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 <[email protected]> > --- > block/mirror.c | 13 +++++-------- > 1 file changed, 5 insertions(+), 8 deletions(-) Reviewed-by: Max Reitz <[email protected]>
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