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Commit 7d40b3134 ("MEDIUM: sched: do not run a same task multiple times
in series") required to slightly reorder a few fields in struct tasklet
and task in order to reuse an existing hole and keep tree nodes aligned.
The problem is that nice+expire were placed in struct task just before
rq, and that a 48-bit hole replaces them in struct tasklet on 64-bit
platforms, just before the struct list. However, on 32-bit platforms,
the hole is only 16-bit and preserves nice, but expire is overwritten
by the first pointer of the list element. This is not a problem for
real tasklets which do not use these fields, but it definitely is a
problem for tasks that are cast to tasklets in the run queues, because
the expire field can be overwritten when the task is woken up, and if
requeued as-is, it will expire at a completely random date.
This is what caused certain regtests to fail on i386 and 32-bit arm
machines.
This fix needs to be backported wherever the patch above was backported.
The bug has no effect on 64-bit platforms. The fix doesn't inflate
structs on 64-bit, but will raise struct tasklet from 40 to 44 bytes on
32-bit platforms.
Thanks to William for spotting the problem, bisecting it and providing
a working reproducer.