Re: [PATCH 3/5] kernel.h: Add non_block_start/end()

[Daniel Vetter]

On Wed, Aug 28, 2019 at 8:56 PM Daniel Vetter <daniel.vetter@xxxxxxxx> wrote:
> On Wed, Aug 28, 2019 at 8:43 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> > On Wed, Aug 28, 2019 at 08:33:13PM +0200, Daniel Vetter wrote:
> > > On Wed, Aug 28, 2019 at 12:50 AM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> > > >
> > > > > diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> > > > > index 4fa360a13c1e..82f84cfe372f 100644
> > > > > +++ b/include/linux/kernel.h
> > > > > @@ -217,7 +217,9 @@ extern void __cant_sleep(const char *file, int line, int preempt_offset);
> > > > >   * might_sleep - annotation for functions that can sleep
> > > > >   *
> > > > >   * this macro will print a stack trace if it is executed in an atomic
> > > > > - * context (spinlock, irq-handler, ...).
> > > > > + * context (spinlock, irq-handler, ...). Additional sections where blocking is
> > > > > + * not allowed can be annotated with non_block_start() and non_block_end()
> > > > > + * pairs.
> > > > >   *
> > > > >   * This is a useful debugging help to be able to catch problems early and not
> > > > >   * be bitten later when the calling function happens to sleep when it is not
> > > > > @@ -233,6 +235,25 @@ extern void __cant_sleep(const char *file, int line, int preempt_offset);
> > > > >  # define cant_sleep() \
> > > > >       do { __cant_sleep(__FILE__, __LINE__, 0); } while (0)
> > > > >  # define sched_annotate_sleep()      (current->task_state_change = 0)
> > > > > +/**
> > > > > + * non_block_start - annotate the start of section where sleeping is prohibited
> > > > > + *
> > > > > + * This is on behalf of the oom reaper, specifically when it is calling the mmu
> > > > > + * notifiers. The problem is that if the notifier were to block on, for example,
> > > > > + * mutex_lock() and if the process which holds that mutex were to perform a
> > > > > + * sleeping memory allocation, the oom reaper is now blocked on completion of
> > > > > + * that memory allocation. Other blocking calls like wait_event() pose similar
> > > > > + * issues.
> > > > > + */
> > > > > +# define non_block_start() \
> > > > > +     do { current->non_block_count++; } while (0)
> > > > > +/**
> > > > > + * non_block_end - annotate the end of section where sleeping is prohibited
> > > > > + *
> > > > > + * Closes a section opened by non_block_start().
> > > > > + */
> > > > > +# define non_block_end() \
> > > > > +     do { WARN_ON(current->non_block_count-- == 0); } while (0)
> > > >
> > > > check-patch does not like these, and I agree
> > > >
> > > > #101: FILE: include/linux/kernel.h:248:
> > > > +# define non_block_start() \
> > > > +       do { current->non_block_count++; } while (0)
> > > >
> > > > /tmp/tmp1spfxufy/0006-kernel-h-Add-non_block_start-end-.patch:108: WARNING: Single statement macros should not use a do {} while (0) loop
> > > > #108: FILE: include/linux/kernel.h:255:
> > > > +# define non_block_end() \
> > > > +       do { WARN_ON(current->non_block_count-- == 0); } while (0)
> > > >
> > > > Please use a static inline?
> > >
> > > We need get_current() plus the task_struct, so this gets real messy
> > > real fast. Not even sure which header this would fit in, or whether
> > > I'd need to create a new one. You're insisting on this or respinning
> > > with the do { } while (0) dropped ok.
> >
> > My prefernce is always a static inline, but if the headers are so
> > twisty we need to use #define to solve a missing include, then I
> > wouldn't insist on it.
>
> Cleanest would be a new header I guess, together with might_sleep().
> But moving that is a bit much I think, there's almost 500 callers of
> that one from a quick git grep
>
> > If dropping do while is the only change then I can edit it in..
> > I think we have the acks now
>
> Yeah sounds simplest, thanks.

Hi Jason,

Do you expect me to resend now, or do you plan to do the patchwork
appeasement when applying? I've seen you merged the other patches
(thanks!), but not these two here.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch