linux-kernel - Re: [RFC-PATCH 1/2] mm: Add __GFP_NO

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Date:   Fri, 14 Aug 2020 09:11:06 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Michal Hocko <mhocko@...e.com>,
        Uladzislau Rezki <urezki@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Joel Fernandes <joel@...lfernandes.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

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