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[26/54] linux-user: Fix locking order in fork_start()

Message ID 20180206191515.25830-27-mdroth@linux.vnet.ibm.com
State New
Headers show
Series [01/54] target/i386: Fix handling of VEX prefixes | expand

Commit Message

Michael Roth Feb. 6, 2018, 7:14 p.m. UTC 
From: Peter Maydell <peter.maydell@linaro.org>


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>

(cherry picked from commit 024949caf32805f4cc3e7d363a80084b47aac1f6)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

---
 linux-user/main.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

-- 
2.11.0
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Patch

diff --git a/linux-user/main.c b/linux-user/main.c
index 6286661bd3..146ee3e4ba 100644
--- a/linux-user/main.c
+++ b/linux-user/main.c
@@ -128,9 +128,9 @@  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)