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Re: [Qemu-devel] [PATCH v4 04/12] migration: avoid concurrent invoke cha
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] [PATCH v4 04/12] migration: avoid concurrent invoke channel_close by different threads |
Date: |
Thu, 31 May 2018 11:52:44 +0100 |
User-agent: |
Mutt/1.9.5 (2018-04-13) |
* 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
> 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
- [Qemu-devel] [PATCH v4 00/12] Enable postcopy RDMA live migration, Lidong Chen, 2018/05/30
- [Qemu-devel] [PATCH v4 06/12] migration: Stop rdma yielding during incoming postcopy, Lidong Chen, 2018/05/30
- [Qemu-devel] [PATCH v4 05/12] migration: implement bi-directional RDMA QIOChannel, Lidong Chen, 2018/05/30
- [Qemu-devel] [PATCH v4 07/12] migration: not wait RDMA_CM_EVENT_DISCONNECTED event after rdma_disconnect, Lidong Chen, 2018/05/30
- [Qemu-devel] [PATCH v4 08/12] migration: implement io_set_aio_fd_handler function for RDMA QIOChannel, Lidong Chen, 2018/05/30
- [Qemu-devel] [PATCH v4 09/12] migration: invoke qio_channel_yield only when qemu_in_coroutine(), Lidong Chen, 2018/05/30
- [Qemu-devel] [PATCH v4 11/12] migration: poll the cm event while wait RDMA work request completion, Lidong Chen, 2018/05/30
- [Qemu-devel] [PATCH v4 10/12] migration: create a dedicated thread to release rdma resource, Lidong Chen, 2018/05/30