Page faults generated walking userspace stacks can call schedule to switch
out the task. When collecting callchains for scheduler tracepoints this
causes a deadlock as the tracepoints can be hit with the runqueue lock held:

[ 8138.159054] WARNING: CPU: 758 PID: 12488 at /opt/dahern/linux.git/arch/sparc/kernel/nmi.c:80 perfctr_irq+0x1f8/0x2b4()

[ 8138.203152] Watchdog detected hard LOCKUP on cpu 758

[ 8138.410969] CPU: 758 PID: 12488 Comm: perf Not tainted 4.0.0-rc6+ #6
[ 8138.437146] Call Trace:
[ 8138.447193]  [000000000045cdd4] warn_slowpath_common+0x7c/0xa0
[ 8138.471238]  [000000000045ce90] warn_slowpath_fmt+0x30/0x40
[ 8138.494189]  [0000000000983e38] perfctr_irq+0x1f8/0x2b4
[ 8138.515716]  [00000000004209f4] tl0_irq15+0x14/0x20
[ 8138.535791]  [00000000009839ec] _raw_spin_trylock_bh+0x68/0x108
[ 8138.560180]  [0000000000980018] __schedule+0xcc/0x710
[ 8138.580981]  [00000000009806dc] preempt_schedule_common+0x10/0x3c
[ 8138.606082]  [000000000098077c] _cond_resched+0x34/0x44
[ 8138.627603]  [0000000000565990] kmem_cache_alloc_node+0x24/0x1a0
[ 8138.652345]  [0000000000450b60] tsb_grow+0xac/0x488
[ 8138.672429]  [0000000000985040] do_sparc64_fault+0x4dc/0x6e4
[ 8138.695736]  [0000000000407c2c] sparc64_realfault_common+0x10/0x20
[ 8138.721202]  [00000000006f2e24] NG4copy_from_user+0xa4/0x3c0
[ 8138.744510]  [000000000044f900] perf_callchain_user+0x5c/0x6c
[ 8138.768182]  [0000000000517b5c] perf_callchain+0x16c/0x19c
[ 8138.790774]  [0000000000515f84] perf_prepare_sample+0x68/0x218
[ 8138.814801] ---[ end trace 42ca6294b1ff7573 ]---

As with PowerPC (b59a1bfc, "powerpc/perf: Disable pagefaults during
callchain stack read") disable pagefaults while walking userspace stacks.
Signed-off-by: David Ahern <david.ahern@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>