Messages in this thread | | | Date | Fri, 10 Jun 2016 15:07:33 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH] events/uprobes: move smp_read_barrier_depends() where needed |
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On 06/10, Andrea Parri wrote: > > On Fri, Jun 10, 2016 at 2:26 PM, Oleg Nesterov <oleg@redhat.com> wrote: > > > On 06/10, Oleg Nesterov wrote: > > > > > > On 06/09, Andrea Parri wrote: > > > > > > > > There is no need to use the barrier if there is no dereference/ > > > > memory access; move it where needed (currently, affecting only > > > > Alpha). > > > > > > OK, although area == NULL is unlikely case, > > > > > > > While touching this, also make the reads _ONCE(). > > > > > > Why? both xol_area/vaddr can't change. > > > > and perhaps it would be better to eliminate this smp_read_barrier_depends() > > altogether, we have a lot of helpers. I mean, > > > > static unsigned long get_trampoline_vaddr(void) > > { > > struct xol_area *area; > > > > area = > > lockless_dereference(current->mm->uprobes_state.xol_area); > > if (area) > > return area->vaddr; > > return -1; > > } > > > > looks a bit more simple/clean. Note also another smp_read_barrier_depends() > > in get_xol_area() which can be changed the same way. > > > > What do you think? > > > > More simply/clean, as you said, maybe; one advantage of keeping > the "raw" smp_read_barrier_depends() in get_trampoline_vaddr() is > that we can avoid it when area is NULL;
Do you really think it makes sense to optimize out read_barrier_depends here?
It can only be NULL in handle_swbp(), and in this case we are going to do a lot of work, and in particular install this xol vma,
> a similar solution is adopt- > ed in kernel/task_work.c:task_work_cancel().
Heh ;) this code was written before we had lockless_dereference(). And I do remember I thought that we need such a helper when read_barrier_depends() was added.
Oleg.
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