Issue 1606: Fixed race condition during expansion of concurrent open hash maps #1607
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sijie
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Aug 16, 2018
eolivelli
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merlimat
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Aug 16, 2018
… hash maps ### Motivation As reported in #1606, there is a race condition in the concurrent open hash maps implementation. The race happens when the maps gets re-hashed after the expansion and the new arrays are substituting the old ones. The race itself is that a thread doing a `get()` on the map is first checking the current `capacity` of the map, uses that to get the bucket and then tries to do optimistic read of the value in that bucket. This assumes `capacity` update is visible only after the `values` array is already swapped, but that is not always the case in current code. ### Changes * Use `volatile` qualifier for `capacity` and `values` arrays to ensure ordering of memory read is respected by compiler * In rehashing, update `capacity` after `values` Author: Matteo Merli <mmerli@apache.org> Reviewers: Enrico Olivelli <eolivelli@gmail.com>, Sijie Guo <sijie@apache.org> This closes #1607 from merlimat/fix-concurrent-maps, closes #1606 (cherry picked from commit a7e66e1) Signed-off-by: Matteo Merli <mmerli@apache.org>
merlimat
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… hash maps ### Motivation As reported in #1606, there is a race condition in the concurrent open hash maps implementation. The race happens when the maps gets re-hashed after the expansion and the new arrays are substituting the old ones. The race itself is that a thread doing a `get()` on the map is first checking the current `capacity` of the map, uses that to get the bucket and then tries to do optimistic read of the value in that bucket. This assumes `capacity` update is visible only after the `values` array is already swapped, but that is not always the case in current code. ### Changes * Use `volatile` qualifier for `capacity` and `values` arrays to ensure ordering of memory read is respected by compiler * In rehashing, update `capacity` after `values` Author: Matteo Merli <mmerli@apache.org> Reviewers: Enrico Olivelli <eolivelli@gmail.com>, Sijie Guo <sijie@apache.org> This closes #1607 from merlimat/fix-concurrent-maps, closes #1606 (cherry picked from commit a7e66e1) Signed-off-by: Matteo Merli <mmerli@apache.org>
sijie
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Aug 17, 2018
…2387) ### Motivation Porting same fix as apache/bookkeeper#1607 to correct issue reported on apache/bookkeeper#1606. There is a race condition in the concurrent open hash maps implementation. The race happens when the maps gets re-hashed after the expansion and the new arrays are substituting the old ones. The race itself is that a thread doing a `get()` on the map is first checking the current capacity of the map, uses that to get the bucket and then tries to do optimistic read of the value in that bucket. This assumes `capacity` update is visible only after the `values` array is already swapped, but that is not always the case in current code. ### Changes * Use `volatile` qualifier for `capacity` and `values` arrays to ensure ordering of memory read is respected by compiler * In rehashing, update `capacity` after `values` where it was not already the case
sijie
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Aug 27, 2018
…2387) ### Motivation Porting same fix as apache/bookkeeper#1607 to correct issue reported on apache/bookkeeper#1606. There is a race condition in the concurrent open hash maps implementation. The race happens when the maps gets re-hashed after the expansion and the new arrays are substituting the old ones. The race itself is that a thread doing a `get()` on the map is first checking the current capacity of the map, uses that to get the bucket and then tries to do optimistic read of the value in that bucket. This assumes `capacity` update is visible only after the `values` array is already swapped, but that is not always the case in current code. ### Changes * Use `volatile` qualifier for `capacity` and `values` arrays to ensure ordering of memory read is respected by compiler * In rehashing, update `capacity` after `values` where it was not already the case
reddycharan
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Oct 17, 2018
…t open hash maps ### Motivation As reported in apache#1606, there is a race condition in the concurrent open hash maps implementation. The race happens when the maps gets re-hashed after the expansion and the new arrays are substituting the old ones. The race itself is that a thread doing a `get()` on the map is first checking the current `capacity` of the map, uses that to get the bucket and then tries to do optimistic read of the value in that bucket. This assumes `capacity` update is visible only after the `values` array is already swapped, but that is not always the case in current code. ### Changes * Use `volatile` qualifier for `capacity` and `values` arrays to ensure ordering of memory read is respected by compiler * In rehashing, update `capacity` after `values` Author: Matteo Merli <mmerli@apache.org> Reviewers: Enrico Olivelli <eolivelli@gmail.com>, Sijie Guo <sijie@apache.org> This closes apache#1607 from merlimat/fix-concurrent-maps, closes apache#1606 (cherry picked from commit a7e66e1) Signed-off-by: Matteo Merli <mmerli@apache.org> (cherry picked from commit 36edaca) Signed-off-by: JV Jujjuri <vjujjuri@salesforce.com>
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Motivation
As reported in #1606, there is a race condition in the concurrent open hash maps implementation. The race happens when the maps gets re-hashed after the expansion and the new arrays are substituting the old ones.
The race itself is that a thread doing a
get()on the map is first checking the currentcapacityof the map, uses that to get the bucket and then tries to do optimistic read of the value in that bucket.This assumes
capacityupdate is visible only after thevaluesarray is already swapped, but that is not always the case in current code.Changes
volatilequalifier forcapacityandvaluesarrays to ensure ordering of memory read is respected by compilercapacityaftervalues