Messages in this thread Patch in this message | | | Date | Tue, 24 Jul 2018 16:43:34 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH -next 0/2] fs/epoll: loosen irq safety when possible |
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On Sat, 21 Jul 2018 11:31:27 -0700 Davidlohr Bueso <dave@stgolabs.net> wrote:
> On Sat, 21 Jul 2018, Peter Zijlstra wrote: > > >On Sat, Jul 21, 2018 at 10:21:20AM -0700, Davidlohr Bueso wrote: > >> On Fri, 20 Jul 2018, Andrew Morton wrote: > >> > >> > We could open-code it locally. Add a couple of > >> > WARN_ON_ONCE(irqs_disabled())? That might need re-benchmarking with > >> > Xen but surely just reading the thing isn't too expensive? > >> > >> We could also pass on the responsibility to lockdep and just use > >> lockdep_assert_irqs_disabled(). But I guess that would be less effective > >> than to just open code it in epoll without lockdep -- note that over 80 > >> places in the kernel do this. > > > >The lockdep thing is relatively recent. I think someone proposed to go > >replace a bunch of the open-coded ones at some point. > > For the open coded checks, I'm seeing a small (1-2% ish) cost for bare > metal on workload 1). I don't see (via code inspection) any additional > overhead in xen either. While negligible in the overall of things, I do > like the idea of lockdep handling it nonetheless.
It's good that the overhead goes away when lockdep is disabled. That's a big advantage over open-coding it.
> I can add the open coded version if people really feel that it would catch > more bugs (no lockdep users out there in production afaik :) in the long > term; but if lockdep is where things are headed... > > ... > > --- a/fs/eventpoll.c > +++ b/fs/eventpoll.c > @@ -670,6 +670,9 @@ static __poll_t ep_scan_ready_list(struct eventpoll *ep, > struct epitem *epi, *nepi; > LIST_HEAD(txlist); > > + /* must not be called with irqs off */ > + lockdep_assert_irqs_enabled();
Although I be the humble comment's most avid fan, these ones are too obvious to be worth the space ;)
Would you survive if I zapped 'em?
--- a/fs/eventpoll.c~fs-epoll-robustify-irq-safety-with-lockdep_assert_irqs_enabled-fix +++ a/fs/eventpoll.c @@ -670,7 +670,6 @@ static __poll_t ep_scan_ready_list(struc struct epitem *epi, *nepi; LIST_HEAD(txlist); - /* must not be called with irqs off */ lockdep_assert_irqs_enabled(); /* @@ -767,7 +766,6 @@ static int ep_remove(struct eventpoll *e { struct file *file = epi->ffd.file; - /* must not be called with irqs off */ lockdep_assert_irqs_enabled(); /* @@ -1418,7 +1416,6 @@ static int ep_insert(struct eventpoll *e struct epitem *epi; struct ep_pqueue epq; - /* must not be called with irqs off */ lockdep_assert_irqs_enabled(); user_watches = atomic_long_read(&ep->user->epoll_watches); @@ -1549,7 +1546,6 @@ static int ep_modify(struct eventpoll *e int pwake = 0; poll_table pt; - /* must not be called with irqs off */ lockdep_assert_irqs_enabled(); init_poll_funcptr(&pt, NULL); _
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