Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

From: "Paul E. McKenney" <paulmck@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Michal Hocko <mhocko@suse.com>,
	Uladzislau Rezki <urezki@gmail.com>,
	LKML <linux-kernel@vger.kernel.org>, RCU <rcu@vger.kernel.org>,
	linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	Matthew Wilcox <willy@infradead.org>,
	"Theodore Y . Ts'o" <tytso@mit.edu>,
	Joel Fernandes <joel@joelfernandes.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Subject: Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag
Date: Fri, 14 Aug 2020 09:11:06 -0700	[thread overview]
Message-ID: <20200814161106.GA13853@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20200814141425.GM4295@paulmck-ThinkPad-P72>

On Fri, Aug 14, 2020 at 07:14:25AM -0700, Paul E. McKenney wrote:
> On Fri, Aug 14, 2020 at 10:30:37AM +0200, Peter Zijlstra wrote:
> > On Fri, Aug 14, 2020 at 01:59:04AM +0200, Thomas Gleixner wrote:

[ . . . ]

> > > > 3.	Reusing existing GFP_ flags/values/whatever to communicate
> > > >	the raw-context information that was to be communicated by
> > > >	the new GFP_ flag.
> > > >
> > > > 4.	Making lockdep forgive acquiring spinlocks while holding
> > > >	raw spinlocks, but only in CONFIG_PREEMPT_NONE=y kernels.
> > 
> > Uhh, !CONFIG_PREEMPT_RT, the rest is 'fine'.
> 
> I would be OK with either.  In CONFIG_PREEMPT_NONE=n kernels, the
> kfree_rcu() code could use preemptible() to determine whether it was safe
> to invoke the allocator.  The code in kfree_rcu() might look like this:
> 
> 	mem = NULL;
> 	if (IS_ENABLED(CONFIG_PREEMPT_NONE) || preemptible())
> 		mem = __get_free_page(...);
> 
> Is your point is that the usual mistakes would then be caught by the
> usual testing on CONFIG_PREEMPT_NONE=n kernels?

Just to make sure we are talking about the same thing, please see below
for an untested patch that illustrates how I was interpreting your words.
Was this what you had in mind?

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index 62a382d..42d0ff1 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -579,7 +579,7 @@ do {									\
 # define lockdep_assert_preemption_disabled() do { } while (0)
 #endif
 
-#ifdef CONFIG_PROVE_RAW_LOCK_NESTING
+#ifdef CONFIG_PROVE_RAW_LOCK_NESTING_EFFECTIVE
 
 # define lockdep_assert_RT_in_threaded_ctx() do {			\
 		WARN_ONCE(debug_locks && !current->lockdep_recursion &&	\
diff --git a/include/linux/lockdep_types.h b/include/linux/lockdep_types.h
index bb35b44..70867d58 100644
--- a/include/linux/lockdep_types.h
+++ b/include/linux/lockdep_types.h
@@ -20,7 +20,7 @@ enum lockdep_wait_type {
 	LD_WAIT_FREE,		/* wait free, rcu etc.. */
 	LD_WAIT_SPIN,		/* spin loops, raw_spinlock_t etc.. */
 
-#ifdef CONFIG_PROVE_RAW_LOCK_NESTING
+#ifdef PROVE_RAW_LOCK_NESTING_EFFECTIVE
 	LD_WAIT_CONFIG,		/* CONFIG_PREEMPT_LOCK, spinlock_t etc.. */
 #else
 	LD_WAIT_CONFIG = LD_WAIT_SPIN,
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index e068c3c..e02de40 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1215,6 +1215,9 @@ config PROVE_RAW_LOCK_NESTING
 
 	 If unsure, select N.
 
+config PROVE_RAW_LOCK_NESTING_EFFECTIVE
+	def_bool PROVE_RAW_LOCK_NESTING && !PREEMPTION
+
 config LOCK_STAT
 	bool "Lock usage statistics"
 	depends on DEBUG_KERNEL && LOCK_DEBUGGING_SUPPORT
