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authorPaolo Bonzini <pbonzini@redhat.com>2016-01-27 08:49:21 +0100
committerPaolo Bonzini <pbonzini@redhat.com>2017-08-08 10:40:09 +0200
commit73c6e4013b4cd92d3d531bc22cc29e6036ef42e0 (patch)
treea6eda7dbf2e2dfb39507cefd5cd0cb95cbe8a640 /include
parentc233a35d3d91af666aa95a6a3ba8244d4ce728c6 (diff)
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rcu: completely disable pthread_atfork callbacks as soon as possible
Because of -daemonize, system mode QEMU sometimes needs to fork() and keep RCU enabled in the child. However, there is a possible deadlock with synchronize_rcu: - the CPU thread is inside a RCU critical section and wants to take the BQL in order to do MMIO - the monitor thread, which is owning the BQL, calls rcu_init_lock which tries to take the rcu_sync_lock - the call_rcu thread has taken rcu_sync_lock in synchronize_rcu, but synchronize_rcu needs the CPU thread to end the critical section before returning. This cannot happen for user-mode emulation, because it does not have a BQL. To fix it, assume that system mode QEMU only forks in preparation for exec (except when daemonizing) and disable pthread_atfork as soon as the double fork has happened. Reported-by: Dr. David Alan Gilbert <dgilbert@redhat.com> Tested-by: Dr. David Alan Gilbert <dgilbert@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Diffstat (limited to 'include')
-rw-r--r--include/qemu/rcu.h6
1 files changed, 6 insertions, 0 deletions
diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h
index 83ae280..c0da990 100644
--- a/include/qemu/rcu.h
+++ b/include/qemu/rcu.h
@@ -105,6 +105,12 @@ extern void synchronize_rcu(void);
*/
extern void rcu_register_thread(void);
extern void rcu_unregister_thread(void);
+
+/*
+ * Support for fork(). fork() support is enabled at startup.
+ */
+extern void rcu_enable_atfork(void);
+extern void rcu_disable_atfork(void);
extern void rcu_after_fork(void);
struct rcu_head;