task_work_add: generic process-context callbacks

Provide a simple mechanism that allows running code in the (nonatomic)
context of the arbitrary task.

The caller does task_work_add(task, task_work) and this task executes
task_work->func() either from do_notify_resume() or from do_exit().  The
callback can rely on PF_EXITING to detect the latter case.

"struct task_work" can be embedded in another struct, still it has "void
*data" to handle the most common/simple case.

This allows us to kill the ->replacement_session_keyring hack, and
potentially this can have more users.

Performance-wise, this adds 2 "unlikely(!hlist_empty())" checks into
tracehook_notify_resume() and do_exit().  But at the same time we can
remove the "replacement_session_keyring != NULL" checks from
arch/*/signal.c and exit_creds().

Note: task_work_add/task_work_run abuses ->pi_lock.  This is only because
this lock is already used by lookup_pi_state() to synchronize with
do_exit() setting PF_EXITING.  Fortunately the scope of this lock in
task_work.c is really tiny, and the code is unlikely anyway.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: David Howells <dhowells@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Richard Kuo <rkuo@codeaurora.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Alexander Gordeev <agordeev@redhat.com>
Cc: Chris Zankel <chris@zankel.net>
Cc: David Smith <dsmith@redhat.com>
Cc: "Frank Ch. Eigler" <fche@redhat.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Larry Woodman <lwoodman@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 4 insertions, 1 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index 910a071..3d93325 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -946,11 +946,14 @@ void do_exit(long code)
 	exit_signals(tsk);  /* sets PF_EXITING */
 	/*
 	 * tsk->flags are checked in the futex code to protect against
-	 * an exiting task cleaning up the robust pi futexes.
+	 * an exiting task cleaning up the robust pi futexes, and in
+	 * task_work_add() to avoid the race with exit_task_work().
 	 */
 	smp_mb();
 	raw_spin_unlock_wait(&tsk->pi_lock);
 
+	exit_task_work(tsk);
+
 	exit_irq_thread();
 
 	if (unlikely(in_atomic()))