Re: [Qemu-devel] [PATCH v2 2/9] migration: Fix a potential issue

[Li, Liang Z]

> Subject: Re: [PATCH v2 2/9] migration: Fix a potential issue
> 
> On (Thu) 05 May 2016 [15:32:52], Liang Li wrote:
> > At the end of live migration and before vm_start() on the destination
> > side, we should make sure all the decompression tasks are finished, if
> > this can not be guaranteed, the VM may get the incorrect memory data,
> > or the updated memory may be overwritten by the decompression thread.
> > Add the code to fix this potential issue.
> >
> > Suggested-by: David Alan Gilbert <address@hidden>
> > Suggested-by: Juan Quintela <address@hidden>
> > Signed-off-by: Liang Li <address@hidden>
> > ---
> >  migration/ram.c | 19 +++++++++++++++++++
> >  1 file changed, 19 insertions(+)
> >
> > diff --git a/migration/ram.c b/migration/ram.c index 7ab6ab5..8a59a08
> > 100644
> > --- a/migration/ram.c
> > +++ b/migration/ram.c
> > @@ -2220,6 +2220,24 @@ static void *do_data_decompress(void *opaque)
> >      return NULL;
> >  }
> >
> > +static void wait_for_decompress_done(void) {
> > +    int idx, thread_count;
> > +
> > +    if (!migrate_use_compression()) {
> > +        return;
> > +    }
> > +
> > +    thread_count = migrate_decompress_threads();
> > +    qemu_mutex_lock(&decomp_done_lock);
> > +    for (idx = 0; idx < thread_count; idx++) {
> > +        while (!decomp_param[idx].done) {
> > +            qemu_cond_wait(&decomp_done_cond, &decomp_done_lock);
> > +        }
> > +    }
> > +    qemu_mutex_unlock(&decomp_done_lock);
> 
> Not sure how this works: in the previous patch, done is set to false under the
> decomp_done_lock.  Here, we take the lock, and wait for done to turn false.
> That can't happen because this thread holds the lock.
> My reading is this is going to lead to a deadlock.  What am I missing?
> 
> 
>               Amit

This is the typical usage of the QemuCond, actually, in qemu_cond_wait() ,
decomp_done_lock will be unlocked at first and then locked again before 
qemu_cond_wait() return.  So deadlock won't happen.

Liang