performance/read-ahead: prevent stale data being returned to application.

Assume that fd is shared by two application threads/processes.

T0 read is triggered from app-thread t1 and read call passes through
   write-behind.
T1 app-thread t2 issues a write. The page on which read from t1 is
   waiting is marked stale
T2 write-behind caches write and indicates to application as write
   complete.
T3 app-thread t2 issues read to same region. Since, there is already a
   page for that region (created as part of read at T0), this read
   request waits on that page to be filled (though it is stale, which
   is a bug).
T4 read (triggered at T0) completes from brick (with write still
   pending). Now both read requests from t1 and t2 are served this data
   (though data is stale from app-thread t2's perspective - which is a
   bug)
T5 write is flushed to brick by write-behind.

Fix is to not to serve data from a stale page, but instead initiate a
fresh read to back-end.

>Change-Id: Id6af733464fa41bb4e81fd29c7451c73d06453fb
>BUG: 1414242
>Signed-off-by: Raghavendra G <rgowdapp@redhat.com>
>Reviewed-on: https://review.gluster.org/7447
>Smoke: Gluster Build System <jenkins@build.gluster.org>
>CentOS-regression: Gluster Build System <jenkins@build.gluster.org>
>Reviewed-by: Csaba Henk <csaba@redhat.com>
>NetBSD-regression: NetBSD Build System <jenkins@build.gluster.org>
>Reviewed-by: Zhou Zhengping <johnzzpcrystal@gmail.com>
>Reviewed-by: Amar Tumballi <amarts@redhat.com>

(cherry picked from commit 2ff39c5cbea6fbda0d7a442f55e6dc2a72efb171)
Change-Id: Id6af733464fa41bb4e81fd29c7451c73d06453fb
BUG: 1449314
Signed-off-by: Raghavendra G <rgowdapp@redhat.com>
Reviewed-on: https://review.gluster.org/17223
NetBSD-regression: NetBSD Build System <jenkins@build.gluster.org>
CentOS-regression: Gluster Build System <jenkins@build.gluster.org>
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Smoke: Gluster Build System <jenkins@build.gluster.org>

0 files changed, 0 insertions, 0 deletions