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List-Post: List-Help: List-Subscribe: , Cc: Qemu-block Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 7/29/19 5:51 PM, Paolo Bonzini wrote: > On 29/07/19 23:46, John Snow wrote: >>> @@ -111,15 +112,12 @@ static void dma_complete(DMAAIOCB *dbs, int ret) >>> { >>> trace_dma_complete(dbs, ret, dbs->common.cb); >>> >>> + assert(!dbs->acb && !dbs->bh); >>> dma_blk_unmap(dbs); >>> if (dbs->common.cb) { >>> dbs->common.cb(dbs->common.opaque, ret); >>> } >>> qemu_iovec_destroy(&dbs->iov); >>> - if (dbs->bh) { >>> - qemu_bh_delete(dbs->bh); >>> - dbs->bh = NULL; >>> - } >> >> Now presumably handled by dma_aio_cancel, > > No, it simply could never happen. dma_complete is called here in dma_blk_cb: > > dbs->acb = NULL; > dbs->offset += dbs->iov.size; > > if (dbs->sg_cur_index == dbs->sg->nsg || ret < 0) { > dma_complete(dbs, ret); > return; > } > > and the only way to reach that when dbs->bh becomes non-NULL is through > reschedule_dma, which clears dbs->bh before invoking dma_blk_cb. > >>> if (dbs->acb) { >>> + /* This will invoke dma_blk_cb. */ >> >> uhh, does it? > > Yes: > > /* Async version of aio cancel. The caller is not blocked if the acb implements > * cancel_async, otherwise we do nothing and let the request normally complete. > * In either case the completion callback must be called. */ > Right, right -- the comment can SAY anything it likes about what the "contract" is ... OK, so it's more as if EITHER the cancel callback will invoke dma_blk_cb, OR the acb object there will ... eventually ... through normal execution. OK, ok, ok. Getting it, slowly, slowly. I think this comment is confusing, actually -- because dma_blk_cb might not actually get invoked in the stack below this call. We only know it might. >> this is maybe where I got lost reading this code. >> Isn't dbs->acb going to be what was returned from e.g. >> dma_blk_read_io_func, which ultimately uses blk_aio_em_aiocb_info, that >> has no cancel callback? > > Right therefore the I/O will complete and the callback will be invoked. > >From the other email: ***oh***. >> Well, here at least I am now on terra-firma that we're going to call the >> original callback with ECANCELED, which is a step towards code that >> isn't surprising my sensibilities. > > Good. :) > > Paolo > OK, this seems right to me then; the last puzzle piece is that Fam added no-op returns for ECANCELED to the IDE originators of such DMA requests, but now that I see the pathways beneath here I think it'd be /never/ right to ignore them. If you cancel IDE's DMA out from under it, I think the IDE state machine ought to treat it as an error, yes. Thanks for the help, Paolo! --js