Re: [Qemu-devel] thread synchronization in qcow2.c and qcow2-cluster.c

["Chunqiang (CQ) Tang" <tangchq@gmail.com>]

> kvm-all.c:kvm_cpu_exec:
>        qemu_mutex_unlock_iothread();
>        ret = kvm_vcpu_ioctl(env, KVM_RUN, 0);
>        qemu_mutex_lock_iothread();

Thank you for the information. I also suspected that
qemu_mutex_lock_iothread() does the synchronization. However, my
profiling showed that qemu-kvm.c:kvm_cpu_exec() in is actually
executed, instead of kvm-all.c:kvm_cpu_exec(). Also I previously
profiled all executions of qemu_mutex_lock_iothread(), and found that
it only protects the vl.c:main_loop_wai() thread but does NOT protect
the qemu-kvm.c:kvm_cpu_exec() thread. Did I miss something or is this
a defect? I did extensive profiling but still don't know the source
code well enough to confidently draw a conclusion.

For example, see the profiled execution sequence below. The
kvm_cpu_exec() thread did not perform qemu_mutex_lock_iothread(). The
locking was only performed by the vl.c:main_loop_wai() thread.

home/ctang/kvm/qemu-kvm-0.12.3/qemu-kvm.c : 2530    thread: b7e056d0
        /home/ctang/kvm/bin/qemu-system-x86_64(qemu_mutex_unlock_iothread+0x1a)
[0x8092242]
        /home/ctang/kvm/bin/qemu-system-x86_64(main_loop_wait+0x221) [0x806edef]
        /home/ctang/kvm/bin/qemu-system-x86_64(kvm_main_loop+0x1ff) [0x80916a1]
        /home/ctang/kvm/bin/qemu-system-x86_64 [0x806f5c2]
        /home/ctang/kvm/bin/qemu-system-x86_64(main+0x2e2c) [0x80736d1]
        /lib/tls/i686/cmov/libc.so.6(__libc_start_main+0xe5) [0xb7e33775]
        /home/ctang/kvm/bin/qemu-system-x86_64 [0x8068bb1]

block/qcow2-cluster.c : 721    thread: b7dc2b90
        /home/ctang/kvm/bin/qemu-system-x86_64(qcow2_alloc_cluster_offset+0x3c)
[0x81175fa]
        /home/ctang/kvm/bin/qemu-system-x86_64(qcow_aio_write_cb+0x158)
[0x8111d73]
        /home/ctang/kvm/bin/qemu-system-x86_64(qcow_aio_writev+0x94) [0x8112054]
        /home/ctang/kvm/bin/qemu-system-x86_64(bdrv_aio_writev+0xe1) [0x80fa8e9]
        /home/ctang/kvm/bin/qemu-system-x86_64 [0x81f4a96]
        /home/ctang/kvm/bin/qemu-system-x86_64 [0x81f4c04]
        /home/ctang/kvm/bin/qemu-system-x86_64(dma_bdrv_write+0x48) [0x81f4cbf]
        /home/ctang/kvm/bin/qemu-system-x86_64 [0x80a437c]
        /home/ctang/kvm/bin/qemu-system-x86_64(bmdma_cmd_writeb+0x73)
[0x80a9503]
        /home/ctang/kvm/bin/qemu-system-x86_64 [0x812b1eb]
        /home/ctang/kvm/bin/qemu-system-x86_64(cpu_outb+0x27) [0x812b4e6]
        /home/ctang/kvm/bin/qemu-system-x86_64 [0x808d267]
        /home/ctang/kvm/bin/qemu-system-x86_64(kvm_run+0x2f4) [0x808f4b8]
        /home/ctang/kvm/bin/qemu-system-x86_64(kvm_cpu_exec+0x56) [0x80907b2]
        /home/ctang/kvm/bin/qemu-system-x86_64 [0x8090f4d]
        /home/ctang/kvm/bin/qemu-system-x86_64 [0x8091098]
        /lib/tls/i686/cmov/libpthread.so.0 [0xb7fd24ff]
        /lib/tls/i686/cmov/libc.so.6(clone+0x5e) [0xb7f0149e]

/home/ctang/kvm/qemu-kvm-0.12.3/qemu-kvm.c : 2537    thread: b7e056d0
        /home/ctang/kvm/bin/qemu-system-x86_64(qemu_mutex_lock_iothread+0x1a)
[0x809229d]
        /home/ctang/kvm/bin/qemu-system-x86_64(main_loop_wait+0x25c) [0x806ee2a]
        /home/ctang/kvm/bin/qemu-system-x86_64(kvm_main_loop+0x1ff) [0x80916a1]
        /home/ctang/kvm/bin/qemu-system-x86_64 [0x806f5c2]
        /home/ctang/kvm/bin/qemu-system-x86_64(main+0x2e2c) [0x80736d1]
        /lib/tls/i686/cmov/libc.so.6(__libc_start_main+0xe5) [0xb7e33775]
        /home/ctang/kvm/bin/qemu-system-x86_64 [0x8068bb1]