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Chuck Lever
2b84ff4da3
[ Upstream commit797943e8bd] The xprtrdma reply path has been the subject of recurring LLM-driven review claims that 'an RPC can complete while receive buffers are still DMA-mapped' or that 'the req can be freed while the HCA still owns the send buffer.' No runtime reproducer has surfaced, but the absence of a written-down invariant set lets each pass of automated review reach the same hypothetical conclusion. Subsequent fixes against ce2f9a4d9ccc ('xprtrdma: Decouple req recycling from RPC completion') closed the underlying races but did not document the closure where future readers will look for it. State the invariants explicitly in a comment above rpcrdma_reply_handler() and back four of them with WARN_ON_ONCE() probes positioned where each invariant is locally checkable on the previous patch's cleaned-up ownership state: - I1 (Receive WR ownership): WARN at rpcrdma_post_recvs() that a rep pulled from rb_free_reps carries rr_rqst == NULL. - I2 (rep attachment): WARN at rpcrdma_reply_put() that req->rl_reply was NULLed before the matching rep_put. - I3 (Registered-MR fence): WARN at rpcrdma_complete_rqst() that req->rl_registered is empty. Strong send-queue ordering of the LocalInv WR chain makes the last completion observe the ib_dma_unmap_sg() of every earlier MR, so 'list empty' implies 'all MRs unmapped'. - I4 (Send-buffer release): WARN at rpcrdma_req_release() that req->rl_sendctx is NULL. Reaching the kref release callback requires both the RPC-layer and Send-side references to have dropped; the Send-side drop runs in rpcrdma_sendctx_unmap(), which clears rl_sendctx (previous patch). A non-NULL rl_sendctx here would mean the Send-side owner had not run -- a contradiction. The XXX comment in xprt_rdma_free() about signal-driven release racing the Send completion described the pre-decouple state. Replace it with a one-line note pointing at the invariant set, since the kref scheme now holds the req across the in-flight Send regardless of which path released the rpc_task. I5 (req lifecycle) is stated in the comment but not probed: making it locally assertible would require moving kref_init out of rpcrdma_req_release(), which in turn requires adding kref_init to the bc_pa_list and backlog-wake reuse paths. That restructuring is deferred -- the invariant is unchanged either way. Signed-off-by: Chuck Lever <chuck.lever@oracle.com> Signed-off-by: Anna Schumaker <anna.schumaker@hammerspace.com> Stable-dep-of:234c0ff695("xprtrdma: Resize reply buffers before reposting receives") Signed-off-by: Sasha Levin <sashal@kernel.org>
Linux kernel
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