From abfcd2760b3e70727bbc0792221b8b98a733dc32 Mon Sep 17 00:00:00 2001 From: Stefan Hajnoczi Date: Tue, 21 Feb 2023 16:22:17 -0500 Subject: [PATCH] dma-helpers: prevent dma_blk_cb() vs dma_aio_cancel() race dma_blk_cb() only takes the AioContext lock around ->io_func(). That means the rest of dma_blk_cb() is not protected. In particular, the DMAAIOCB field accesses happen outside the lock. There is a race when the main loop thread holds the AioContext lock and invokes scsi_device_purge_requests() -> bdrv_aio_cancel() -> dma_aio_cancel() while an IOThread executes dma_blk_cb(). The dbs->acb field determines how cancellation proceeds. If dma_aio_cancel() sees dbs->acb == NULL while dma_blk_cb() is still running, the request can be completed twice (-ECANCELED and the actual return value). The following assertion can occur with virtio-scsi when an IOThread is used: ../hw/scsi/scsi-disk.c:368: scsi_dma_complete: Assertion `r->req.aiocb != NULL' failed. Fix the race by holding the AioContext across dma_blk_cb(). Now dma_aio_cancel() under the AioContext lock will not see inconsistent/intermediate states. Cc: Paolo Bonzini Reviewed-by: Eric Blake Signed-off-by: Stefan Hajnoczi Message-Id: <20230221212218.1378734-3-stefanha@redhat.com> Signed-off-by: Kevin Wolf --- hw/scsi/scsi-disk.c | 4 +--- softmmu/dma-helpers.c | 12 +++++++----- 2 files changed, 8 insertions(+), 8 deletions(-) diff --git a/hw/scsi/scsi-disk.c b/hw/scsi/scsi-disk.c index 115584f8b9..97c9b1c8cd 100644 --- a/hw/scsi/scsi-disk.c +++ b/hw/scsi/scsi-disk.c @@ -354,13 +354,12 @@ done: scsi_req_unref(&r->req); } +/* Called with AioContext lock held */ static void scsi_dma_complete(void *opaque, int ret) { SCSIDiskReq *r = (SCSIDiskReq *)opaque; SCSIDiskState *s = DO_UPCAST(SCSIDiskState, qdev, r->req.dev); - aio_context_acquire(blk_get_aio_context(s->qdev.conf.blk)); - assert(r->req.aiocb != NULL); r->req.aiocb = NULL; @@ -370,7 +369,6 @@ static void scsi_dma_complete(void *opaque, int ret) block_acct_done(blk_get_stats(s->qdev.conf.blk), &r->acct); } scsi_dma_complete_noio(r, ret); - aio_context_release(blk_get_aio_context(s->qdev.conf.blk)); } static void scsi_read_complete_noio(SCSIDiskReq *r, int ret) diff --git a/softmmu/dma-helpers.c b/softmmu/dma-helpers.c index 7820fec54c..2463964805 100644 --- a/softmmu/dma-helpers.c +++ b/softmmu/dma-helpers.c @@ -113,17 +113,19 @@ static void dma_complete(DMAAIOCB *dbs, int ret) static void dma_blk_cb(void *opaque, int ret) { DMAAIOCB *dbs = (DMAAIOCB *)opaque; + AioContext *ctx = dbs->ctx; dma_addr_t cur_addr, cur_len; void *mem; trace_dma_blk_cb(dbs, ret); + aio_context_acquire(ctx); dbs->acb = NULL; dbs->offset += dbs->iov.size; if (dbs->sg_cur_index == dbs->sg->nsg || ret < 0) { dma_complete(dbs, ret); - return; + goto out; } dma_blk_unmap(dbs); @@ -164,9 +166,9 @@ static void dma_blk_cb(void *opaque, int ret) if (dbs->iov.size == 0) { trace_dma_map_wait(dbs); - dbs->bh = aio_bh_new(dbs->ctx, reschedule_dma, dbs); + dbs->bh = aio_bh_new(ctx, reschedule_dma, dbs); cpu_register_map_client(dbs->bh); - return; + goto out; } if (!QEMU_IS_ALIGNED(dbs->iov.size, dbs->align)) { @@ -174,11 +176,11 @@ static void dma_blk_cb(void *opaque, int ret) QEMU_ALIGN_DOWN(dbs->iov.size, dbs->align)); } - aio_context_acquire(dbs->ctx); dbs->acb = dbs->io_func(dbs->offset, &dbs->iov, dma_blk_cb, dbs, dbs->io_func_opaque); - aio_context_release(dbs->ctx); assert(dbs->acb); +out: + aio_context_release(ctx); } static void dma_aio_cancel(BlockAIOCB *acb)