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Sriharsha (sriharsha.basavapatna@broadcom.com) reports an occasional double DMA unmap of an FRWR MR when a connection is lost. I see one way this can happen. When a request requires more than one segment or chunk, rpcrdma_marshal_req loops, invoking ->frwr_op_map for each segment (MR) in each chunk. Each call posts a FASTREG Work Request to register one MR. Now suppose that the transport connection is lost part-way through marshaling this request. As part of recovering and resetting that req, rpcrdma_marshal_req invokes ->frwr_op_unmap_safe, which hands all the req's registered FRWRs to the MR recovery thread. But note: FRWR registration is asynchronous. So it's possible that some of these "already registered" FRWRs are fully registered, and some are still waiting for their FASTREG WR to complete. When the connection is lost, the "already registered" frmrs are marked FRMR_IS_VALID, and the "still waiting" WRs flush. Then frwr_wc_fastreg marks these frmrs FRMR_FLUSHED_FR. But thanks to ->frwr_op_unmap_safe, the MR recovery thread is doing an unreg / alloc_mr, a DMA unmap, and marking each of these frwrs FRMR_IS_INVALID, at the same time frwr_wc_fastreg might be running. - If the recovery thread runs last, then the frmr is marked FRMR_IS_INVALID, and life continues. - If frwr_wc_fastreg runs last, the frmr is marked FRMR_FLUSHED_FR, but the recovery thread has already DMA unmapped that MR. When ->frwr_op_map later re-uses this frmr, it sees it is not marked FRMR_IS_INVALID, and tries to recover it before using it, resulting in a second DMA unmap of the same MR. The fix is to guarantee in-flight FASTREG WRs have flushed before MR recovery runs on those FRWRs. Thus we depend on ro_unmap_safe (called from xprt_rdma_send_request on retransmit, or from xprt_rdma_free) to clean up old registrations as needed. Reported-by: Sriharsha Basavapatna <sriharsha.basavapatna@broadcom.com> Signed-off-by: Chuck Lever <chuck.lever@oracle.com> Tested-by: Sriharsha Basavapatna <sriharsha.basavapatna@broadcom.com> Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com> |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.