powerpc/icswx: Fix race condition with IPI setting ACOP

There is a race where a thread causes a coprocessor type to be valid
in its own ACOP _and_ in the current context, but it does not
propagate to the ACOP register of other threads in time for them to
use it.  The original code tries to solve this by sending an IPI to
all threads on the system, which is heavy handed, but unfortunately
still provides a window where the icswx is issued by other threads and
the ACOP is not up to date.

This patch detects that the ACOP DSI fault was a "false positive" and
syncs the ACOP and causes the icswx to be replayed.
Signed-off-by: Jimi Xenidis <jimix@pobox.com>
Cc: Anton Blanchard <anton@samba.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/mm/icswx.c

@@ -163,7 +163,7 @@ EXPORT_SYMBOL_GPL(drop_cop);
 
 static int acop_use_cop(int ct)
 {
-	/* todo */
+	/* There is no alternate policy, yet */
 	return -1;
 }
 
@@ -227,11 +227,30 @@ int acop_handle_fault(struct pt_regs *regs, unsigned long address,
 		ct = (ccw >> 16) & 0x3f;
 	}
 
+	/*
+	 * We could be here because another thread has enabled acop
+	 * but the ACOP register has yet to be updated.
+	 *
+	 * This should have been taken care of by the IPI to sync all
+	 * the threads (see smp_call_function(sync_cop, mm, 1)), but
+	 * that could take forever if there are a significant amount
+	 * of threads.
+	 *
+	 * Given the number of threads on some of these systems,
+	 * perhaps this is the best way to sync ACOP rather than whack
+	 * every thread with an IPI.
+	 */
+	if ((acop_copro_type_bit(ct) & current->active_mm->context.acop) != 0) {
+		sync_cop(current->active_mm);
+		return 0;
+	}
+
+	/* check for alternate policy */
 	if (!acop_use_cop(ct))
 		return 0;
 
 	/* at this point the CT is unknown to the system */
-	pr_warn("%s[%d]: Coprocessor %d is unavailable",
+	pr_warn("%s[%d]: Coprocessor %d is unavailable\n",
 		current->comm, current->pid, ct);
 
 	/* get inst if we don't already have it */

arch/powerpc/mm/icswx.h

@@ -59,4 +59,10 @@ extern void free_cop_pid(int free_pid);
 
 extern int acop_handle_fault(struct pt_regs *regs, unsigned long address,
 			     unsigned long error_code);
+
+static inline u64 acop_copro_type_bit(unsigned int type)
+{
+	return 1ULL << (63 - type);
+}
+
 #endif /* !_ARCH_POWERPC_MM_ICSWX_H_ */