[Bugs] [Bug 1333266] SMB:while running I/O on cifs mount and doing graph switch causes cifs mount to hang.

[bugzilla at redhat.com]

https://bugzilla.redhat.com/show_bug.cgi?id=1333266



--- Comment #2 from Vijay Bellur <vbellur at redhat.com> ---
COMMIT: http://review.gluster.org/14221 committed in release-3.8 by Niels de
Vos (ndevos at redhat.com) 
------
commit 938dfb9d021db20dc3b511b78ec8c137b8ff3e7c
Author: Poornima G <pgurusid at redhat.com>
Date:   Fri Apr 29 12:24:24 2016 -0400

    gfapi: Fix a deadlock caused by graph switch while aio in progress

    RCA:
    Currently async nature is achieved by submitting a syncop operation to
    synctask threads. Consider a scenario where the graph switch is triggered,
    the next write fop checks for the next available graph and sets
    fs->migration_in_progess and triggers the migration of fds and other
    things, which can cause some syncop_lookup operation. While this fop (on
    synctask thread) is waiting for syncop_lookup to return, lets say there
    are another 17 write async calls submitted, all these writes are blocked
    waiting for fs->migration_in_progress to be unset, hence all the 16
    synctask threads are blocked waiting for fs->migration_in_progress to be
    unset. Now the syncop_lookup returns, but there are no synctask threads to
    process the lookup_cbk. If this syncop_lookup doesn't return,
    then fs->migration_in_progress can not be unset by the first fop.
    Thus causing a deadlock.

    To fix this deadlock, changing all the async APIs to use STACK_WIND,
    instead of syntask to achieve async nature. glfs_preadv_async is already
    implemented using STACK_WIND, now changing all the other async APIs
    also to do the same.

    This patch as such will not reduce the performance of async IO, the only
    thing that can affect is that, in case of write, the buf passed by
    application is copied onto iobuf in the same thread wheras before it
    was being copied in synctask thread.

    Since, the syncop + graph switch logic (lock across fops) is not a good
    candidate for synctask, changing the async APIs to use STACK_WIND

    Backport of http://review.gluster.org/#/c/14148/

    Change-Id: Idf665cae0a8e27697fbfc5ec8d93a6d6bae3a4f1
    BUG: 1333266
    Signed-off-by: Poornima G <pgurusid at redhat.com>
    Reviewed-on: http://review.gluster.org/14221
    Smoke: Gluster Build System <jenkins at build.gluster.com>
    NetBSD-regression: NetBSD Build System <jenkins at build.gluster.org>
    CentOS-regression: Gluster Build System <jenkins at build.gluster.com>
    Reviewed-by: Raghavendra Talur <rtalur at redhat.com>
    Reviewed-by: Rajesh Joseph <rjoseph at redhat.com>
    Reviewed-by: Niels de Vos <ndevos at redhat.com>

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