linux-kernel - Re: [PATCH] module: Annotate nested sleep in resolve

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Message-ID: <20150210171255.GQ24151@twins.programming.kicks-ass.net>
Date:	Tue, 10 Feb 2015 18:12:55 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Dave Jones <davej@...emonkey.org.uk>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Cc:	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH] module: Annotate nested sleep in resolve_symbol()



On which, we should probably do this.

---
Subject: module: Replace over-engineered nested sleep 

Since the introduction of the nested sleep warning; we've established
that the occasional sleep inside a wait_event() is fine.

wait_event() loops are invariant wrt. spurious wakeups, and the
occasional sleep has a similar effect on them. As long as its occasional
its harmless.

Therefore replace the 'correct' but verbose wait_woken() thing with
a simple annotation to shut up the warning.

Cc: Rusty Russell <rusty@...tcorp.com.au>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 kernel/module.c | 36 ++++++++----------------------------
 1 file changed, 8 insertions(+), 28 deletions(-)

diff --git a/kernel/module.c b/kernel/module.c
index d856e96a3cce..98231bf59b6e 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2978,6 +2978,12 @@ static bool finished_loading(const char *name)
 	struct module *mod;
 	bool ret;
 
+	/*
+	 * The module_mutex should not be a heavily contended lock;
+	 * if we get the occasional sleep here, we'll go an extra iteration
+	 * in the wait_event_interruptible(), which is harmless.
+	 */
+	sched_annotate_sleep();
 	mutex_lock(&module_mutex);
 	mod = find_module_all(name, strlen(name), true);
 	ret = !mod || mod->state == MODULE_STATE_LIVE
@@ -3120,32 +3126,6 @@ static int may_init_module(void)
 }
 
 /*
- * Can't use wait_event_interruptible() because our condition
- * 'finished_loading()' contains a blocking primitive itself (mutex_lock).
- */
-static int wait_finished_loading(struct module *mod)
-{
-	DEFINE_WAIT_FUNC(wait, woken_wake_function);
-	int ret = 0;
-
-	add_wait_queue(&module_wq, &wait);
-	for (;;) {
-		if (finished_loading(mod->name))
-			break;
-
-		if (signal_pending(current)) {
-			ret = -ERESTARTSYS;
-			break;
-		}
-
-		wait_woken(&wait, TASK_INTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
-	}
-	remove_wait_queue(&module_wq, &wait);
-
-	return ret;
-}
-
-/*
  * We try to place it in the list now to make sure it's unique before
  * we dedicate too many resources.  In particular, temporary percpu
  * memory exhaustion.
@@ -3165,8 +3145,8 @@ static int add_unformed_module(struct module *mod)
 		    || old->state == MODULE_STATE_UNFORMED) {
 			/* Wait in case it fails to load. */
 			mutex_unlock(&module_mutex);
-
-			err = wait_finished_loading(mod);
+			err = wait_event_interruptible(module_wq,
+					       finished_loading(mod->name));
 			if (err)
 				goto out_unlocked;
 			goto again;
--
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