backends/iommufd: Remove mutex

Coverity reports a concurrent data access violation because be->users
is being accessed in iommufd_backend_can_be_deleted() without holding
the mutex.

However, these routines are called from the QEMU main thread when a
device is created. In this case, the code paths should be protected by
the BQL lock and it should be safe to drop the IOMMUFD backend mutex.
Simply remove it.

Fixes: CID 1531550
Fixes: CID 1531549
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>

backends/iommufd.c

@@ -29,7 +29,6 @@ static void iommufd_backend_init(Object *obj)
     be->fd = -1;
     be->users = 0;
     be->owned = true;
-    qemu_mutex_init(&be->lock);
 }
 
 static void iommufd_backend_finalize(Object *obj)
@@ -52,10 +51,8 @@ static void iommufd_backend_set_fd(Object *obj, const char *str, Error **errp)
         error_prepend(errp, "Could not parse remote object fd %s:", str);
         return;
     }
-    qemu_mutex_lock(&be->lock);
     be->fd = fd;
     be->owned = false;
-    qemu_mutex_unlock(&be->lock);
     trace_iommu_backend_set_fd(be->fd);
 }
 
@@ -79,7 +76,6 @@ int iommufd_backend_connect(IOMMUFDBackend *be, Error **errp)
 {
     int fd, ret = 0;
 
-    qemu_mutex_lock(&be->lock);
     if (be->owned && !be->users) {
         fd = qemu_open_old("/dev/iommu", O_RDWR);
         if (fd < 0) {
@@ -93,13 +89,11 @@ int iommufd_backend_connect(IOMMUFDBackend *be, Error **errp)
 out:
     trace_iommufd_backend_connect(be->fd, be->owned,
                                   be->users, ret);
-    qemu_mutex_unlock(&be->lock);
     return ret;
 }
 
 void iommufd_backend_disconnect(IOMMUFDBackend *be)
 {
-    qemu_mutex_lock(&be->lock);
     if (!be->users) {
         goto out;
     }
@@ -110,7 +104,6 @@ void iommufd_backend_disconnect(IOMMUFDBackend *be)
     }
 out:
     trace_iommufd_backend_disconnect(be->fd, be->users);
-    qemu_mutex_unlock(&be->lock);
 }
 
 int iommufd_backend_alloc_ioas(IOMMUFDBackend *be, uint32_t *ioas_id,

include/sysemu/iommufd.h

@@ -2,7 +2,6 @@
 #define SYSEMU_IOMMUFD_H
 
 #include "qom/object.h"
-#include "qemu/thread.h"
 #include "exec/hwaddr.h"
 #include "exec/cpu-common.h"
 
@@ -19,7 +18,6 @@ struct IOMMUFDBackend {
     /*< protected >*/
     int fd;            /* /dev/iommu file descriptor */
     bool owned;        /* is the /dev/iommu opened internally */
-    QemuMutex lock;
     uint32_t users;
 
     /*< public >*/