On Sun, 16 Dec 2018, Gaël PORTAY wrote: > Dear kernel hackers, > > I faced a deadlock in CMA (using usb-storage and FAT) between two tasks that > want to allocate CMA. All task involved are in D-state. I am running 4.19.1 > mainline on an imx6q platform. > > My understanding of that issue is as follow: > > The first task gets in cma_alloc(), acquires the mutex, triggers page > migrations and yields. ... > The second task wants to writeback/flush the pages through USB, which, I > assume, is due to the page migration. The usb-storage triggers a CMA allocation > but get locked in cma_alloc since the first task hold the mutex (It is a FAT > formatted partition, if it helps). > > usb-storage D 0 349 2 0x00000000 > Backtrace: ... > [<bf1c7550>] (usb_sg_wait [usbcore]) from [<bf2bd618>] (usb_stor_bulk_transfer_sglist.part.2+0x80/0xdc [usb_storage]) r9:0001e000 r8:eca594ac r7:0001e000 r6:c0008200 r5:eca59514 r4:eca59488 It looks like there is a logical problem in the CMA allocator. The call in usb_sg_wait() specifies GFP_NOIO, which is supposed to prevent allocations from blocking on any I/O operations. Therefore we shouldn't be waiting for the CMA mutex. Perhaps the CMA allocator needs to drop the mutex while doing writebacks/flushes, or perhaps it needs to be reorganized some other way. I don't know anything about it. Does the CMA code have any maintainers who might need to know about this, or is it all handled by the MM maintainers? Alan Stern
