[PATCH v2 0/1] uprobes: Use percpu_rw_semaphore to fix register/unregister vs dup_mmap() race

[Oleg Nesterov <oleg@redhat.com>]

Hello.

The same patch I sent before, the only difference is that it uses
percpu_rw_semaphore instead of brw_mutex.

Srikar, I'll hope you can ack v2 too, and unless someone objects
I'll ask Ingo to pull this fix.

Tejun, recently we briefly discussed signal->group_rwsem, please
see the note about cgroups below.

Note:

	- The current implementation of percpu_rw_semaphore is not
	  optimal, register/unregister will block fork() completely
	  while it sleeps in msleep() and synchronize_sched().

	  So this patch assumes that

	  	percpu_rw_semaphore-reimplement-to-not-block-the-readers-unnecessarily.patch
	  	http://marc.info/?l=linux-mm-commits&m=135240650828875

	  will be applied eventually (and the new implementation can be
	  improved).

	- This patch adds percpu_down_read/up_read around dup_mmap()
	  for uprobes.

	  Given that it is very cheap (and assuming the optimizations
	  above), _perhaps_ we can turn this dup_mmap_sem into fork_sem
	  and use it instead of threadgroup_change_begin/end, so that
	  uprobes and cgroups can use it.

	- Compared to v1, percpu_rw_semaphore doesn't support multi-
	  writers. I hope we can tolerate this, register/unregister
	  are system-wide and rare events anyway. And _perhaps_ we
	  can add percpu_down_write_nonexclusive() later.

	- Given that currently percpu_down_write() is exclusive, this
	  patch almost dismisses the purpose of uprobes_mutex[] array.

	  Yes, but we need to rework this locking anyway for filtering.

Oleg.