Hi Paolo,
Am 24.11.2015 um 16:54 schrieb Paolo Bonzini:
On 24/11/2015 16:43, David Engraf wrote:
Commit afbe70535ff1a8a7a32910cc15ebecc0ba92e7da introduced the function
qemu_mutex_iothread_locked to avoid recursive locking of the iothread
lock. The iothread_locked variable uses the __thread attribute which
results in a per thread storage location whereas the qemu_global_mutex
is not thread specific. This leads to race conditions because the
variable is not in sync between threads.
Frankly, this makes no sense. You're modifying the
qemu_mutex_iothread_locked function to return whether _some_ thread has
taken the mutex, but the function tells you whether _the calling_ thread
has taken the mutex (that's the point about recursive locking). This
breaks the callers of qemu_mutex_iothread_locked completely.
The variable need not be in sync between the different threads; each
thread only needs to know about itself. The current code works because:
>
1) the iothread mutex is not locked before qemu_mutex_lock_iothread
2) the iothread mutex is not locked after qemu_mutex_lock_iothread
3) qemu_cond_wait doesn't matter because the thread does not run during
a qemu_cond_wait.
But this is my issue on Windows:
qemu_tcg_cpu_thread_fn
-> qemu_tcg_wait_io_event
-> qemu_cond_wait acquires the mutex
qemu_tcg_cpu_thread_fn
-> tcg_exec_all -> tcg_cpu_exec -> cpu_exec
-> cpu_exec ends up in calling prepare_mmio_access
prepare_mmio_access uses qemu_mutex_iothread_locked to check if
the lock is already held for this thread, but it returns 0
because qemu_cond_wait doesn't set iothread_locked but the mutex
is locked. Thus the function calls qemu_mutex_lock_iothread which
tries to acquire the mutex again. On Windows this results in an
assertion:
Assertion failed: mutex->owner == 0, file util/qemu-thread-win32.c, line 60
I'm using a self compiled, cross gcc-5.2.0 mingw compiler.
David