Re: [Qemu-devel] [PATCH v4 04/12] migration: avoid concurrent invoke channel_close by different threads

[858585 jemmy]

On Thu, May 31, 2018 at 6:52 PM, Dr. David Alan Gilbert
<address@hidden> wrote:
> * 858585 jemmy (address@hidden) wrote:
>> On Wed, May 30, 2018 at 10:45 PM, Dr. David Alan Gilbert
>> <address@hidden> wrote:
>> > * Lidong Chen (address@hidden) wrote:
>> >> From: Lidong Chen <address@hidden>
>> >>
>> >> The channel_close maybe invoked by different threads. For example, source
>> >> qemu invokes qemu_fclose in main thread, migration thread and return path
>> >> thread. Destination qemu invokes qemu_fclose in main thread, listen thread
>> >> and COLO incoming thread.
>> >>
>> >> Add a mutex in QEMUFile struct to avoid concurrent invoke channel_close.
>> >>
>> >> Signed-off-by: Lidong Chen <address@hidden>
>> >> ---
>> >>  migration/qemu-file.c | 5 +++++
>> >>  1 file changed, 5 insertions(+)
>> >>
>> >> diff --git a/migration/qemu-file.c b/migration/qemu-file.c
>> >> index 977b9ae..87d0f05 100644
>> >> --- a/migration/qemu-file.c
>> >> +++ b/migration/qemu-file.c
>> >> @@ -52,6 +52,7 @@ struct QEMUFile {
>> >>      unsigned int iovcnt;
>> >>
>> >>      int last_error;
>> >> +    QemuMutex lock;
>> >
>> > That could do with a comment saying what you're protecting
>> >
>> >>  };
>> >>
>> >>  /*
>> >> @@ -96,6 +97,7 @@ QEMUFile *qemu_fopen_ops(void *opaque, const 
>> >> QEMUFileOps *ops)
>> >>
>> >>      f = g_new0(QEMUFile, 1);
>> >>
>> >> +    qemu_mutex_init(&f->lock);
>> >>      f->opaque = opaque;
>> >>      f->ops = ops;
>> >>      return f;
>> >> @@ -328,7 +330,9 @@ int qemu_fclose(QEMUFile *f)
>> >>      ret = qemu_file_get_error(f);
>> >>
>> >>      if (f->ops->close) {
>> >> +        qemu_mutex_lock(&f->lock);
>> >>          int ret2 = f->ops->close(f->opaque);
>> >> +        qemu_mutex_unlock(&f->lock);
>> >
>> > OK, and at least for the RDMA code, if it calls
>> > close a 2nd time, rioc->rdma is checked so it wont actually free stuff a
>> > 2nd time.
>> >
>> >>          if (ret >= 0) {
>> >>              ret = ret2;
>> >>          }
>> >> @@ -339,6 +343,7 @@ int qemu_fclose(QEMUFile *f)
>> >>      if (f->last_error) {
>> >>          ret = f->last_error;
>> >>      }
>> >> +    qemu_mutex_destroy(&f->lock);
>> >>      g_free(f);
>> >
>> > Hmm but that's not safe; if two things really do call qemu_fclose()
>> > on the same structure they race here and can end up destroying the lock
>> > twice, or doing f->lock  after the 1st one has already g_free(f).
>>
>> >
>> >
>> > So lets go back a step.
>> > I think:
>> >   a) There should always be a separate QEMUFile* for
>> >      to_src_file and from_src_file - I don't see where you open
>> >      the 2nd one; I don't see your implementation of
>> >      f->ops->get_return_path.
>>
>> yes, current qemu version use a separate QEMUFile* for to_src_file and
>> from_src_file.
>> and the two QEMUFile point to one QIOChannelRDMA.
>>
>> the f->ops->get_return_path is implemented by channel_output_ops or
>> channel_input_ops.
>
> Ah OK, yes that makes sense.
>
>> >   b) I *think* that while the different threads might all call
>> >      fclose(), I think there should only ever be one qemu_fclose
>> >      call for each direction on the QEMUFile.
>> >
>> > But now we have two problems:
>> >   If (a) is true then f->lock  is separate on each one so
>> >    doesn't really protect if the two directions are closed
>> >    at once. (Assuming (b) is true)
>>
>> yes, you are right.  so I should add a QemuMutex in QIOChannel structure, not
>> QEMUFile structure. and qemu_mutex_destroy the QemuMutex in
>> qio_channel_finalize.
>
> OK, that sounds better.
>
> Dave
>

Hi Dave:
    Another way is protect channel_close in migration part, like
QemuMutex rp_mutex.
    As Daniel mentioned, QIOChannel impls are only intended to a single thread.
    https://www.mail-archive.com/address@hidden/msg530100.html

    which way is better? Does QIOChannel have the plan to support multi thread?
    Not only channel_close need lock between different threads,
writev_buffer write also
    need.

    thanks.


>> Thank you.
>>
>> >
>> >   If (a) is false and we actually share a single QEMUFile then
>> >  that race at the end happens.
>> >
>> > Dave
>> >
>> >
>> >>      trace_qemu_file_fclose();
>> >>      return ret;
>> >> --
>> >> 1.8.3.1
>> >>
>> > --
>> > Dr. David Alan Gilbert / address@hidden / Manchester, UK
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK