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Re: [Qemu-devel] [PATCH v2 0/4] coroutine-lock: polymorphic CoQueue


From: Fam Zheng
Subject: Re: [Qemu-devel] [PATCH v2 0/4] coroutine-lock: polymorphic CoQueue
Date: Thu, 25 Jan 2018 23:05:27 +0800

On Tue, Jan 16, 2018 at 10:23 PM, Paolo Bonzini <address@hidden> wrote:
> There are cases in which a queued coroutine must be restarted from
> non-coroutine context (with qemu_co_enter_next).  In this cases,
> qemu_co_enter_next also needs to be thread-safe, but it cannot use a
> CoMutex and so cannot qemu_co_queue_wait.  This happens in curl (which
> right now is rolling its own list of Coroutines) and will happen in
> Fam's NVMe driver as well.
>
> This series extracts the idea of a polymorphic lockable object
> from my "scoped lock guard" proposal, and applies it to CoQueue.
> The implementation of QemuLockable is similar to C11 _Generic, but
> redone using the preprocessor and GCC builtins for compatibility.
>
> In general, while a bit on the esoteric side, the functionality used
> to emulate _Generic is fairly old in GCC, and the builtins are already
> used by include/qemu/atomic.h; the series was tested with Fedora 27 (boot
> Damn Small Linux via http) and CentOS 6 (compiled only).

I'm seeing this crash with the series:

(gdb) bt
#0  0x00007ff76204d66b in raise () at /lib64/libc.so.6
#1  0x00007ff76204f381 in abort () at /lib64/libc.so.6
#2  0x00007ff7620458fa in __assert_fail_base () at /lib64/libc.so.6
#3  0x00007ff762045972 in  () at /lib64/libc.so.6
#4  0x000055eaab249c68 in qemu_co_mutex_unlock (mutex=0x7ff750bf7b40)
at /stor/work/qemu/util/qemu-coroutine-lock.c:320
#5  0x000055eaab249da3 in qemu_lockable_unlock (x=0x7ff750bf7b40) at
/stor/work/qemu/include/qemu/lockable.h:72
#6  0x000055eaab249da3 in qemu_co_queue_wait_impl
(queue=0x55eaaef41a08, address@hidden) at
/stor/work/qemu/util/qemu-coroutine-lock.c:49
#7  0x000055eaab19f2b9 in handle_dependencies
(address@hidden, address@hidden,
address@hidden, address@hidden
50bf7c58) at /stor/work/qemu/block/qcow2-cluster.c:1067
#8  0x000055eaab1a1b85 in qcow2_alloc_cluster_offset
(address@hidden, address@hidden,
address@hidden, address@hidden
=0x7ff750bf7c50, address@hidden) at
/stor/work/qemu/block/qcow2-cluster.c:1497
#9  0x000055eaab19411e in qcow2_co_pwritev (bs=0x55eaad9c6620,
offset=1597440, bytes=8192, qiov=0x55eaaedb4880, flags=<optimized
out>) at /stor/work/qemu/block/qcow2.c:1896
#10 0x000055eaab1c2962 in bdrv_driver_pwritev
(address@hidden, address@hidden,
address@hidden, address@hidden, address@hidden
try=0) at /stor/work/qemu/block/io.c:976
#11 0x000055eaab1c3985 in bdrv_aligned_pwritev
(address@hidden, address@hidden,
address@hidden, address@hidden, align=ali
address@hidden, address@hidden, flags=0) at
/stor/work/qemu/block/io.c:1534
#12 0x000055eaab1c4ca5 in bdrv_co_pwritev (child=0x55eaad92bd00,
address@hidden, address@hidden,
address@hidden, address@hidden)
   at /stor/work/qemu/block/io.c:1785
#13 0x000055eaab1b4f06 in blk_co_pwritev (blk=0x55eaad9c63c0,
offset=1597440, bytes=8192, qiov=0x55eaaedb4880, flags=0) at
/stor/work/qemu/block/block-backend.c:1135
#14 0x000055eaab1b4fff in blk_aio_write_entry (opaque=0x55eaaefc5eb0)
at /stor/work/qemu/block/block-backend.c:1326
#15 0x000055eaab24a77a in coroutine_trampoline (i0=<optimized out>,
i1=<optimized out>) at /stor/work/qemu/util/coroutine-ucontext.c:79
#16 0x00007ff762066bc0 in __start_context () at /lib64/libc.so.6
#17 0x00007ffdf69102d0 in  ()
#18 0x0000000000000000 in  ()


It's late today so I'll take a closer look tomorrow.

Fam



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