From: Alberto Garcia <be...@igalia.com> In 27ccdd52598290f0f8b58be56e235aff7aebfaf3 the throttling fields were moved from BlockDriverState to BlockBackend. However in a few cases the code started using throttling fields from the active BlockBackend instead of the round-robin token, making the algorithm behave incorrectly.
This can cause starvation if there's a throttling group with several drives but only one of them has I/O. Cc: qemu-sta...@nongnu.org Reported-by: Paolo Bonzini <pbonz...@redhat.com> Signed-off-by: Alberto Garcia <be...@igalia.com> Reviewed-by: Paolo Bonzini <pbonz...@redhat.com> Signed-off-by: Kevin Wolf <kw...@redhat.com> --- block/throttle-groups.c | 27 +++++++++++++++++++++++---- 1 file changed, 23 insertions(+), 4 deletions(-) diff --git a/block/throttle-groups.c b/block/throttle-groups.c index 59545e2..17b2efb 100644 --- a/block/throttle-groups.c +++ b/block/throttle-groups.c @@ -168,6 +168,22 @@ static BlockBackend *throttle_group_next_blk(BlockBackend *blk) return blk_by_public(next); } +/* + * Return whether a BlockBackend has pending requests. + * + * This assumes that tg->lock is held. + * + * @blk: the BlockBackend + * @is_write: the type of operation (read/write) + * @ret: whether the BlockBackend has pending requests. + */ +static inline bool blk_has_pending_reqs(BlockBackend *blk, + bool is_write) +{ + const BlockBackendPublic *blkp = blk_get_public(blk); + return blkp->pending_reqs[is_write]; +} + /* Return the next BlockBackend in the round-robin sequence with pending I/O * requests. * @@ -188,7 +204,7 @@ static BlockBackend *next_throttle_token(BlockBackend *blk, bool is_write) /* get next bs round in round robin style */ token = throttle_group_next_blk(token); - while (token != start && !blkp->pending_reqs[is_write]) { + while (token != start && !blk_has_pending_reqs(token, is_write)) { token = throttle_group_next_blk(token); } @@ -196,10 +212,13 @@ static BlockBackend *next_throttle_token(BlockBackend *blk, bool is_write) * then decide the token is the current bs because chances are * the current bs get the current request queued. */ - if (token == start && !blkp->pending_reqs[is_write]) { + if (token == start && !blk_has_pending_reqs(token, is_write)) { token = blk; } + /* Either we return the original BB, or one with pending requests */ + assert(token == blk || blk_has_pending_reqs(token, is_write)); + return token; } @@ -257,7 +276,7 @@ static void schedule_next_request(BlockBackend *blk, bool is_write) /* Check if there's any pending request to schedule next */ token = next_throttle_token(blk, is_write); - if (!blkp->pending_reqs[is_write]) { + if (!blk_has_pending_reqs(token, is_write)) { return; } @@ -271,7 +290,7 @@ static void schedule_next_request(BlockBackend *blk, bool is_write) qemu_co_queue_next(&blkp->throttled_reqs[is_write])) { token = blk; } else { - ThrottleTimers *tt = &blkp->throttle_timers; + ThrottleTimers *tt = &blk_get_public(token)->throttle_timers; int64_t now = qemu_clock_get_ns(tt->clock_type); timer_mod(tt->timers[is_write], now + 1); tg->any_timer_armed[is_write] = true; -- 1.8.3.1