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[Qemu-devel] [PATCH 2/2] migration/rdma.c: Swap synchronize_rcu for call
From: |
Dr. David Alan Gilbert (git) |
Subject: |
[Qemu-devel] [PATCH 2/2] migration/rdma.c: Swap synchronize_rcu for call_rcu |
Date: |
Fri, 13 Sep 2019 17:35:07 +0100 |
From: "Dr. David Alan Gilbert" <address@hidden>
This fixes a deadlock that can occur on the migration source after
a failed RDMA migration; as the source tries to cleanup it
clears a pair of pointers and uses synchronize_rcu to wait; this
is happening on the main thread. With the CPUs running
a CPU thread can be an rcu reader and attempt to grab the main lock
(kvm_handle_io->address_space_write->flatview_write->flatview_write_continue->
prepare_mmio_access->qemu_mutex_lock_iothread_impl)
Replace the synchronize_rcu with a call_rcu to postpone the freeing.
Fixes: 74637e6f08fceda98806 ("migration: implement bi-directional RDMA
QIOChannel")
( https://bugzilla.redhat.com/show_bug.cgi?id=1746787 )
Signed-off-by: Dr. David Alan Gilbert <address@hidden>
---
migration/rdma.c | 39 +++++++++++++++++++++++++++------------
1 file changed, 27 insertions(+), 12 deletions(-)
diff --git a/migration/rdma.c b/migration/rdma.c
index 0fcf02f48e..4c74e88a37 100644
--- a/migration/rdma.c
+++ b/migration/rdma.c
@@ -3017,11 +3017,35 @@ static void
qio_channel_rdma_set_aio_fd_handler(QIOChannel *ioc,
}
}
+struct rdma_close_rcu {
+ struct rcu_head rcu;
+ RDMAContext *rdmain;
+ RDMAContext *rdmaout;
+};
+
+/* callback from qio_channel_rdma_close via call_rcu */
+static void qio_channel_rdma_close_rcu(struct rdma_close_rcu *rcu)
+{
+ if (rcu->rdmain) {
+ qemu_rdma_cleanup(rcu->rdmain);
+ }
+
+ if (rcu->rdmaout) {
+ qemu_rdma_cleanup(rcu->rdmaout);
+ }
+
+ g_free(rcu->rdmain);
+ g_free(rcu->rdmaout);
+ g_free(rcu);
+}
+
static int qio_channel_rdma_close(QIOChannel *ioc,
Error **errp)
{
QIOChannelRDMA *rioc = QIO_CHANNEL_RDMA(ioc);
RDMAContext *rdmain, *rdmaout;
+ struct rdma_close_rcu *rcu = g_new(struct rdma_close_rcu, 1);
+
trace_qemu_rdma_close();
rdmain = rioc->rdmain;
@@ -3034,18 +3058,9 @@ static int qio_channel_rdma_close(QIOChannel *ioc,
atomic_rcu_set(&rioc->rdmaout, NULL);
}
- synchronize_rcu();
-
- if (rdmain) {
- qemu_rdma_cleanup(rdmain);
- }
-
- if (rdmaout) {
- qemu_rdma_cleanup(rdmaout);
- }
-
- g_free(rdmain);
- g_free(rdmaout);
+ rcu->rdmain = rdmain;
+ rcu->rdmaout = rdmaout;
+ call_rcu(rcu, qio_channel_rdma_close_rcu, rcu);
return 0;
}
--
2.21.0