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From: Michael Tokarev <mjt@tls.msk.ru>
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Date: Sat, 19 Dec 2015 12:16:58 +0300
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Subject: Re: [Qemu-devel] 9pfs is broken in 2.5 (SIGSEGV) (bisected)
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To: qemu-devel <qemu-devel@nongnu.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>, "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>, Greg Kurz <gkurz@linux.vnet.ibm.com>

19.12.2015 11:15, Michael Tokarev wrote:
>> commit ebac1202c95a4f1b76b6ef3f0f63926fa76e753e
>> Author: Paolo Bonzini <pbonzini@redhat.com>
>> Date:   Fri Nov 27 12:43:06 2015 +0100
>>     virtio-9p: use QEMU thread pool
> 
> Program received signal SIGSEGV, Segmentation fault.
> thread_pool_submit_aio (pool=0x0, func=0x55555580f6a0 <coroutine_enter_func>, arg=0x555557698870, cb=<optimized out>,
>     opaque=<optimized out>) at /build/kvm/git/thread-pool.c:246
> 246	    QLIST_INSERT_HEAD(&pool->head, req, all);
> (gdb) bt
> #0  thread_pool_submit_aio (pool=0x0, func=0x55555580f6a0 <coroutine_enter_func>, arg=0x555557698870, cb=<optimized out>,
>     opaque=<optimized out>) at /build/kvm/git/thread-pool.c:246
> #1  0x000055555592c9ed in aio_bh_call (bh=<optimized out>) at /build/kvm/git/async.c:64
> #2  aio_bh_poll (ctx=ctx@entry=0x5555563c9360) at /build/kvm/git/async.c:92
> #3  0x0000555555939390 in aio_dispatch (ctx=0x5555563c9360) at /build/kvm/git/aio-posix.c:305
> #4  0x000055555592c7de in aio_ctx_dispatch (source=<optimized out>, callback=<optimized out>, user_data=<optimized out>)
>     at /build/kvm/git/async.c:231
> #5  0x00007ffff49d1c5d in g_main_context_dispatch () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
> #6  0x0000555555937cf8 in glib_pollfds_poll () at /build/kvm/git/main-loop.c:211
> #7  os_host_main_loop_wait (timeout=<optimized out>) at /build/kvm/git/main-loop.c:256
> #8  main_loop_wait (nonblocking=<optimized out>) at /build/kvm/git/main-loop.c:504
> #9  0x00005555556dbe3e in main_loop () at /build/kvm/git/vl.c:1923
> #10 main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at /build/kvm/git/vl.c:4684
> (gdb) p pool
> $2 = (ThreadPool *) 0x0

In 9pfs/virtio-9p-coth.c:co_run_in_worker_bh(), in this context:

void co_run_in_worker_bh(void *opaque)
{
    Coroutine *co = opaque;
    thread_pool_submit_aio(qemu_get_aio_context()->thread_pool,
                           coroutine_enter_func, co, coroutine_enter_cb, co);
}

qemu_get_aio_context()->thread_pool is NULL.

Thanks,

/mjt