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only re-enable I/O when all reasons have cleared. sys/dev/xen/blkfront/block.h: In the block front driver softc, replace the boolean XBDF_FROZEN flag with a count of commands and driver global issues that freeze the I/O queue. So long xbd_qfrozen_cnt is non-zero, I/O is halted. Add flags to xbd_flags for tracking grant table entry and free command resource shortages. Each of these classes can increment xbd_qfrozen_cnt at most once. Add a command flag (XBDCF_ASYNC_MAPPING) that is set whenever the initial mapping attempt of a command fails with EINPROGRESS. sys/dev/xen/blkfront/blkfront.c: In xbd_queue_cb(), use new XBDCF_ASYNC_MAPPING flag to definitively know if an async bus dmamap load has occurred. Add xbd_freeze() and xbd_thaw() helper methods for managing xbd_qfrozen_cnt and use them to implement all queue freezing logic. Add missing "thaw" to restart I/O processing once grant references become available. Sponsored by: Spectra Logic Corporation
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