• Paolo Valente's avatar
    block, bfq: reset in_service_entity if it becomes idle · 6ab1d8da
    Paolo Valente authored
    BFQ implements hierarchical scheduling by representing each group of
    queues with a generic parent entity. For each parent entity, BFQ
    maintains an in_service_entity pointer: if one of the child entities
    happens to be in service, in_service_entity points to it.  The
    resetting of these pointers happens only on queue expirations: when
    the in-service queue is expired, i.e., stops to be the queue in
    service, BFQ resets all in_service_entity pointers along the
    parent-entity path from this queue to the root entity.
    Functions handling the scheduling of entities assume, naturally, that
    in-service entities are active, i.e., have pending I/O requests (or,
    as a special case, even if they have no pending requests, they are
    expected to receive a new request very soon, with the scheduler idling
    the storage device while waiting for such an event). Unfortunately,
    the above resetting scheme of the in_service_entity pointers may cause
    this assumption to be violated.  For example, the in-service queue may
    happen to remain without requests because of a request merge. In this
    case the queue does become idle, and all related data structures are
    updated accordingly. But in_service_entity still points to the queue
    in the parent entity. This inconsistency may even propagate to
    higher-level parent entities, if they happen to become idle as well,
    as a consequence of the leaf queue becoming idle. For this queue and
    parent entities, scheduling functions have an undefined behaviour,
    and, as reported, may easily lead to kernel crashes or hangs.
    This commit addresses this issue by simply resetting the
    in_service_entity field also when it is detected to point to an entity
    becoming idle (regardless of why the entity becomes idle).
    Reported-by: default avatarLaurentiu Nicola <lnicola@dend.ro>
    Signed-off-by: default avatarPaolo Valente <paolo.valente@linaro.org>
    Tested-by: default avatarLaurentiu Nicola <lnicola@dend.ro>
    Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
bfq-wf2q.c 50.3 KB