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Implement a proper rw_tryupgrade #554
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@tuxoko could you open a dummy zfs PR so we can get a test run of this patch. That should give us some build coverage for a few architectures and kernels, plus some functional testing. That said, this is a quite a bit smaller than I was expecting it to be which is a nice surprise. Maybe this won't be as fragile as I expected. |
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As I alluded to in openzfs/zfs#4692 my only real concern was non-x86 architectures (ARM mainly) and where they are and may be going with their implementations. |
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While it may seems that many archs have their own implementations. Actually, they are just optimization for the fast path. They all share the same structure and same slow path. So they should all be logically the same. |
Current rw_tryupgrade does rw_exit and then rw_tryenter(RW_RWITER), and then does rw_enter(RW_READER) if it fails. This violate the assumption that rw_tryupgrade should be atomic and could cause extra contention or even lock inversion. This patch we implement a proper rw_tryupgrade. For rwsem-spinlock, we take the spinlock to check rwsem->count and rwsem->wait_list. For normal rwsem, we use cmpxchg on rwsem->count to change the value from single reader to single writer. Signed-off-by: Chunwei Chen <[email protected]>
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update: fix splat use RWSEM_COUNT |
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This LGTM. I'm OK with supporting this approach at least until it's no longer maintainable for some reason. |
nedbass
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Current rw_tryupgrade does rw_exit and then rw_tryenter(RW_RWITER), and then does rw_enter(RW_READER) if it fails. This violate the assumption that rw_tryupgrade should be atomic and could cause extra contention or even lock inversion. This patch we implement a proper rw_tryupgrade. For rwsem-spinlock, we take the spinlock to check rwsem->count and rwsem->wait_list. For normal rwsem, we use cmpxchg on rwsem->count to change the value from single reader to single writer. Signed-off-by: Chunwei Chen <[email protected]> Signed-off-by: Brian Behlendorf <[email protected]> Signed-off-by: Tim Chase <[email protected]> Closes openzfs/zfs#4692 Closes openzfs#554
tuxoko
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Sep 8, 2016
Current rw_tryupgrade does rw_exit and then rw_tryenter(RW_RWITER), and then does rw_enter(RW_READER) if it fails. This violate the assumption that rw_tryupgrade should be atomic and could cause extra contention or even lock inversion. This patch we implement a proper rw_tryupgrade. For rwsem-spinlock, we take the spinlock to check rwsem->count and rwsem->wait_list. For normal rwsem, we use cmpxchg on rwsem->count to change the value from single reader to single writer. Signed-off-by: Chunwei Chen <[email protected]> Signed-off-by: Brian Behlendorf <[email protected]> Signed-off-by: Tim Chase <[email protected]> Closes openzfs/zfs#4692 Closes openzfs#554
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Current rw_tryupgrade does rw_exit and then rw_tryenter(RW_RWITER), and then
does rw_enter(RW_READER) if it fails. This violate the assumption that
rw_tryupgrade should be atomic and could cause extra contention or even lock
inversion.
This patch we implement a proper rw_tryupgrade. For rwsem-spinlock, we take
the spinlock to check rwsem->count and rwsem->wait_list. For normal rwsem, we
use cmpxchg on rwsem->count to change the value from single reader to single
writer.
Signed-off-by: Chunwei Chen [email protected]