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[Qemu-devel] [PATCH 1/2] rcu: completely disable pthread_atfork callback
From: |
Paolo Bonzini |
Subject: |
[Qemu-devel] [PATCH 1/2] rcu: completely disable pthread_atfork callbacks as soon as possible |
Date: |
Fri, 4 Aug 2017 18:14:51 +0200 |
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: David Gilbert <address@hidden>
Signed-off-by: Paolo Bonzini <address@hidden>
---
include/qemu/rcu.h | 6 ++++++
util/rcu.c | 20 ++++++++++++++++++++
vl.c | 1 +
3 files changed, 27 insertions(+)
diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h
index 83ae2808be..c0da9907e8 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;
diff --git a/util/rcu.c b/util/rcu.c
index 9adc5e4a36..2142ddd93b 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);
}
diff --git a/vl.c b/vl.c
index 99fcfa0442..8967115514 100644
--- a/vl.c
+++ b/vl.c
@@ -4121,6 +4121,7 @@ int main(int argc, char **argv, char **envp)
set_memory_options(&ram_slots, &maxram_size, machine_class);
os_daemonize();
+ rcu_disable_atfork();
if (pid_file && qemu_create_pidfile(pid_file) != 0) {
error_report("could not acquire pid file: %s", strerror(errno));
--
2.13.3