[Bugs] [Bug 1365821] IO ERROR when multiple graph switches

[bugzilla at redhat.com]

https://bugzilla.redhat.com/show_bug.cgi?id=1365821



--- Comment #2 from Vijay Bellur <vbellur at redhat.com> ---
COMMIT: http://review.gluster.org/15134 committed in release-3.8 by Rajesh
Joseph (rjoseph at redhat.com) 
------
commit 26a0b60a01cebda3e88406dd8e13b1b9ed136108
Author: Poornima G <pgurusid at redhat.com>
Date:   Tue Jul 19 15:20:09 2016 +0530

    gfapi: Fix IO error caused when there is consecutive graph switches

    Backport of http://review.gluster.org/#/c/14722/

    This is part 2 of the fix, the part 1 can be found at:
    http://review.gluster.org/#/c/14656/

    Problem:
    =======
    Consider a race between, __glfs_active_subvol() and graph_setup().
    Lets say @TIME T1:
    fs->active_subvol = A
    fs->next_subvol = B
    __glfs_active_subvol()                //under lock fs->mutex
    {
      ....
      new_subvol = fs->next_subvol       //which is B
      ....                               //Start migration from A to B
      __glfs_first_lookup(){
         ....
         unlock fs->mutex                //@TIME T2
         network fop
         lock fs->mutex
         ....
      }
      ....                                //migration continue on B
      fs->active_subvol = fs->next_subvol //which is C (explained below)
      ....
    }

    @Time T2, lets say in another thread, graph_setup() is called with C,
    note that at T2, fs->mutex is unlocked.

    graph_stup(C...)
    {
      lock fs->mutex
      ....
      if (fs->next_subvol)                // which is B
          destroy subvol (fs->next_subvol)
      ....
      fs->next_subvol = C
      ....
      unlock fs->mutex
    }

    Thus at the end of this,
    fs->old_subvol = A;
    fs->active_subvol = C;
    fs->next_subvol = NULL;
    which is wrong, as B completed migration, but was destroyed by
    graph_setup, and C never was migrated.

    Solution:
    =========
    Any new graph can be in one of the 2 states:
    - Picked for migration, migration in progress (fs->mip_subvol)
    - Not picked so far for migration (fs->next_subvol)
    graph_setup() updates fs->next_subvol only, __glfs_active_subvol()
    moves fs->next_subvol to fs->mip_subvol and fs->next_subvol = NULL
    atomically, and then once the migration is complete, make that the
    fs->active_subvol

    Change-Id: Ib6ff0565105c5eedb912a43da4017cd413243612
    BUG: 1365821
    Signed-off-by: Poornima G <pgurusid at redhat.com>
    Reviewed-on: http://review.gluster.org/15134
    Smoke: Gluster Build System <jenkins at build.gluster.org>
    CentOS-regression: Gluster Build System <jenkins at build.gluster.org>
    NetBSD-regression: NetBSD Build System <jenkins at build.gluster.org>
    Reviewed-by: Rajesh Joseph <rjoseph at redhat.com>

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