[PATCH 3.10 22/71] MIPS: Remove BUG_ON(!is_fpu_owner()) in do_ade()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

3.10-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Huacai Chen <chenhc@lemote.com>

commit 2e5767a27337812f6850b3fa362419e2f085e5c3 upstream.

In do_ade(), is_fpu_owner() isn't preempt-safe. For example, when an
unaligned ldc1 is executed, do_cpu() is called and then FPU will be
enabled (and TIF_USEDFPU will be set for the current process). Then,
do_ade() is called because the access is unaligned.  If the current
process is preempted at this time, TIF_USEDFPU will be cleard.  So when
the process is scheduled again, BUG_ON(!is_fpu_owner()) is triggered.

This small program can trigger this BUG in a preemptible kernel:

int main (int argc, char *argv[])
{
        double u64[2];

        while (1) {
                asm volatile (
                        ".set push \n\t"
                        ".set noreorder \n\t"
                        "ldc1 $f3, 4(%0) \n\t"
                        ".set pop \n\t"
                        ::"r"(u64):
                );
        }

        return 0;
}

V2: Remove the BUG_ON() unconditionally due to Paul's suggestion.

Signed-off-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Jie Chen <chenj@lemote.com>
Signed-off-by: Rui Wang <wangr@lemote.com>
Cc: John Crispin <john@phrozen.org>
Cc: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: Fuxin Zhang <zhangfx@lemote.com>
Cc: Zhangjin Wu <wuzhangjin@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 arch/mips/kernel/unaligned.c |    1 -
 1 file changed, 1 deletion(-)

--- a/arch/mips/kernel/unaligned.c
+++ b/arch/mips/kernel/unaligned.c
@@ -604,7 +604,6 @@ static void emulate_load_store_insn(stru
 	case sdc1_op:
 		die_if_kernel("Unaligned FP access in kernel code", regs);
 		BUG_ON(!used_math());
-		BUG_ON(!is_fpu_owner());
 
 		lose_fpu(1);	/* Save FPU state for the emulator. */
 		res = fpu_emulator_cop1Handler(regs, &current->thread.fpu, 1,