[Bug 111541] Cursor sprite sometimes not showed since linux 5.2

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=111541

--- Comment #14 from Jonas Ådahl <jadahl at gmail.com> ---
Created attachment 145329
  --> https://bugs.freedesktop.org/attachment.cgi?id=145329&action=edit
dump-gbm-bo.c

Using the attached function (compile, break the compositor process using gdb,
then run "print dlopen("/path/to/compiled/dump-gbm-bo.so", 2)") I attempted to
look at the contents of the cursor buffer before it was passed to
drmModeSetCursor2().

When reproducing, it seems that it contains only 0s, but the rest is correct
(e.g. size). When it's not reproducing, the dump at the equivalent timing shows
correct content. I haven't verified that a dump of the same gbm_bo is correct
immediately after writing pixels.

In the cursor renderer code in mutter, we only ever write to a gbm_bo
immediately after its construction, and we never map the memory after that. As
the case is that this only reproduces with some kernel versions, I have my
doubt it's that we upload empty pixels, but will add some code that dumps after
construction too to be sure. What could cause memory of a gbm_bo to be cleared
after its construction, but before its destruction?

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