硬件： 飞凌 imx6dl 开发板 软件： linux 4.1.15 X11 qt5.6.2

问题的现象： QT应用程序运行一段时间后会卡死（界面不刷新）， 此时， top命令可以看到程序进程还在，cpu占用率为0 （图中Impella进程）， 内核状态也正常，内存资源看起来也正常， 触摸屏事件也能捕捉到，再运行其他qt程序也可以工作，唯独跑了一段时间后的Impella卡死了。 复现时间间隔不固定，可能两到三小时，或者一到两天。 解决过程： 问题debug了 很久，基于对开发板厂商的信任， 一开始 怀疑QT 程序内有死锁，或陷入睡眠，死循环等。 测试发现死锁或者睡眠，的现象与问题的现象一致，都是进程卡死，cpu占用率为0 死循环 也会导致卡死，但是cpu占用率很高，与问题不一致， 梳理代码后我认为，死锁，或陷入睡眠，死循环的概率很小。 代码中没有用信号量或者sleep等

又开始怀疑 是不是某槽函数阻塞导致假死， 一段时间百度折腾后，网上答案五花八门，都没解决我的问题。

最后想到用gdb去查看程序卡死时的状态，调用栈等。 程序卡死后，我们可以gdb attach 程序对应的pid 然后通过 bt full查看程序调用栈，

(gdb) bt full #0 0x75e5fa0c in pthread_cond_wait () from /lib/libpthread.so.0 No symbol table info available. #1 0x75856b3c in _XReply () from /usr/lib/libX11.so.6 No symbol table info available. #2 0x758593e4 in ?? () from /usr/lib/libX11.so.6 No symbol table info available. Backtrace stopped: previous frame identical to this frame (corrupt stack?)

从上面log可以看到, 程序卡死时，进入了自下往上 _XReply () > pthread_cond_wait 最终陷入了等待， 原来是 libX11 的锅。

查看 libX11 的问题记录发现， 官方在1.6.9以后的版本中都修复了此问题， https://gitlab.freedesktop.org/xorg/lib/libx11/-/merge_requests/13/diffs

Avoid recursing through _XError due to sequence adjustment This patch is based on research done by Dmitry Osipenko to uncover the cause of a large class of Xlib lockups.

_XError must unlock and re-lock the display around the call to the user error handler function. When re-locking the display, two functions are called to ensure that the display is ready to generate a request:

The first ensures that there is at least one XID available to use (possibly calling _xcb_generate_id to do so). The second makes sure a reply is received at least every 65535 requests to keep sequence numbers in sync (possibly generating a GetInputFocus request and synchronously awaiting the reply).

If the second of these does generate a GetInputFocus request and wait for the reply, then a pending error will cause recursion into _XError, which deadlocks the display.

One seemingly easy fix is to have _XError avoid those calls by invoking InternalLockDisplay instead of LockDisplay…

1.7.0版本中又进行了调整如下： https://gitlab.freedesktop.org/xorg/lib/libx11/-/commit/30ccef3a48029bf4fc31d4abda2d2778d0ad6277

Avoid recursing through _XError due to sequence adjustment This patch is based on research done by Dmitry Osipenko to uncover the cause of a large class of Xlib lockups.

_XError must unlock and re-lock the display around the call to the user error handler function. When re-locking the display, two functions are called to ensure that the display is ready to generate a request:

The first ensures that there is at least one XID available to use (possibly calling _xcb_generate_id to do so). The second makes sure a reply is received at least every 65535 requests to keep sequence numbers in sync (possibly generating a GetInputFocus request and synchronously awaiting the reply).

If the second of these does generate a GetInputFocus request and wait for the reply, then a pending error will cause recursion into _XError, which deadlocks the display.

One seemingly easy fix is to have _XError avoid those calls by invoking InternalLockDisplay instead of LockDisplay…

解决方法：

我目前的bsp中的libX11 版本 是 1.6.3 ， 所以把libX11 的版本更新到 1.6.9 以上的版本即可 我最终选择更新到当前最新版本1.7.2 libX11 源码下载地址 https://www.x.org/releases/individual/lib/

我一般都对平台比较信任的，但出了问题，就纯纯要人命啊。 希望此记录能帮助有缘人避坑。