[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>

2 files changed, 10 insertions, 2 deletions

diff --git a/arch/um/kernel/skas/process_kern.c b/arch/um/kernel/skas/process_kern.c
index 3f70a2e12f06..14360ac17f02 100644
--- a/arch/um/kernel/skas/process_kern.c
+++ b/arch/um/kernel/skas/process_kern.c
@@ -35,6 +35,8 @@ void switch_to_skas(void *prev, void *next)
         switch_threads(&from->thread.mode.skas.switch_buf,
                        to->thread.mode.skas.switch_buf);
 
+        arch_switch_to_skas(current->thread.prev_sched, current);
+
         if(current->pid == 0)
                 switch_timers(1);
 }
diff --git a/arch/um/kernel/tt/process_kern.c b/arch/um/kernel/tt/process_kern.c
index 295c1ac817b3..a9c1443fc548 100644
--- a/arch/um/kernel/tt/process_kern.c
+++ b/arch/um/kernel/tt/process_kern.c
@@ -51,6 +51,13 @@ void switch_to_tt(void *prev, void *next)
 
         c = 0;
 
+        /* Notice that here we "up" the semaphore on which "to" is waiting, and
+         * below (the read) we wait on this semaphore (which is implemented by
+         * switch_pipe) and go sleeping. Thus, after that, we have resumed in
+         * "to", and can't use any more the value of "from" (which is outdated),
+         * nor the value in "to" (since it was the task which stole us the CPU,
+         * which we don't care about). */
+
         err = os_write_file(to->thread.mode.tt.switch_pipe[1], &c, sizeof(c));
         if(err != sizeof(c))
                 panic("write of switch_pipe failed, err = %d", -err);
@@ -77,7 +84,7 @@ void switch_to_tt(void *prev, void *next)
         change_sig(SIGALRM, alrm);
         change_sig(SIGPROF, prof);
 
-        arch_switch();
+        arch_switch_to_tt(prev_sched, current);
 
         flush_tlb_all();
         local_irq_restore(flags);
@@ -141,7 +148,6 @@ static void new_thread_handler(int sig)
         set_cmdline("(kernel thread)");
 
         change_sig(SIGUSR1, 1);
-        change_sig(SIGVTALRM, 1);
         change_sig(SIGPROF, 1);
         local_irq_enable();
         if(!run_kernel_thread(fn, arg, &current->thread.exec_buf))