On 4/4/18 10:35 AM, Ming Lei wrote: > This patch orders getting budget and driver tag by making sure to acquire > driver tag after budget is got, this way can help to avoid the following > race: > > 1) before dispatch request from scheduler queue, get one budget first, then > dequeue a request, call it request A. > > 2) in another IO path for dispatching request B which is from hctx->dispatch, > driver tag is got, then try to get budget in blk_mq_dispatch_rq_list(), > unfortunately the budget is held by request A. > > 3) meantime blk_mq_dispatch_rq_list() is called for dispatching request > A, and try to get driver tag first, unfortunately no driver tag is > available because the driver tag is held by request B > > 4) both two IO pathes can't move on, and IO stall is caused. > > This issue can be observed when running dbench on USB storage.
Good catch, this can trigger on anything potentially, but of course more likely with limited budget and/or tag space. Classic ABBA deadlock. -- Jens Axboe
