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Re: [PATCH-for-7.0 v2] qemu/main-loop: Disable block backend global stat


From: Akihiko Odaki
Subject: Re: [PATCH-for-7.0 v2] qemu/main-loop: Disable block backend global state assertion on Cocoa
Date: Tue, 22 Mar 2022 19:56:49 +0900
User-agent: Mozilla/5.0 (X11; Linux aarch64; rv:91.0) Gecko/20100101 Thunderbird/91.7.0

On 2022/03/22 18:35, Philippe Mathieu-Daudé wrote:
On 22/3/22 09:32, Paolo Bonzini wrote:
On 3/22/22 08:54, Philippe Mathieu-Daudé wrote:
From: Philippe Mathieu-Daudé <f4bug@amsat.org>

Since commit 0439c5a462 ("block/block-backend.c: assertions for
block-backend") QEMU crashes when using Cocoa on Darwin hosts.

Example on macOS:

   $ qemu-system-i386
   Assertion failed: (qemu_in_main_thread()), function blk_all_next, file block-backend.c, line 552.
   Abort trap: 6

Looking with lldb:

   Assertion failed: (qemu_in_main_thread()), function blk_all_next, file block-backend.c, line 552.
   Process 76914 stopped
   * thread #1, queue = 'com.apple.main-thread', stop reason = hit program assert
      frame #4: 0x000000010057c2d4 qemu-system-i386`blk_all_next.cold.1
   at block-backend.c:552:5 [opt]
       549    */
       550   BlockBackend *blk_all_next(BlockBackend *blk)
       551   {
   --> 552       GLOBAL_STATE_CODE();
       553       return blk ? QTAILQ_NEXT(blk, link)
       554                  : QTAILQ_FIRST(&block_backends);
       555   }
   Target 1: (qemu-system-i386) stopped.

   (lldb) bt
   * thread #1, queue = 'com.apple.main-thread', stop reason = hit program assert       frame #0: 0x00000001908c99b8 libsystem_kernel.dylib`__pthread_kill + 8       frame #1: 0x00000001908fceb0 libsystem_pthread.dylib`pthread_kill + 288
      frame #2: 0x000000019083a314 libsystem_c.dylib`abort + 164
      frame #3: 0x000000019083972c libsystem_c.dylib`__assert_rtn + 300
    * frame #4: 0x000000010057c2d4 qemu-system-i386`blk_all_next.cold.1 at block-backend.c:552:5 [opt]       frame #5: 0x00000001003c00b4 qemu-system-i386`blk_all_next(blk=<unavailable>) at block-backend.c:552:5 [opt]       frame #6: 0x00000001003d8f04 qemu-system-i386`qmp_query_block(errp=0x0000000000000000) at qapi.c:591:16 [opt]       frame #7: 0x000000010003ab0c qemu-system-i386`main [inlined] addRemovableDevicesMenuItems at cocoa.m:1756:21 [opt]       frame #8: 0x000000010003ab04 qemu-system-i386`main(argc=<unavailable>, argv=<unavailable>) at cocoa.m:1980:5 [opt]
      frame #9: 0x00000001012690f4 dyld`start + 520

As we are in passed release 7.0 hard freeze, disable the block
backend assertion which, while being valuable during development,
is not helpful to users. We'll restore this assertion immediately
once 7.0 is released and work on a fix.

Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Suggested-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
---
Supersedes: <20220321145537.98924-1-philippe.mathieu.daude@gmail.com>
---
  include/qemu/main-loop.h | 12 ++++++++++++
  1 file changed, 12 insertions(+)

diff --git a/include/qemu/main-loop.h b/include/qemu/main-loop.h
index 7a4d6a0920..48061f736b 100644
--- a/include/qemu/main-loop.h
+++ b/include/qemu/main-loop.h
@@ -270,10 +270,22 @@ bool qemu_mutex_iothread_locked(void);
  bool qemu_in_main_thread(void);
  /* Mark and check that the function is part of the global state API. */
+#ifdef CONFIG_COCOA
+/*
+ * When using Cocoa ui, addRemovableDevicesMenuItems() calls qmp_query_block() + * while expecting the main thread to still hold the BQL, triggering this + * assertions in the block layer (commit 0439c5a462). As the Cocoa fix is not + * trivial, disable this assertion for the v7.0.0 release when using Cocoa; it + * will be restored immediately after the release. This issue is tracked as
+ * https://gitlab.com/qemu-project/qemu/-/issues/926
+ */
+#define GLOBAL_STATE_CODE()
+#else
  #define GLOBAL_STATE_CODE()                                         \
      do {                                                            \
          assert(qemu_in_main_thread());                              \
      } while (0)
+#endif /* CONFIG_DARWIN */
  /* Mark and check that the function is part of the I/O API. */
  #define IO_CODE()                                                   \

I don't know, it seems to me that the reorganized initialization code had only advantages.

For now, it fixes the regression and makes the Cocoa build much more similar to the others.  There is an easy way to fix the -runas regression, by moving the code up to the call of -sharedApplication in cocoa_display_init.

So the options are:

#1 disabling the assert for cocoa (this patch)

#2 run qemu_init() in the main thread​ from Paolo [*] with this (?)
    patch on top:

-- >8 --
diff --git a/ui/cocoa.m b/ui/cocoa.m
index e69ce97f44..867c222e18 100644
--- a/ui/cocoa.m
+++ b/ui/cocoa.m
@@ -1946,7 +1946,6 @@ static void cocoa_clipboard_request(QemuClipboardInfo *info,
  int main(int argc, char **argv, char **envp)
  {
      COCOA_DEBUG("Entered main()\n");
-    qemu_event_init(&cbevent, false);

     /* Takes iothread lock, released in applicationDidFinishLaunching:.  */
      qemu_init(argc, argv, envp);
@@ -1958,13 +1957,6 @@ int main(int argc, char **argv, char **envp)

      NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

-    // Pull this console process up to being a fully-fledged graphical
-    // app with a menubar and Dock icon
-    ProcessSerialNumber psn = { 0, kCurrentProcess };
-    TransformProcessType(&psn, kProcessTransformToForegroundApplication);
-
-    [QemuApplication sharedApplication];
-
      create_initial_menus();

      /*
@@ -1976,7 +1968,6 @@ int main(int argc, char **argv, char **envp)
       */
      add_console_menu_entries();
      addRemovableDevicesMenuItems();
-    cbowner = [[QemuCocoaPasteboardTypeOwner alloc] init];

      // Create an Application controller
     QemuCocoaAppController *appController = [[QemuCocoaAppController alloc] init]; @@ -2089,6 +2080,17 @@ static void cocoa_display_init(DisplayState *ds, DisplayOptions *opts)      if (opts->u.cocoa.has_left_command_key && !opts->u.cocoa.left_command_key) {
          left_command_key_enabled = 0;
      }
+
+    qemu_event_init(&cbevent, false);
+
+    cbowner = [[QemuCocoaPasteboardTypeOwner alloc] init];
+
+    // Pull this console process up to being a fully-fledged graphical
+    // app with a menubar and Dock icon
+    ProcessSerialNumber psn = { 0, kCurrentProcess };
+    TransformProcessType(&psn, kProcessTransformToForegroundApplication);
+
+    [QemuApplication sharedApplication];

I think the changes for cbevent and cbowner are irrelevant, but basically this should do.

  }

---

# 3 "Create menus in iothread" approach from Akihiko:
20220321041043.24112-1-akihiko.odaki@gmail.com/">https://lore.kernel.org/qemu-devel/20220321041043.24112-1-akihiko.odaki@gmail.com/

Is that correct? (#2 patch and the 3 different options)

What is preferred between #2 and #3? I don't have enough knowledge to
take the decision, which is why I suggested the chicken-hearted "disable
assert" option #1.

#2 can help allowing ui/cocoa to live in the same binary with ui/gtk and ui/sdl2 in the future and I believe it should be eventually (possible after 7.0) done, but requires relatively large code change.

#3 aims to solve only the problem with iothread dependency when creating menus and its scope of modification is minimal. It is concerned that #3 may confuse developers since it touches Cocoa interfaces from iothread although it is still according to its API convention.

I'm pretty sure both of #2 and #3 work, but I would like to rather insist #1 as the safest. As a developer, I don't want see such an ugly workaround like #1 of course, but such feeling does not matter much in practice. On the other hand, having the entire next release cycle for testing #2 is a real advantage even if the possibility of discovering a new problem is very tiny.

Regards,
Akihiko Odaki


Thanks,

Phil.

[*] 20220317125534.38706-1-philippe.mathieu.daude@gmail.com/">https://lore.kernel.org/qemu-devel/20220317125534.38706-1-philippe.mathieu.daude@gmail.com/




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