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authorPeter Maydell <peter.maydell@linaro.org>2014-06-23 14:54:10 +0100
committerPeter Maydell <peter.maydell@linaro.org>2014-06-23 16:20:41 +0100
commitec11c435431ad37cb6b6571ed0d648621dfbf606 (patch)
tree7b171a8ed40c3335666f345424f3acd9377d9a35
parentf8ce98e3efdf52d991cec3395fc1238c6c49e0fe (diff)
downloadqemu-arm-ec11c435431ad37cb6b6571ed0d648621dfbf606.tar.gz
coroutine-win32.c: Add noinline attribute to work around gcc bug
A gcc codegen bug in x86_64-w64-mingw32-gcc (GCC) 4.6.3 means that non-debug builds of QEMU for Windows tend to assert when using coroutines. Work around this by marking qemu_coroutine_switch as noinline. If we allow gcc to inline qemu_coroutine_switch into coroutine_trampoline, then it hoists the code to get the address of the TLS variable "current" out of the while() loop. This is an invalid transformation because the SwitchToFiber() call may be called when running thread A but return in thread B, and so we might be in a different thread context each time round the loop. This can happen quite often. Typically. a coroutine is started when a VCPU thread does bdrv_aio_readv: VCPU thread main VCPU thread coroutine I/O coroutine bdrv_aio_readv -----> start I/O operation thread_pool_submit_co <------------ yields back to emulation Then I/O finishes and the thread-pool.c event notifier triggers in the I/O thread. event_notifier_ready calls thread_pool_co_cb, and the I/O coroutine now restarts *in another thread*: iothread main iothread coroutine I/O coroutine (formerly in VCPU thread) event_notifier_ready thread_pool_co_cb -----> current = I/O coroutine; call AIO callback But on Win32, because of the bug, the "current" being set here the current coroutine of the VCPU thread, not the iothread. noinline is a good-enough workaround, and quite unlikely to break in the future. (Thanks to Paolo Bonzini for assistance in diagnosing the problem and providing the detailed example/ascii art quoted above.) Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
-rw-r--r--coroutine-win32.c13
1 files changed, 11 insertions, 2 deletions
diff --git a/coroutine-win32.c b/coroutine-win32.c
index edc1f72c18..17ace37dee 100644
--- a/coroutine-win32.c
+++ b/coroutine-win32.c
@@ -36,8 +36,17 @@ typedef struct
static __thread CoroutineWin32 leader;
static __thread Coroutine *current;
-CoroutineAction qemu_coroutine_switch(Coroutine *from_, Coroutine *to_,
- CoroutineAction action)
+/* This function is marked noinline to prevent GCC from inlining it
+ * into coroutine_trampoline(). If we allow it to do that then it
+ * hoists the code to get the address of the TLS variable "current"
+ * out of the while() loop. This is an invalid transformation because
+ * the SwitchToFiber() call may be called when running thread A but
+ * return in thread B, and so we might be in a different thread
+ * context each time round the loop.
+ */
+CoroutineAction __attribute__((noinline))
+qemu_coroutine_switch(Coroutine *from_, Coroutine *to_,
+ CoroutineAction action)
{
CoroutineWin32 *from = DO_UPCAST(CoroutineWin32, base, from_);
CoroutineWin32 *to = DO_UPCAST(CoroutineWin32, base, to_);