[PATCH] uml: add arch_switch_to for newly forked thread

Newly forked threads have no arch_switch_to_skas() called before their first
run, because when schedule() switches to them they're resumed in the body of
thread_wait() inside fork_handler() rather than in switch_threads() in
switch_to_skas().  Compensate this missing call.

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>

3 files changed, 25 insertions, 4 deletions

diff --git a/arch/um/kernel/skas/process_kern.c b/arch/um/kernel/skas/process_kern.c
index 38b185370c42..2135eaf98a93 100644
--- a/arch/um/kernel/skas/process_kern.c
+++ b/arch/um/kernel/skas/process_kern.c
@@ -91,10 +91,17 @@ void fork_handler(int sig)
                 panic("blech");
 
         schedule_tail(current->thread.prev_sched);
+
+        /* XXX: if interrupt_end() calls schedule, this call to
+         * arch_switch_to_skas isn't needed. We could want to apply this to
+         * improve performance. -bb */
+        arch_switch_to_skas(current->thread.prev_sched, current);
+
         current->thread.prev_sched = NULL;
 
 /* Handle any immediate reschedules or signals */
         interrupt_end();
+
         userspace(&current->thread.regs.regs);
 }
 
diff --git a/arch/um/sys-i386/ptrace.c b/arch/um/sys-i386/ptrace.c
index 6a23cc6947c3..6028bc7cc01b 100644
--- a/arch/um/sys-i386/ptrace.c
+++ b/arch/um/sys-i386/ptrace.c
@@ -23,7 +23,14 @@ void arch_switch_to_tt(struct task_struct *from, struct task_struct *to)
 
 void arch_switch_to_skas(struct task_struct *from, struct task_struct *to)
 {
-        arch_switch_tls_skas(from, to);
+        int err = arch_switch_tls_skas(from, to);
+        if (!err)
+                return;
+
+        if (err != -EINVAL)
+                printk(KERN_WARNING "arch_switch_tls_skas failed, errno %d, not EINVAL\n", -err);
+        else
+                printk(KERN_WARNING "arch_switch_tls_skas failed, errno = EINVAL\n");
 }
 
 int is_syscall(unsigned long addr)
diff --git a/arch/um/sys-i386/tls.c b/arch/um/sys-i386/tls.c
index e3c5bc593fae..2251654c6b45 100644
--- a/arch/um/sys-i386/tls.c
+++ b/arch/um/sys-i386/tls.c
@@ -70,8 +70,6 @@ static int get_free_idx(struct task_struct* task)
         return -ESRCH;
 }
 
-#define O_FORCE 1
-
 static inline void clear_user_desc(struct user_desc* info)
 {
         /* Postcondition: LDT_empty(info) returns true. */
@@ -84,6 +82,8 @@ static inline void clear_user_desc(struct user_desc* info)
         info->seg_not_present = 1;
 }
 
+#define O_FORCE 1
+
 static int load_TLS(int flags, struct task_struct *to)
 {
         int ret = 0;
@@ -162,7 +162,13 @@ void clear_flushed_tls(struct task_struct *task)
  * SKAS patch. */
 int arch_switch_tls_skas(struct task_struct *from, struct task_struct *to)
 {
-        return load_TLS(O_FORCE, to);
+        /* We have no need whatsoever to switch TLS for kernel threads; beyond
+         * that, that would also result in us calling os_set_thread_area with
+         * userspace_pid[cpu] == 0, which gives an error. */
+        if (likely(to->mm))
+                return load_TLS(O_FORCE, to);
+
+        return 0;
 }
 
 int arch_switch_tls_tt(struct task_struct *from, struct task_struct *to)
@@ -324,3 +330,4 @@ int ptrace_get_thread_area(struct task_struct *child, int idx,
 out:
         return ret;
 }
+