linux-kernel - RE: [PATCH 1/2 v2] cleanup: Add cond

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Message-ID: <65c15b025484f_3744792942a@iweiny-mobl.notmuch>
Date: Mon, 5 Feb 2024 14:02:42 -0800
From: Ira Weiny <ira.weiny@...el.com>
To: Dan Williams <dan.j.williams@...el.com>, "Fabio M. De Francesco"
	<fabio.maria.de.francesco@...ux.intel.com>, Peter Zijlstra
	<peterz@...radead.org>, <linux-kernel@...r.kernel.org>
CC: <linux-cxl@...r.kernel.org>, Ingo Molnar <mingo@...nel.org>, "Jonathan
 Cameron" <Jonathan.Cameron@...wei.com>, "Fabio M. De Francesco"
	<fabio.maria.de.francesco@...ux.intel.com>, Ira Weiny <ira.weiny@...el.com>
Subject: RE: [PATCH 1/2 v2] cleanup: Add cond_guard() to conditional guards

Dan Williams wrote:
> Fabio M. De Francesco wrote:
> > Add cond_guard() macro to conditional guards.
> > 
> > cond_guard() is a guard to be used with the conditional variants of locks,
> > like down_read_trylock() or mutex_lock_interruptible().
> > 
> > It takes a statement (or more statements in a block) that is passed to its
> > second argument. That statement (or block) is executed if waiting for a
> > lock is interrupted or if a _trylock() fails in case of contention.
> > 
> > Usage example:
> > 
> > 	cond_guard(rwsem_read_try, { printk(...); return 0; }, &semaphore);
> > 
> > Consistently with the other guards, locks are unlocked at the exit of the
> > scope where cond_guard() is called.
> > 
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Suggested-by: Dan Williams <dan.j.williams@...el.com>
> > Suggested-by: Ira Weiny <ira.weiny@...el.com>
> > Signed-off-by: Fabio M. De Francesco <fabio.maria.de.francesco@...ux.intel.com>
> > ---
> >  include/linux/cleanup.h | 14 ++++++++++++++
> >  1 file changed, 14 insertions(+)
> > 
> > diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
> > index c2d09bc4f976..88af56600325 100644
> > --- a/include/linux/cleanup.h
> > +++ b/include/linux/cleanup.h
> > @@ -134,6 +134,16 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
> >   *	an anonymous instance of the (guard) class, not recommended for
> >   *	conditional locks.
> >   *
> > + * cond_guard(name, fail, args...):
> > + *	a guard to be used with the conditional variants of locks, like
> > + *	down_read_trylock() or mutex_lock_interruptible. 'fail' are one or more
> > + *	statements that are executed when waiting for a lock is interrupted or
> > + *	when a _trylock() fails in case of contention.
> > + *
> > + *	Example:
> > + *
> > + *		cond_guard(rwsem_read_try, { printk(...); return 0; }, &semaphore);
> 
> That _fail argument likely needs to be a statement expression for the
> multi-statement case.

You mean ({ ... }) as discussed here?

https://lore.kernel.org/all/65c1578c76def_37447929456@iweiny-mobl.notmuch/

> 
> > + *
> >   * scoped_guard (name, args...) { }:
> >   *	similar to CLASS(name, scope)(args), except the variable (with the
> >   *	explicit name 'scope') is declard in a for-loop such that its scope is
> > @@ -165,6 +175,10 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
> >  
> >  #define __guard_ptr(_name) class_##_name##_lock_ptr
> >  
> > +#define cond_guard(_name, _fail, args...) \
> > +	CLASS(_name, scope)(args); \
> > +	if (!__guard_ptr(_name)(&scope)) _fail
> 
> No, as I stated before this is broken for usages of:
> 
>     if () cond_guard() else if ()
> 
> The 'else' in the definition is critical, this builds for me (untested):

I did not test Fabios work directly but I don't understand this example.
It seems like your suggestion does nothing useful.  The cond_guard()
becomes a single statement like...

	if ()
		cond_guard();
	else ...

.. And can't protect anything.  NOTE From my understanding of
cond_guard() as defined, the ';' must be used as part of cond_guard() and
should complete the internal macro 'if' statement.

I think this would work:

	if () {
		cond_guard();
		... do locked stuff ...
	} else ...

> 
> diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
> index 88af56600325..665407498781 100644
> --- a/include/linux/cleanup.h
> +++ b/include/linux/cleanup.h
> @@ -142,7 +142,7 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
>   *
>   *	Example:
>   *
> - *		cond_guard(rwsem_read_try, { printk(...); return 0; }, &semaphore);
> + *		cond_guard(rwsem_read_try, ({ printk(...); return 0; }), &semaphore);
>   *
>   * scoped_guard (name, args...) { }:
>   *	similar to CLASS(name, scope)(args), except the variable (with the
> @@ -177,7 +177,8 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
>  
>  #define cond_guard(_name, _fail, args...) \
>  	CLASS(_name, scope)(args); \
> -	if (!__guard_ptr(_name)(&scope)) _fail
> +	if (!__guard_ptr(_name)(&scope)) _fail; \

Building on what I found for scoped_cond_guard() this should be

> +	if (!__guard_ptr(_name)(&scope)) { _fail; }

And drop the else.  The else needs to clearly be part of an outside if in
your example.

Ira

> +	else
>  
>  #define scoped_guard(_name, args...)					\
>  	for (CLASS(_name, scope)(args),					\