Re: [PATCH] system/physmem: mark io_mem_unassigned lockless

[Peter Xu <peterx@redhat.com>]

On Wed, Oct 29, 2025 at 02:52:24PM -0400, Stefan Hajnoczi wrote:
> When the Bus Master bit is disabled in a PCI device's Command Register,
> the device's DMA address space becomes unassigned memory (i.e. the
> io_mem_unassigned MemoryRegion).
> 
> This can lead to deadlocks with IOThreads since io_mem_unassigned
> accesses attempt to acquire the Big QEMU Lock (BQL). For example,
> virtio-pci devices deadlock in virtio_write_config() ->
> virtio_pci_stop_ioeventfd() when waiting for the IOThread while holding
> the BQL. The IOThread is unable to acquire the BQL but the vcpu thread
> won't release the BQL while waiting for the IOThread.
> 
> io_mem_unassigned is trivially thread-safe since it has no state, it
> simply rejects all load/store accesses. Therefore it is safe to enable
> lockless I/O on io_mem_unassigned to eliminate this deadlock.
> 
> Here is the backtrace described above:
> 
>   Thread 9 (Thread 0x7fccfcdff6c0 (LWP 247832) "CPU 4/KVM"):
>   #0  0x00007fcd11529d46 in ppoll () from target:/lib64/libc.so.6
>   #1  0x000056468a1a9bad in ppoll (__fds=<optimized out>, __nfds=<optimized out>, __timeout=0x0, __ss=0x0) at /usr/include/bits/poll2.h:88
>   #2  0x000056468a18f9d9 in fdmon_poll_wait (ctx=0x5646c6a1dc30, ready_list=0x7fccfcdfb310, timeout=-1) at ../util/fdmon-poll.c:79
>   #3  0x000056468a18f14f in aio_poll (ctx=<optimized out>, blocking=blocking@entry=true) at ../util/aio-posix.c:730
>   #4  0x000056468a1ad842 in aio_wait_bh_oneshot (ctx=<optimized out>, cb=cb@entry=0x564689faa420 <virtio_blk_ioeventfd_stop_vq_bh>, opaque=<optimized out>) at ../util/aio-wait.c:85
>   #5  0x0000564689faaa89 in virtio_blk_stop_ioeventfd (vdev=0x5646c8fd7e90) at ../hw/block/virtio-blk.c:1644
>   #6  0x0000564689d77880 in virtio_bus_stop_ioeventfd (bus=bus@entry=0x5646c8fd7e08) at ../hw/virtio/virtio-bus.c:264
>   #7  0x0000564689d780db in virtio_bus_stop_ioeventfd (bus=bus@entry=0x5646c8fd7e08) at ../hw/virtio/virtio-bus.c:256
>   #8  0x0000564689d7d98a in virtio_pci_stop_ioeventfd (proxy=0x5646c8fcf8e0) at ../hw/virtio/virtio-pci.c:413
>   #9  virtio_write_config (pci_dev=0x5646c8fcf8e0, address=4, val=<optimized out>, len=<optimized out>) at ../hw/virtio/virtio-pci.c:803
>   #10 0x0000564689dcb45a in memory_region_write_accessor (mr=mr@entry=0x5646c6dc2d30, addr=3145732, value=value@entry=0x7fccfcdfb528, size=size@entry=2, shift=<optimized out>, mask=mask@entry=65535, attrs=...) at ../system/memory.c:491
>   #11 0x0000564689dcaeb0 in access_with_adjusted_size (addr=addr@entry=3145732, value=value@entry=0x7fccfcdfb528, size=size@entry=2, access_size_min=<optimized out>, access_size_max=<optimized out>, access_fn=0x564689dcb3f0 <memory_region_write_accessor>, mr=0x5646c6dc2d30, attrs=...) at ../system/memory.c:567
>   #12 0x0000564689dcb156 in memory_region_dispatch_write (mr=mr@entry=0x5646c6dc2d30, addr=addr@entry=3145732, data=<optimized out>, op=<optimized out>, attrs=attrs@entry=...) at ../system/memory.c:1554
>   #13 0x0000564689dd389a in flatview_write_continue_step (attrs=..., attrs@entry=..., buf=buf@entry=0x7fcd05b87028 "", mr_addr=3145732, l=l@entry=0x7fccfcdfb5f0, mr=0x5646c6dc2d30, len=2) at ../system/physmem.c:3266
>   #14 0x0000564689dd3adb in flatview_write_continue (fv=0x7fcadc0d8930, addr=3761242116, attrs=..., ptr=0xe0300004, len=2, mr_addr=<optimized out>, l=<optimized out>, mr=<optimized out>) at ../system/physmem.c:3296
>   #15 flatview_write (fv=0x7fcadc0d8930, addr=addr@entry=3761242116, attrs=attrs@entry=..., buf=buf@entry=0x7fcd05b87028, len=len@entry=2) at ../system/physmem.c:3327
>   #16 0x0000564689dd7191 in address_space_write (as=0x56468b433600 <address_space_memory>, addr=3761242116, attrs=..., buf=0x7fcd05b87028, len=2) at ../system/physmem.c:3447
>   #17 address_space_rw (as=0x56468b433600 <address_space_memory>, addr=3761242116, attrs=attrs@entry=..., buf=buf@entry=0x7fcd05b87028, len=2, is_write=<optimized out>) at ../system/physmem.c:3457
>   #18 0x0000564689ff1ef6 in kvm_cpu_exec (cpu=cpu@entry=0x5646c6dab810) at ../accel/kvm/kvm-all.c:3248
>   #19 0x0000564689ff32f5 in kvm_vcpu_thread_fn (arg=arg@entry=0x5646c6dab810) at ../accel/kvm/kvm-accel-ops.c:53
>   #20 0x000056468a19225c in qemu_thread_start (args=0x5646c6db6190) at ../util/qemu-thread-posix.c:393
>   #21 0x00007fcd114c5b68 in start_thread () from target:/lib64/libc.so.6
>   #22 0x00007fcd115364e4 in clone () from target:/lib64/libc.so.6
> 
>   Thread 3 (Thread 0x7fcd0503a6c0 (LWP 247825) "IO iothread1"):
>   #0  0x00007fcd114c2d30 in __lll_lock_wait () from target:/lib64/libc.so.6
>   #1  0x00007fcd114c8fe2 in pthread_mutex_lock@@GLIBC_2.2.5 () from target:/lib64/libc.so.6
>   #2  0x000056468a192538 in qemu_mutex_lock_impl (mutex=0x56468b432e60 <bql>, file=0x56468a1e26a5 "../system/physmem.c", line=3198) at ../util/qemu-thread-posix.c:94
>   #3  0x0000564689dc12e2 in bql_lock_impl (file=file@entry=0x56468a1e26a5 "../system/physmem.c", line=line@entry=3198) at ../system/cpus.c:566
>   #4  0x0000564689ddc151 in prepare_mmio_access (mr=0x56468b433800 <io_mem_unassigned>) at ../system/physmem.c:3198
>   #5  address_space_lduw_internal_cached_slow (cache=<optimized out>, addr=2, attrs=..., result=0x0, endian=DEVICE_LITTLE_ENDIAN) at ../system/memory_ldst.c.inc:211
>   #6  address_space_lduw_le_cached_slow (cache=<optimized out>, addr=addr@entry=2, attrs=attrs@entry=..., result=result@entry=0x0) at ../system/memory_ldst.c.inc:253
>   #7  0x0000564689fd692c in address_space_lduw_le_cached (result=0x0, cache=<optimized out>, addr=2, attrs=...) at /var/tmp/qemu/include/exec/memory_ldst_cached.h.inc:35
>   #8  lduw_le_phys_cached (cache=<optimized out>, addr=2) at /var/tmp/qemu/include/exec/memory_ldst_phys.h.inc:66
>   #9  virtio_lduw_phys_cached (vdev=<optimized out>, cache=<optimized out>, pa=2) at /var/tmp/qemu/include/hw/virtio/virtio-access.h:166
>   #10 vring_avail_idx (vq=0x5646c8fe2470) at ../hw/virtio/virtio.c:396
>   #11 virtio_queue_split_set_notification (vq=0x5646c8fe2470, enable=0) at ../hw/virtio/virtio.c:534
>   #12 virtio_queue_set_notification (vq=0x5646c8fe2470, enable=0) at ../hw/virtio/virtio.c:595
>   #13 0x000056468a18e7a8 in poll_set_started (ctx=ctx@entry=0x5646c6c74e30, ready_list=ready_list@entry=0x7fcd050366a0, started=started@entry=true) at ../util/aio-posix.c:247
>   #14 0x000056468a18f2bb in poll_set_started (ctx=0x5646c6c74e30, ready_list=0x7fcd050366a0, started=true) at ../util/aio-posix.c:226
>   #15 try_poll_mode (ctx=0x5646c6c74e30, ready_list=0x7fcd050366a0, timeout=<synthetic pointer>) at ../util/aio-posix.c:612
>   #16 aio_poll (ctx=0x5646c6c74e30, blocking=blocking@entry=true) at ../util/aio-posix.c:689
>   #17 0x000056468a032c26 in iothread_run (opaque=opaque@entry=0x5646c69f3380) at ../iothread.c:63
>   #18 0x000056468a19225c in qemu_thread_start (args=0x5646c6c75410) at ../util/qemu-thread-posix.c:393
>   #19 0x00007fcd114c5b68 in start_thread () from target:/lib64/libc.so.6
>   #20 0x00007fcd115364e4 in clone () from target:/lib64/libc.so.6
> 
> Buglink: https://issues.redhat.com/browse/RHEL-71933
> Reported-by: Peixiu Hou <phou@redhat.com>
> Cc: Kevin Wolf <kwolf@redhat.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

queued, thanks!

-- 
Peter Xu