tree f9435bed2d582e4cd3e91e4d6fb18a18f62aa019
parent cd45daffd1f7b53aac0835b23e97f814ec3f10dc
author Mikulas Patocka <mpatocka@redhat.com> 1216638034 +0100
committer Alasdair G Kergon <agk@redhat.com> 1216638034 +0100

dm snapshot: fix race during exception creation

Fix a race condition that returns incorrect data when a write causes an
exception to be allocated whilst a read is still in flight.

The race condition happens as follows:
* A read to non-reallocated sector in the snapshot is submitted so that the
  read is routed to the original device.
* A write to the original device is submitted. The write causes an exception
  that reallocates the block.  The write proceeds.
* The original read is dequeued and reads the wrong data.

This race can be triggered with CFQ scheduler and one thread writing and
multiple threads reading simultaneously.

(This patch relies upon the earlier dm-kcopyd-per-device.patch to avoid a
deadlock.)

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>