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authorPeter Maydell <peter.maydell@linaro.org>2017-12-04 14:22:11 +0000
committerLaurent Vivier <laurent@vivier.eu>2018-01-23 14:20:52 +0100
commit024949caf32805f4cc3e7d363a80084b47aac1f6 (patch)
tree28b0ba5b4593d78eacdc49b9627e9cf0e61e54a6
parent52483b067cce4a88ffbf8fbeea26de7549d2ad23 (diff)
linux-user: Fix locking order in fork_start()
Our locking order is that the tb lock should be taken inside the mmap_lock, but fork_start() grabs locks the other way around. This means that if a heavily multithreaded guest process (such as Java) calls fork() it can deadlock, with the thread that called fork() stuck in fork_start() with the tb lock and waiting for the mmap lock, but some other thread in tb_find() with the mmap lock and waiting for the tb lock. The cpu_list_lock() should also always be taken last, not first. Fix this by making fork_start() grab the locks in the right order. The order in which we drop locks doesn't matter, so we leave fork_end() the way it is. Signed-off-by: Peter Maydell <peter.maydell@linaro.org> Cc: qemu-stable@nongnu.org Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Alex Bennée <alex.bennee@linaro.org> Message-Id: <1512397331-15238-1-git-send-email-peter.maydell@linaro.org> Signed-off-by: Laurent Vivier <laurent@vivier.eu>
-rw-r--r--linux-user/main.c4
1 files changed, 2 insertions, 2 deletions
diff --git a/linux-user/main.c b/linux-user/main.c
index 450eb3ce65..e8406917e3 100644
--- a/linux-user/main.c
+++ b/linux-user/main.c
@@ -127,9 +127,9 @@ int cpu_get_pic_interrupt(CPUX86State *env)
/* Make sure everything is in a consistent state for calling fork(). */
void fork_start(void)
{
- cpu_list_lock();
- qemu_mutex_lock(&tb_ctx.tb_lock);
mmap_fork_start();
+ qemu_mutex_lock(&tb_ctx.tb_lock);
+ cpu_list_lock();
}
void fork_end(int child)