this thread | | | Date | Wed, 24 Nov 1999 23:34:47 +0100 (CET) | From | Andrea Arcangeli <> | Subject | Re: spin_unlock optimization(i386) |
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On Wed, 24 Nov 1999, Manfred Spraul wrote:
>What's the definition of a read memory barrier?
That's not the point. What you should tell me is that I can't reimplement rmb() without a lock on the bus because the minium common divisor of a spin_unlock() in pseudocode is not like in the Alpha implementation:
mb(); spinlock.lock = 0;
as I basically stated (I was thinking wrong) in my previous email.
On Alpha the spin_unlock can't be more finegrined because there's no way you can _only_ avoid memory accesses before the barrier to be executed after the barrier and letting accesses after the barrier to be executed before the barrier.
I didn't understood well what IA32 enforces in a write: I thought the write was enforcing order in both directions (up and down).
>--------- >AFAICS, the rmb() during set_current_state() is still required:
Not only the rmb() as it's common code (on i386 the rmb would be enough of course). mb() is necessary instead.
BTW, In the example you provide below moving add_wait_queue in the middle would remove the need of the barrier as an add_wait_queue executes both the spin_lock and spin_unlock so it will sobstitute an mb() no matter how the spin_lock/unlock are implemented.
>__wait_on_inode() >add_wait_queue(); >current->state=TASK_UNINTERRUPTIBLE; >if(inode->i_state & I_LOCK) { > schedule();
Thanks.
Andrea
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