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Re: [Qemu-devel] [RFC PATCH] qemu_mutux: make the iothread recursive (MT

From: Frederic Konrad
Subject: Re: [Qemu-devel] [RFC PATCH] qemu_mutux: make the iothread recursive (MTTCG)
Date: Tue, 23 Jun 2015 16:21:31 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0 
On 23/06/2015 14:21, Alex Bennée wrote:

While I was testing multi-threaded TCG I discovered once consequence of
using locking around memory_region_dispatch is that virt-io transactions
could dead lock trying to grab the main mutex. This is due to the
virt-io driver writing data back into the system memory:


Hi,

Thanks for that.
Didn't qemu abort in this case with a pthread error? Maybe that did change since 
the last time I had this error.

Thanks,
Fred


     #0  0x00007ffff119dcc9 in __GI_raise (address@hidden) at 
../nptl/sysdeps/unix/sysv/linux/raise.c:56
     #1  0x00007ffff11a10d8 in __GI_abort () at abort.c:89
     #2  0x00005555555f9b24 in error_exit (err=<optimised out>, address@hidden 
<__func__.6011> "qemu_mutex_lock") at util/qemu-thread-posix.c:48
     #3  0x000055555594d630 in qemu_mutex_lock (address@hidden 
<qemu_global_mutex>) at util/qemu-thread-posix.c:79
     #4  0x0000555555631a84 in qemu_mutex_lock_iothread () at 
/home/alex/lsrc/qemu/qemu.git/cpus.c:1128
     #5  0x000055555560dd1a in stw_phys_internal (endian=DEVICE_LITTLE_ENDIAN, val=1, 
addr=<optimised out>, as=0x555555e08060 <address_space_memory>) at 
/home/alex/lsrc/qemu/qemu.git/exec.c:3010
     #6  stw_le_phys (address@hidden <address_space_memory>, addr=<optimised 
out>, val=1) at /home/alex/lsrc/qemu/qemu.git/exec.c:3024
     #7  0x0000555555696ae5 in virtio_stw_phys (vdev=<optimised out>, value=<optimised 
out>, pa=<optimised out>) at 
/home/alex/lsrc/qemu/qemu.git/include/hw/virtio/virtio-access.h:61
     #8  vring_avail_event (vq=0x55555648dc00, vq=0x55555648dc00, vq=0x55555648dc00, 
val=<optimised out>) at /home/alex/lsrc/qemu/qemu.git/hw/virtio/virtio.c:214
     #9  virtqueue_pop (vq=0x55555648dc00, address@hidden) at 
/home/alex/lsrc/qemu/qemu.git/hw/virtio/virtio.c:472
     #10 0x0000555555653cd1 in virtio_blk_get_request (s=0x555556486830) at 
/home/alex/lsrc/qemu/qemu.git/hw/block/virtio-blk.c:122
     #11 virtio_blk_handle_output (vdev=<optimised out>, vq=<optimised out>) at 
/home/alex/lsrc/qemu/qemu.git/hw/block/virtio-blk.c:446
     #12 0x00005555556414e1 in access_with_adjusted_size (address@hidden, 
address@hidden, address@hidden, access_size_min=<optimised out>,
         access_size_max=<optimised out>, access=0x5555556413e0 
<memory_region_write_accessor>, mr=0x555556b80388) at 
/home/alex/lsrc/qemu/qemu.git/memory.c:461
     #13 0x00005555556471b7 in memory_region_dispatch_write (size=4, data=0, 
addr=80, mr=0x555556b80388) at /home/alex/lsrc/qemu/qemu.git/memory.c:1103
     #14 io_mem_write (address@hidden, addr=80, val=<optimised out>, 
address@hidden) at /home/alex/lsrc/qemu/qemu.git/memory.c:2003
     #15 0x000055555560ad6b in address_space_rw_internal (as=<optimised out>, addr=167788112, 
address@hidden "", len=4, address@hidden, unlocked=<optimised out>,
         address@hidden) at /home/alex/lsrc/qemu/qemu.git/exec.c:2318
     #16 0x000055555560aea8 in address_space_rw (is_write=true, len=<optimised out>, 
buf=0x7fffa9305380 "", addr=<optimised out>, as=<optimised out>) at 
/home/alex/lsrc/qemu/qemu.git/exec.c:2392
     #17 address_space_write (len=<optimised out>, buf=0x7fffa9305380 "", 
addr=<optimised out>, as=<optimised out>) at /home/alex/lsrc/qemu/qemu.git/exec.c:2404
     #18 subpage_write (opaque=<optimised out>, addr=<optimised out>, value=<optimised 
out>, len=<optimised out>) at /home/alex/lsrc/qemu/qemu.git/exec.c:1963
     #19 0x00005555556414e1 in access_with_adjusted_size (address@hidden, 
address@hidden, address@hidden, access_size_min=<optimised out>,
         access_size_max=<optimised out>, access=0x5555556413e0 
<memory_region_write_accessor>, mr=0x555556bfca20) at 
/home/alex/lsrc/qemu/qemu.git/memory.c:461
     #20 0x00005555556471b7 in memory_region_dispatch_write (size=4, data=0, 
addr=592, mr=0x555556bfca20) at /home/alex/lsrc/qemu/qemu.git/memory.c:1103
     #21 io_mem_write (address@hidden, address@hidden, address@hidden, 
address@hidden) at /home/alex/lsrc/qemu/qemu.git/memory.c:2003
     #22 0x000055555564ce16 in io_writel (retaddr=140736492182797, 
addr=4027616848, val=0, physaddr=592, env=0x55555649e9b0) at 
/home/alex/lsrc/qemu/qemu.git/softmmu_template.h:386
     #23 helper_le_stl_mmu (env=0x55555649e9b0, addr=<optimised out>, val=0, 
mmu_idx=<optimised out>, retaddr=140736492182797) at 
/home/alex/lsrc/qemu/qemu.git/softmmu_template.h:426
     #24 0x00007fffc49f9d0f in code_gen_buffer ()
     #25 0x00005555556109dc in cpu_tb_exec (tb_ptr=0x7fffc49f9c60 
<code_gen_buffer+8371296> "A\213n\374\205\355\017\205\233\001", 
cpu=0x555556496750)
         at /home/alex/lsrc/qemu/qemu.git/cpu-exec.c:179
     #26 cpu_arm_exec (address@hidden) at 
/home/alex/lsrc/qemu/qemu.git/cpu-exec.c:524
     #27 0x0000555555630f28 in tcg_cpu_exec (env=0x55555649e9b0) at 
/home/alex/lsrc/qemu/qemu.git/cpus.c:1344
     #28 tcg_exec_all (cpu=0x555556496750) at 
/home/alex/lsrc/qemu/qemu.git/cpus.c:1392
     #29 qemu_tcg_cpu_thread_fn (arg=0x555556496750) at 
/home/alex/lsrc/qemu/qemu.git/cpus.c:1037
     #30 0x00007ffff1534182 in start_thread (arg=0x7fffa9306700) at 
pthread_create.c:312
     #31 0x00007ffff126147d in clone () at 
../sysdeps/unix/sysv/linux/x86_64/clone.S:111

The fix in this patch makes the global/iothread mutex recursive (only if
called from the same thread-id). As other threads are still blocked
memory is still consistent all the way through.

This seems neater than having to do a trylock each time.

Tested-by: Alex Bennée <address@hidden>
---
  cpus.c                   |  2 +-
  include/qemu/thread.h    |  1 +
  util/qemu-thread-posix.c | 12 ++++++++++++
  util/qemu-thread-win32.c |  5 +++++
  4 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/cpus.c b/cpus.c
index d161bb9..412ab04 100644
--- a/cpus.c
+++ b/cpus.c
@@ -804,7 +804,7 @@ void qemu_init_cpu_loop(void)
      qemu_cond_init(&qemu_pause_cond);
      qemu_cond_init(&qemu_work_cond);
      qemu_cond_init(&qemu_io_proceeded_cond);
-    qemu_mutex_init(&qemu_global_mutex);
+    qemu_mutex_init_recursive(&qemu_global_mutex);
qemu_thread_get_self(&io_thread); 
  }
diff --git a/include/qemu/thread.h b/include/qemu/thread.h
index c9389f4..4519d2f 100644
--- a/include/qemu/thread.h
+++ b/include/qemu/thread.h
@@ -22,6 +22,7 @@ typedef struct QemuThread QemuThread;
  #define QEMU_THREAD_DETACHED 1
void qemu_mutex_init(QemuMutex *mutex); 
+void qemu_mutex_init_recursive(QemuMutex *mutex);
  void qemu_mutex_destroy(QemuMutex *mutex);
  void __qemu_mutex_lock(QemuMutex *mutex, const char *func, int line);
  #define qemu_mutex_lock(mutex) __qemu_mutex_lock(mutex, __func__, __LINE__)
diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
index 98eb0f0..ba2fb97 100644
--- a/util/qemu-thread-posix.c
+++ b/util/qemu-thread-posix.c
@@ -57,6 +57,18 @@ void qemu_mutex_init(QemuMutex *mutex)
          error_exit(err, __func__);
  }
+void qemu_mutex_init_recursive(QemuMutex *mutex) 
+{
+    int err;
+    pthread_mutexattr_t attr;
+
+    pthread_mutexattr_init(&attr);
+    pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE_NP);
+    err = pthread_mutex_init(&mutex->lock, &attr);
+    if (err)
+        error_exit(err, __func__);
+}
+
  void qemu_mutex_destroy(QemuMutex *mutex)
  {
      int err;
diff --git a/util/qemu-thread-win32.c b/util/qemu-thread-win32.c
index 406b52f..f055067 100644
--- a/util/qemu-thread-win32.c
+++ b/util/qemu-thread-win32.c
@@ -44,6 +44,11 @@ void qemu_mutex_init(QemuMutex *mutex)
      InitializeCriticalSection(&mutex->lock);
  }
+void qemu_mutex_init_recursive(QemuMutex *mutex) 
+{
+    error_exit(0, "%s: not implemented for win32\n", __func__);
+}
+
  void qemu_mutex_destroy(QemuMutex *mutex)
  {
      assert(mutex->owner == 0);
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