[Bugs] [Bug 1205186] RCU changes wrt peers to be done for GlusterFS-3.7.0

[bugzilla at redhat.com]

https://bugzilla.redhat.com/show_bug.cgi?id=1205186



--- Comment #11 from Anand Avati <aavati at redhat.com> ---
COMMIT: http://review.gluster.org/10147 committed in master by Krishnan
Parthasarathi (kparthas at redhat.com) 
------
commit 1efa50861b2cee68de9c9b751d9fc5eed08f5e5b
Author: Kaushal M <kaushal at redhat.com>
Date:   Thu Mar 26 15:18:54 2015 +0530

    glusterd: Replace transaction peers lists

    Transaction peer lists were used in GlusterD to peers belonging to a
    transaction. This was needed to prevent newly added peers performing
    partial transactions, which could be incorrect.

    This was accomplished by creating a seperate transaction peers list at
    the beginning of every transaction. A transaction peers list referenced
    the peerinfo data structures of the peers which were present at the
    beginning of the transaction. RCU protection of peerinfos referenced by
    the transaction peers list is a hard problem and difficult to do
    correctly.

    To have proper RCU protection of peerinfos, the transaction peers lists
    have been replaced by an alternative method to identify peers that
    belong to a transaction. The alternative method is to the global peers
    list along with generation numbers to identify peers that should belong
    to a transaction.

    This change introduces a global peer list generation number, and a
    generation number for each peerinfo object. Whenever a peerinfo object
    is created, the global generation number is bumped, and the peerinfos
    generation number is set to the bumped global generation.

    With the above changes, the algorithm to identify peers belonging to a
    transaction with RCU protection is as follows,
    - At the beginning of a transaction, the current global generation
      number is saved
    - To identify if a peers belonging to the transaction,
      - Start a RCU read critical section
      - For each peer in the global peers list,
        - If the peers generation number is not greater than the saved
          generation number, continue with the action on the peer
      - End the RCU read critical section

    The above algorithm guarantees that,
    - The peer list is not modified when a transaction is iterating through
      it
    - The transaction actions are only done on peers that were present when
      the transaction started

    But, as a transaction could iterate over the peers list multiple times,
    the algorithm cannot guarantee that same set of peers will be selected
    every time. A peer could get deleted between two iterations of the list
    within a transaction. This problem existed with transaction peers list
    as well, but unlike before now it will not lead to invalid memory access
    and potential crashes. This problem will be addressed seprately.

    This change was developed on the git branch at [1]. This commit is a
    combination of the following commits on the development branch.
      52ded5b Add timespec_cmp
      44aedd8 Add create timestamp to peerinfo
      7bcbea5 Fix some silly mistakes
      13e3241 Add start time to opinfo
      17a6727 Use timestamp comparisions to identify xaction peers instead
              of a xaction peer list
      3be05b6 Correct check for peerinfo age
      70d5b58 Use read-critical sections for peer list iteration
      ba4dbca Use peerinfo timestamp checks in op-sm instead of xaction peer
              list
      d63f811 Add more peer status checks when iterating peers list in
              glusterd-syncop
      1998a2a Timestamp based peer list traversal of mgmtv3 xactions
      f3c1a42 Remove transaction peer lists
      b8b08ee Remove unused labels
      32e5f5b Remove 'npeers' usage
      a075fb7 Remove 'npeers' from mgmt-v3 framework
      12c9df2 Use generation number instead of timestamps.
      9723021 Remove timespec_cmp
      80ae2c6 Remove timespec.h include
      a9479b0 Address review comments on 10147/4

    [1]: https://github.com/kshlm/glusterfs/tree/urcu

    Change-Id: I9be1033525c0a89276f5b5d83dc2eb061918b97f
    BUG: 1205186
    Signed-off-by: Kaushal M <kaushal at redhat.com>
    Reviewed-on: http://review.gluster.org/10147
    Tested-by: Gluster Build System <jenkins at build.gluster.com>
    Reviewed-by: Atin Mukherjee <amukherj at redhat.com>
    Reviewed-by: Anand Nekkunti <anekkunt at redhat.com>
    Reviewed-by: Krishnan Parthasarathi <kparthas at redhat.com>
    Tested-by: Krishnan Parthasarathi <kparthas at redhat.com>

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