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migration: Make from_dst_file accesses thread-safe

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Accessing from_dst_file is potentially racy in current code base like below:

  if (s->from_dst_file)
    do_something(s->from_dst_file);

Because from_dst_file can be reset right after the check in another
thread (rp_thread).  One example is migrate_fd_cancel().

Use the same qemu_file_lock to protect it too, just like to_dst_file.

When it's safe to access without lock, comment it.

There's one special reference in migration_thread() that can be replaced by
the newly introduced rp_thread_created flag.

Reported-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Lukas Straub <lukasstraub2@web.de>
Message-Id: <20210722175841.938739-3-peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
  with Peter's fixup
This commit is contained in:
Peter Xu 2021-07-22 13:58:38 -04:00 • committed by Dr. David Alan Gilbert
parent 53021ea165
commit 43044ac0ee
3 changed files with 35 additions and 13 deletions
Download patch file Download diff file Expand all files Collapse all files

8
migration/migration.h
View file

@ -154,12 +154,13 @@ struct MigrationState {
QemuThread thread;
QEMUBH *vm_start_bh;
QEMUBH *cleanup_bh;
/* Protected by qemu_file_lock */
QEMUFile *to_dst_file;
QIOChannelBuffer *bioc;
/*
* Protects to_dst_file pointer. We need to make sure we won't
* yield or hang during the critical section, since this lock will
* be used in OOB command handler.
* Protects to_dst_file/from_dst_file pointers. We need to make sure we
* won't yield or hang during the critical section, since this lock will be
* used in OOB command handler.
*/
QemuMutex qemu_file_lock;
@ -192,6 +193,7 @@ struct MigrationState {
/* State related to return path */
struct {
/* Protected by qemu_file_lock */
QEMUFile *from_dst_file;
QemuThread rp_thread;
bool error;