task_work: Make task_work_add() lockless
task_work: Make task_work_add() lockless

Change task_work's to use llist-like code to avoid pi_lock
in task_work_add(), this makes it useable under rq->lock.

task_work_cancel() and task_work_run() still use pi_lock
to synchronize with each other.

(This is in preparation for a deadlock fix.)

Suggested-by: Peter Zijlstra <3fddac958924aef220f202ca567388ddab3f14a8@infradead.org>
Signed-off-by: Oleg Nesterov <20b70f0af00562e63758b9ee42012ecc96c58590@redhat.com>
Signed-off-by: Peter Zijlstra <645ca7d3a8d3d4f60557176cd361ea8351edc32b@chello.nl>
Cc: Al Viro <de609eb4d5d70b1d38ec6642adbfc33a2781f63c@zeniv.linux.org.uk>
Cc: Linus Torvalds <69652caca27c8b940640ad396ab71f93cacec34f@linux-foundation.org>
Cc: Andrew Morton <5c1e68b099950c134891f0b6e179498a8ebe9cf9@linux-foundation.org>
Cc: Thomas Gleixner <00e4cf8f46a57000a44449bf9dd8cbbcc209fd2a@linutronix.de>
Link: a1727dfc74e4256411f8765122d4f69e02000632@redhat.com
Signed-off-by: Ingo Molnar <9dbbbf0688fedc85ad4da37637f1a64b8c718ee2@kernel.org>

KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas