Merge branch 'sparc-perf-stack'

David Ahern says:

====================
sparc64: perf fixes for userspace stacks

Coming back to the perf userspace callchain problem. As a reminder there are
a series of problems trying to use perf to collect callchains with scheduling
tracepoints, e.g., perf sched record -g -- <cmd>.

The first patch disables pagefaults while walking the user stack. As discussed
a couple of months ago this is the right fix, but I was puzzled as to why
processes were terminating with sigbus (and sometimes sigsegv). I believe the
root of this problem is bad addresses trying to walk the frames using frame
pointers. The bad addresses lead to faults that get handled by do_sparc64_fault
and it aborts the task though I am still puzzled as to why it gets past this
check in do_sparc64_fault:

        if (in_atomic() || !mm)
                goto intr_or_no_mm;

pagefault_disable bumps the preempt_count which should make in_atomic return != 0
(building kernels with preemption set to voluntar, CONFIG_PREEMPT_VOLUNTARY=y).

While this set does not fully solve the problem it does prevent a number of
pain points with the current code, most notably able to lock up the system.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>

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