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

[858585 jemmy]

On Sun, Jun 3, 2018 at 9:50 PM, 858585 jemmy <address@hidden> wrote:
> 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.
>
>

I find qemu not call qemu_mutex_destroy to release rp_mutex in
migration_instance_finalize:(
although qemu_mutex_destroy is not necceesary, but it is a good practice to do.
it's better we fixed it.

>>> 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