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

[Daniel Vetter]

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.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch