On Fri, Jun 29, 2018 at 03:40:50PM +0300, Denis Plotnikov wrote: > There are cases when a request to a block driver state shouldn't have > appeared producing dangerous race conditions. > This misbehaviour is usually happens with storage devices emulated > without eventfd for guest to host notifications like IDE. > > The issue arises when the context is in the "drained" section > and doesn't expect the request to come, but request comes from the > device not using iothread and which context is processed by the main loop. > > The main loop apart of the iothread event loop isn't blocked by the > "drained" section. > The request coming and processing while in "drained" section can spoil the > block driver state consistency. > > This behavior can be observed in the following KVM-based case: > > 1. Setup a VM with an IDE disk. > 2. Inside a VM start a disk writing load for the IDE device > e.g: dd if=<file> of=<file> bs=X count=Y oflag=direct > 3. On the host create a mirroring block job for the IDE device > e.g: drive_mirror <your_IDE> <your_path> > 4. On the host finish the block job > e.g: block_job_complete <your_IDE> > > Having done the 4th action, you could get an assert: > assert(QLIST_EMPTY(&bs->tracked_requests)) from mirror_run. > On my setup, the assert is 1/3 reproducible. > > The patch series introduces the mechanism to postpone the requests > until the BDS leaves "drained" section for the devices not using iothreads. > Also, it modifies the asynchronous block backend infrastructure to use > that mechanism to release the assert bug for IDE devices.
I don't understand the scenario. IDE emulation runs in the vcpu and main loop threads. These threads hold the global mutex when executing QEMU code. If thread A is in a drained region with the global mutex, then thread B cannot run QEMU code since it would need to global mutex. So I guess the problem is not that thread B will submit new requests, but maybe that the IDE DMA code will run a completion in thread A and submit another request in the drained region? Stefan
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