arch/sh: use slow path for get_user_pages_fast() on SMP

The fast path of get_user_pages_fast() disables IRQs and then does:

 - gup_pud_range()
   - gup_pmd_range()
     - gup_pte_range()
       - flush_dcache_page()

However, flush_dcache_page() makes a smp_call_function(), and using
smp_call_function() when IRQs are disabled is not allowed.

In order to work around this problem, this commit prevents the fast
path from get_user_pages_fast() from being used on SMP, and directly
uses the slow path, which doesn't disable interrupts.

Fixes the following warning when get_futex_key() is called:

WARNING: CPU: 1 PID: 410 at kernel/smp.c:416 smp_call_function_many+0x1fc/0x2ac
Modules linked in:

CPU: 1 PID: 410 Comm: wdc-pfmu-test Tainted: G           O    4.13.0-00035-g0d3023c733a0-dirty torvalds#363
task: 97d9a120 task.stack: 97d2e000
PC is at smp_call_function_many+0x1fc/0x2ac
PR is at smp_call_function_many+0x1e2/0x2ac
PC  : 88071fd4 SP  : 97d2fcd4 SR  : 400080f1
TEA : c06d6840
R0  : 000000f0 R1  : 00000002 R2  : 00000000 R3  : 8849c89c
R4  : 886d62c4 R5  : 886d62c4 R6  : 886db928 R7  : 884a9dec
R8  : 00000000 R9  : ffffffff R10 : 8844667c R11 : 29ee9000
R12 : 8844667c R13 : 00000001 R14 : 97d2fcd4
MACH: 000177e7 MACL: 0000018c GBR : 295742d0 PR  : 88071fba

Call trace:
 [<8800fe2c>] sh4_flush_dcache_page+0x0/0xa4
 [<880720a2>] smp_call_function+0x1e/0x50
 [<8800fe2c>] sh4_flush_dcache_page+0x0/0xa4
 [<8800ecc0>] flush_dcache_page+0x34/0x74
 [<88010d1c>] gup_pte_range+0xd0/0x140
 [<88010e04>] gup_pmd_range+0x78/0xa8
 [<88010c4c>] gup_pte_range+0x0/0x140
 [<88011006>] get_user_pages_fast+0xda/0x130
 [<8806f376>] get_futex_key+0x72/0x3a0
 [<8806f6a4>] futex_wait_setup+0x0/0x10c
 [<88055a08>] __rcu_read_unlock+0x0/0x54
 [<880559c8>] __rcu_read_lock+0x0/0x1c
 [<88010f2c>] get_user_pages_fast+0x0/0x130
 [<8806f6de>] futex_wait_setup+0x3a/0x10c
 [<8806f6a4>] futex_wait_setup+0x0/0x10c
 [<8806f852>] futex_wait+0xa2/0x1b8
 [<8806f6a4>] futex_wait_setup+0x0/0x10c
 [<88071c00>] smp_call_function_single+0xac/0x108
 [<8800e908>] cache_noop+0x0/0xc
 [<8800e908>] cache_noop+0x0/0xc
 [<88071018>] do_futex+0x108/0x8f8
 [<880bdfe4>] finish_fault+0x38/0x70
 [<880bdff2>] finish_fault+0x46/0x70
 [<880bf212>] handle_mm_fault+0x2f6/0x954
 [<880bf21a>] handle_mm_fault+0x2fe/0x954
 [<88030e64>] resched_curr+0x54/0x6c
 [<8832be9e>] _raw_spin_lock_irqsave+0xa/0x18
 [<880718dc>] SyS_futex+0xd4/0x124
 [<8801488e>] SyS_clone+0x16/0x24
 [<8800727e>] syscall_call+0x18/0x1e
 [<88071808>] SyS_futex+0x0/0x124

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

arch/sh/mm/gup.c

@@ -234,6 +234,9 @@ int get_user_pages_fast(unsigned long start, int nr_pages, int write,
 	if (end < start)
 		goto slow_irqon;
 
+	if (IS_ENABLED(CONFIG_SMP))
+		goto slow_irqon;
+
 	local_irq_disable();
 	pgdp = pgd_offset(mm, addr);
 	do {