linux-kernel - Re: [PATCH v2 0/8] Add rcu_sync infrastructure to avoid

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Date:	Mon, 24 Aug 2015 17:34:31 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/8] Add rcu_sync infrastructure to avoid
	_expedited() in percpu-rwsem

On 08/22, Paul E. McKenney wrote:
>
> Queued for testing, thank you, Oleg!

Thanks Paul!

> Right now, this is mostly relying on 0day and -next testing.  Any thoughts
> for a useful torture test for this?

Right now I do not have any idea how to write the meaningful test for
rcu_sync... Perhaps something like

        struct rcu_sync_struct rss;
        spinlock_t lock;

        int A, B;

        void read(void)
        {
                rcu_read_lock();

                bool need_lock = !rcu_sync_is_idle(&rss);
                if (need_lock)
                        spin_lock(&lock);

                BUG_ON(A != B);

                if (need_lock)
                        spin_unlock(&lock);

                rcu_read_unlock();
        }

        void modify(void)
        {
                rcu_sync_enter(&rss);

                spin_lock(&lock);
                A++; B++;
                spin_unlock(&lock);

                rcu_sync_exit(&rss);
        }

makes sense... I'll try to think.

> One approach would be to treat it
> like a reader-writer lock.  Other thoughts?

I booted the kernel with the additional patch below, and nothing bad has
happened, it continues to print

	Writes:  Total: 2  Max/Min: 0/0   Fail: 0
	Reads :  Total: 2  Max/Min: 0/0   Fail: 0

However, I do not know what this code actually does, so currently I have
no idea if this test makes any sense for percpu_rw_semaphore.

Oleg.
---

diff --git a/kernel/locking/locktorture.c b/kernel/locking/locktorture.c
index ec8cce2..62561ec 100644
--- a/kernel/locking/locktorture.c
+++ b/kernel/locking/locktorture.c
@@ -55,7 +55,7 @@ torture_param(int, stutter, 5, "Number of jiffies to run/halt test, 0=disable");
 torture_param(bool, verbose, true,
 	     "Enable verbose debugging printk()s");
 
-static char *torture_type = "spin_lock";
+static char *torture_type = "rwsem_lock";
 module_param(torture_type, charp, 0444);
 MODULE_PARM_DESC(torture_type,
 		 "Type of lock to torture (spin_lock, spin_lock_irq, mutex_lock, ...)");
@@ -361,10 +361,12 @@ static struct lock_torture_ops mutex_lock_ops = {
 	.name		= "mutex_lock"
 };
 
-static DECLARE_RWSEM(torture_rwsem);
-static int torture_rwsem_down_write(void) __acquires(torture_rwsem)
+#include <linux/percpu-rwsem.h>
+static struct percpu_rw_semaphore pcpu_rwsem;
+
+static int torture_rwsem_down_write(void) __acquires(pcpu_rwsem)
 {
-	down_write(&torture_rwsem);
+	percpu_down_write(&pcpu_rwsem);
 	return 0;
 }
 
@@ -384,14 +386,14 @@ static void torture_rwsem_write_delay(struct torture_random_state *trsp)
 #endif
 }
 
-static void torture_rwsem_up_write(void) __releases(torture_rwsem)
+static void torture_rwsem_up_write(void) __releases(pcpu_rwsem)
 {
-	up_write(&torture_rwsem);
+	percpu_up_write(&pcpu_rwsem);
 }
 
-static int torture_rwsem_down_read(void) __acquires(torture_rwsem)
+static int torture_rwsem_down_read(void) __acquires(pcpu_rwsem)
 {
-	down_read(&torture_rwsem);
+	percpu_down_read(&pcpu_rwsem);
 	return 0;
 }
 
@@ -411,9 +413,9 @@ static void torture_rwsem_read_delay(struct torture_random_state *trsp)
 #endif
 }
 
-static void torture_rwsem_up_read(void) __releases(torture_rwsem)
+static void torture_rwsem_up_read(void) __releases(pcpu_rwsem)
 {
-	up_read(&torture_rwsem);
+	percpu_up_read(&pcpu_rwsem);
 }
 
 static struct lock_torture_ops rwsem_lock_ops = {
@@ -645,6 +647,11 @@ static int __init lock_torture_init(void)
 		&rwsem_lock_ops,
 	};
 
+	/*
+	 * TODO: DECLARE_PERCPU_RWSEM(). The patch already exists.
+	 */
+	BUG_ON(percpu_init_rwsem(&pcpu_rwsem));
+
 	if (!torture_init_begin(torture_type, verbose, &torture_runnable))
 		return -EBUSY;
 

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