[PATCH] uml: clean arch_switch usage

Call arch_switch also in switch_to_skas, even if it's, for now, a no-op for
that case (and mark this in the comment); this will change soon.

Also, arch_switch for TT mode is actually useless when the PT proxy (a
complicate debugging instrumentation for TT mode) is not enabled.  In fact, it
only calls update_debugregs, which checks debugregs_seq against seq (to check
if the registers are up-to-date - seq here means a "version number" of the
registers).

If the ptrace proxy is not enabled, debugregs_seq always stays 0 and
update_debugregs will be a no-op.  So, optimize this out (the compiler can't
do it).

Also, I've been disappointed by the fact that it would make a lot of sense if,
after calling a successful
update_debugregs(current->thread.arch.debugregs_seq),
current->thread.arch.debugregs_seq were updated with the new debugregs_seq.
But this is not done.  Is this a bug or a feature?  For all purposes, it seems
a bug (otherwise the whole mechanism does not make sense, which is also a
possibility to check), which causes some performance only problems (not
correctness), since we write_debugregs when not needed.

Also, as suggested by Jeff, remove a redundant enabling of SIGVTALRM,
comprised in the subsequent local_irq_enable().  I'm just a bit dubious if
ordering matters there...

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Acked-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/um/sys-i386/ptrace_user.c

@@ -14,6 +14,7 @@
 #include "sysdep/thread.h"
 #include "user.h"
 #include "os.h"
+#include "uml-config.h"
 
 int ptrace_getregs(long pid, unsigned long *regs_out)
 {
@@ -43,6 +44,7 @@ int ptrace_setfpregs(long pid, unsigned long *regs)
 	return 0;
 }
 
+/* All the below stuff is of interest for TT mode only */
 static void write_debugregs(int pid, unsigned long *regs)
 {
 	struct user *dummy;
@@ -75,7 +77,6 @@ static void read_debugregs(int pid, unsigned long *regs)
 
 /* Accessed only by the tracing thread */
 static unsigned long kernel_debugregs[8] = { [ 0 ... 7 ] = 0 };
-static int debugregs_seq = 0;
 
 void arch_enter_kernel(void *task, int pid)
 {
@@ -89,6 +90,11 @@ void arch_leave_kernel(void *task, int pid)
 	write_debugregs(pid, TASK_DEBUGREGS(task));
 }
 
+#ifdef UML_CONFIG_PT_PROXY
+/* Accessed only by the tracing thread */
+static int debugregs_seq;
+
+/* Only called by the ptrace proxy */
 void ptrace_pokeuser(unsigned long addr, unsigned long data)
 {
 	if((addr < offsetof(struct user, u_debugreg[0])) ||
@@ -109,6 +115,7 @@ static void update_debugregs_cb(void *arg)
 	write_debugregs(pid, kernel_debugregs);
 }
 
+/* Optimized out in its header when not defined */
 void update_debugregs(int seq)
 {
 	int me;
@@ -118,6 +125,7 @@ void update_debugregs(int seq)
 	me = os_getpid();
 	initial_thread_cb(update_debugregs_cb, &me);
 }
+#endif
 
 /*
  * Overrides for Emacs so that we follow Linus's tabbing style.