[PATCH 3.16 158/366] x86: Call fixup_exception() before notify_die() in math_error()

[Ben Hutchings]

3.16.61-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Siarhei Liakh <Siarhei.Liakh@xxxxxxxxxxxxxxxxx>

commit 3ae6295ccb7cf6d344908209701badbbbb503e40 upstream.

fpu__drop() has an explicit fwait which under some conditions can trigger a
fixable FPU exception while in kernel. Thus, we should attempt to fixup the
exception first, and only call notify_die() if the fixup failed just like
in do_general_protection(). The original call sequence incorrectly triggers
KDB entry on debug kernels under particular FPU-intensive workloads.

Andy noted, that this makes the whole conditional irq enable thing even
more inconsistent, but fixing that it outside the scope of this.

Signed-off-by: Siarhei Liakh <siarhei.liakh@xxxxxxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: "Borislav  Petkov" <bpetkov@xxxxxxx>
Link: https://lkml.kernel.org/r/DM5PR11MB201156F1CAB2592B07C79A03B17D0@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 arch/x86/kernel/traps.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -556,17 +556,19 @@ static void math_error(struct pt_regs *r
 	char *str = (trapnr == X86_TRAP_MF) ? "fpu exception" :
 						"simd exception";
 
-	if (notify_die(DIE_TRAP, str, regs, error_code, trapnr, SIGFPE) == NOTIFY_STOP)
-		return;
 	conditional_sti(regs);
 
 	if (!user_mode_vm(regs))
 	{
-		if (!fixup_exception(regs)) {
-			task->thread.error_code = error_code;
-			task->thread.trap_nr = trapnr;
+		if (fixup_exception(regs))
+			return;
+
+		task->thread.error_code = error_code;
+		task->thread.trap_nr = trapnr;
+
+		if (notify_die(DIE_TRAP, str, regs, error_code,
+					trapnr, SIGFPE) != NOTIFY_STOP)
 			die(str, regs, error_code);
-		}
 		return;
 	}