kthreads: kill CLONE_KERNEL, change kernel_thread(kernel_init) to avoid CLONE_SIGHAND

1. Remove CLONE_KERNEL, it has no users and it is dangerous.

   The (old) comment says "List of flags we want to share for kernel
   threads" but this is not true, we do not want to share ->sighand by
   default. This flag can only be used if the caller is sure that both
   parent/child will never play with signals (say, allow_signal/etc).

2. Change rest_init() to clone kernel_init() without CLONE_SIGHAND.

   In this case CLONE_SIGHAND does not really hurt, and it looks like
   optimization because copy_sighand() can avoid kmem_cache_alloc().

   But in fact this only adds the minor pessimization. kernel_init()
   is going to exec the init process, and de_thread() will need to
   unshare ->sighand and do kmem_cache_alloc(sighand_cachep) anyway,
   but it needs to do more work and take tasklist_lock and siglock.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 77c941f43413d4e5299a6e4b5880933da72ce92c..c1efe3926d1750923f8714f9de846671651e45ce 100644 (file)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -136,12 +136,6 @@ struct filename;
 #define VMACACHE_SIZE (1U << VMACACHE_BITS)
 #define VMACACHE_MASK (VMACACHE_SIZE - 1)
 
-/*
- * List of flags we want to share for kernel threads,
- * if only because they are not used by them anyway.
- */
-#define CLONE_KERNEL   (CLONE_FS | CLONE_FILES | CLONE_SIGHAND)
-
 /*
  * These are the constant used to fake the fixed-point load-average
  * counting. Some notes:

diff --git a/init/main.c b/init/main.c
index 70dd27766adfc50956d9d90721555e80b08e26af..155976ff912ca5cceac0b09fc34e3be3cddc91d8 100644 (file)
--- a/init/main.c
+++ b/init/main.c
@@ -380,7 +380,7 @@ static noinline void __init_refok rest_init(void)
         * the init task will end up wanting to create kthreads, which, if
         * we schedule it before we create kthreadd, will OOPS.
         */
-       kernel_thread(kernel_init, NULL, CLONE_FS | CLONE_SIGHAND);
+       kernel_thread(kernel_init, NULL, CLONE_FS);
        numa_default_policy();
        pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);
        rcu_read_lock();