[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>

1 files changed, 6 insertions, 2 deletions

diff --git a/arch/um/sys-i386/ptrace.c b/arch/um/sys-i386/ptrace.c
index ff94eded93eb..927fcd955651 100644
--- a/arch/um/sys-i386/ptrace.c
+++ b/arch/um/sys-i386/ptrace.c
@@ -15,9 +15,13 @@
 #include "sysdep/sigcontext.h"
 #include "sysdep/sc.h"
 
-void arch_switch(void)
+void arch_switch_to_tt(struct task_struct *from, struct task_struct *to)
+{
+        update_debugregs(to->thread.arch.debugregs_seq);
+}
+
+void arch_switch_to_skas(struct task_struct *from, struct task_struct *to)
 {
-        update_debugregs(current->thread.arch.debugregs_seq);
 }
 
 int is_syscall(unsigned long addr)