Frank Rowand wrote:
john cooper wrote:
... There is also a bug fix
contained for tlb_gather_mmu() which was causing debug
assertions to be generated in a path which attempted to
sleep with a non-zero preempt count.
Manish Lachwani mentioned to me that he faced the same issue
with the MIPS RT support and that when he discussed
it with Ingo that the solution was for include/asm-ppc/tlb.h
to include/asm-generic/tlb-simple.h when PREEMPT_RT is turned on.
The patch does this for the #ifdef CONFIG_PPC_STD_MMU case,
but not for the #else case. I don't know which case is used
for the Ampro board.
It appeared to me a generic issue though I believe a number
of solutions are possible. asm-generic/tlb.h:tlb_gather_mmu()
expands to linux/percpu.h:get_cpu_var() which does a
preempt_disable() and __get_cpu_var(). This caused the debug
assertion to kick when __page_cache_release() and to a lesser
extent activate_page() attempted to block on a mutex (though
other paths may well exist). My approach was to replace the
outer layer preempt_disable/enable calls with a mutex-spinlock.
The fix was fairly easy once it was known from where the
gratuitous call to preempt_disable() existed. I cobbled
together a logging mechanism which detected the problem. As it
wasn't very general I removed it from the patch. I didn't see
an alternate means of diagnosing such a scenario so I'll likely
address generalizing the code.
-john
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