• Paolo Valente's avatar
    block, bfq: consider also in_service_entity to state whether an entity is active · 46d556e6
    Paolo Valente authored
    Groups of BFQ queues are represented by generic entities in BFQ. When
    a queue belonging to a parent entity is deactivated, the parent entity
    may need to be deactivated too, in case the deactivated queue was the
    only active queue for the parent entity. This deactivation may need to
    be propagated upwards if the entity belongs, in its turn, to a further
    higher-level entity, and so on. In particular, the upward propagation
    of deactivation stops at the first parent entity that remains active
    even if one of its child entities has been deactivated.
    To decide whether the last non-deactivation condition holds for a
    parent entity, BFQ checks whether the field next_in_service is still
    not NULL for the parent entity, after the deactivation of one of its
    child entity. If it is not NULL, then there are certainly other active
    entities in the parent entity, and deactivations can stop.
    Unfortunately, this check misses a corner case: if in_service_entity
    is not NULL, then next_in_service may happen to be NULL, although the
    parent entity is evidently active. This happens if: 1) the entity
    pointed by in_service_entity is the only active entity in the parent
    entity, and 2) according to the definition of next_in_service, the
    in_service_entity cannot be considered as next_in_service. See the
    comments on the definition of next_in_service for details on this
    second point.
    Hitting the above corner case causes crashes.
    To address this issue, this commit:
    1) Extends the above check on only next_in_service to controlling both
    next_in_service and in_service_entity (if any of them is not NULL,
    then no further deactivation is performed)
    2) Improves the (important) comments on how next_in_service is defined
    and updated; in particular it fixes a few rather obscure paragraphs
    Reported-by: default avatarEric Wheeler <bfq-sched@lists.ewheeler.net>
    Reported-by: default avatarRick Yiu <rick_yiu@htc.com>
    Reported-by: default avatarTom X Nguyen <tom81094@gmail.com>
    Signed-off-by: default avatarPaolo Valente <paolo.valente@linaro.org>
    Tested-by: default avatarEric Wheeler <bfq-sched@lists.ewheeler.net>
    Tested-by: default avatarRick Yiu <rick_yiu@htc.com>
    Tested-by: default avatarLaurentiu Nicola <lnicola@dend.ro>
    Tested-by: default avatarTom X Nguyen <tom81094@gmail.com>
    Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
bfq-wf2q.c 51 KB