dm snapshot: fix race during exception creation

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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>

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Mikulas Patocka 2008-07-21 12:00:34 +01:00 • committed by Alasdair G Kergon

parent cd45daffd1

commit a8d41b59f3

1 changed files with 28 additions and 0 deletions

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drivers/md/dm-snap.c

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