Re: [Qemu-block] [PATCH v2 1/2] mirror: fix dead-lock

[Max Reitz]

On 29.11.18 11:18, Vladimir Sementsov-Ogievskiy wrote:
> Let start from the beginning:
> 
> Commit b9e413dd375 (in 2.9)
> "block: explicitly acquire aiocontext in aio callbacks that need it"
> added pairs of aio_context_acquire/release to mirror_write_complete and
> mirror_read_complete, when they were aio callbacks for blk_aio_* calls.
> 
> Then, commit 2e1990b26e5 (in 3.0) "block/mirror: Convert to coroutines"
> dropped these blk_aio_* calls, than mirror_write_complete and
> mirror_read_complete are not callbacks more, and don't need additional
> aiocontext acquiring.

Yes, that alone makes enough sense to me.

> Furthermore, mirror_read_complete calls
> blk_co_pwritev inside these pair of aio_context_acquire/release, which
> leads to the following dead-lock with mirror:
> 
>  (gdb) info thr
>    Id   Target Id         Frame
>    3    Thread (LWP 145412) "qemu-system-x86" syscall ()
>    2    Thread (LWP 145416) "qemu-system-x86" __lll_lock_wait ()
>  * 1    Thread (LWP 145411) "qemu-system-x86" __lll_lock_wait ()
> 
>  (gdb) bt
>  #0  __lll_lock_wait ()
>  #1  _L_lock_812 ()
>  #2  __GI___pthread_mutex_lock
>  #3  qemu_mutex_lock_impl (mutex=0x561032dce420 <qemu_global_mutex>,
>      file=0x5610327d8654 "util/main-loop.c", line=236) at
>      util/qemu-thread-posix.c:66
>  #4  qemu_mutex_lock_iothread_impl
>  #5  os_host_main_loop_wait (timeout=480116000) at util/main-loop.c:236
>  #6  main_loop_wait (nonblocking=0) at util/main-loop.c:497
>  #7  main_loop () at vl.c:1892
>  #8  main
> 
> Printing contents of qemu_global_mutex, I see that "__owner = 145416",
> so, thr1 is main loop, and now it wants BQL, which is owned by thr2.
> 
>  (gdb) thr 2
>  (gdb) bt
>  #0  __lll_lock_wait ()
>  #1  _L_lock_870 ()
>  #2  __GI___pthread_mutex_lock
>  #3  qemu_mutex_lock_impl (mutex=0x561034d25dc0, ...
>  #4  aio_context_acquire (ctx=0x561034d25d60)
>  #5  dma_blk_cb
>  #6  dma_blk_io
>  #7  dma_blk_read
>  #8  ide_dma_cb
>  #9  bmdma_cmd_writeb
>  #10 bmdma_write
>  #11 memory_region_write_accessor
>  #12 access_with_adjusted_size
>  #15 flatview_write
>  #16 address_space_write
>  #17 address_space_rw
>  #18 kvm_handle_io
>  #19 kvm_cpu_exec
>  #20 qemu_kvm_cpu_thread_fn
>  #21 qemu_thread_start
>  #22 start_thread
>  #23 clone ()
> 
> Printing mutex in fr 2, I see "__owner = 145411", so thr2 wants aio
> context mutex, which is owned by thr1. Classic dead-lock.
> 
> Then, let's check that aio context is hold by mirror coroutine: just
> print coroutine stack of first tracked request in mirror job target:
> 
>  (gdb) [...]
>  (gdb) qemu coroutine 0x561035dd0860
>  #0  qemu_coroutine_switch
>  #1  qemu_coroutine_yield
>  #2  qemu_co_mutex_lock_slowpath
>  #3  qemu_co_mutex_lock
>  #4  qcow2_co_pwritev
>  #5  bdrv_driver_pwritev
>  #6  bdrv_aligned_pwritev
>  #7  bdrv_co_pwritev
>  #8  blk_co_pwritev
>  #9  mirror_read_complete () at block/mirror.c:232
>  #10 mirror_co_read () at block/mirror.c:370
>  #11 coroutine_trampoline
>  #12 __start_context
> 
> Yes it is mirror_read_complete calling blk_co_pwritev after acquiring
> aio context.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> ---
>  block/mirror.c | 13 +++++--------
>  1 file changed, 5 insertions(+), 8 deletions(-)

Reviewed-by: Max Reitz <address@hidden>

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