linux-kernel - Re: [PATCH -next 0/2] fs/epoll: loosen irq safety when possible

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Date:   Tue, 24 Jul 2018 16:43:34 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     Peter Zijlstra <peterz@...radead.org>, jbaron@...mai.com,
        viro@...iv.linux.org.uk, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next 0/2] fs/epoll: loosen irq safety when possible

On Sat, 21 Jul 2018 11:31:27 -0700 Davidlohr Bueso <dave@...olabs.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);
_