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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 /util
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 'util')
-rw-r--r--util/rcu.c20
1 files changed, 20 insertions, 0 deletions
diff --git a/util/rcu.c b/util/rcu.c
index 9adc5e4..2142ddd 100644
--- a/util/rcu.c
+++ b/util/rcu.c
@@ -318,15 +318,35 @@ static void rcu_init_complete(void)
rcu_register_thread();
}
+static int atfork_depth = 1;
+
+void rcu_enable_atfork(void)
+{
+ atfork_depth++;
+}
+
+void rcu_disable_atfork(void)
+{
+ atfork_depth--;
+}
+
#ifdef CONFIG_POSIX
static void rcu_init_lock(void)
{
+ if (atfork_depth < 1) {
+ return;
+ }
+
qemu_mutex_lock(&rcu_sync_lock);
qemu_mutex_lock(&rcu_registry_lock);
}
static void rcu_init_unlock(void)
{
+ if (atfork_depth < 1) {
+ return;
+ }
+
qemu_mutex_unlock(&rcu_registry_lock);
qemu_mutex_unlock(&rcu_sync_lock);
}