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Date: Wed, 18 Apr 2012 01:33:56 -0700
From: Anton Vorontsov <anton.vorontsov@...aro.org>
To: Pekka Enberg <penberg@...nel.org>
Cc: Leonid Moiseichuk <leonid.moiseichuk@...ia.com>,
John Stultz <john.stultz@...aro.org>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, linaro-kernel@...ts.linaro.org,
patches@...aro.org
Subject: [PATCH 1/2] vmevent: Should not grab mutex in the atomic context
vmevent grabs a mutex in the atomic context, and so this pops up:
BUG: sleeping function called from invalid context at kernel/mutex.c:271
in_atomic(): 1, irqs_disabled(): 0, pid: 0, name: swapper/0
1 lock held by swapper/0/0:
#0: (&watch->timer){+.-...}, at: [<ffffffff8103eb80>] call_timer_fn+0x0/0xf0
Pid: 0, comm: swapper/0 Not tainted 3.2.0+ #6
Call Trace:
<IRQ> [<ffffffff8102f5da>] __might_sleep+0x12a/0x1e0
[<ffffffff810bd990>] ? vmevent_match+0xe0/0xe0
[<ffffffff81321f2c>] mutex_lock_nested+0x3c/0x340
[<ffffffff81064b33>] ? lock_acquire+0xa3/0xc0
[<ffffffff8103eb80>] ? internal_add_timer+0x110/0x110
[<ffffffff810bd990>] ? vmevent_match+0xe0/0xe0
[<ffffffff810bda21>] vmevent_timer_fn+0x91/0xf0
[<ffffffff810bd990>] ? vmevent_match+0xe0/0xe0
[<ffffffff8103ebf5>] call_timer_fn+0x75/0xf0
[<ffffffff8103eb80>] ? internal_add_timer+0x110/0x110
[<ffffffff81062fdd>] ? trace_hardirqs_on_caller+0x7d/0x120
[<ffffffff8103ee9f>] run_timer_softirq+0x10f/0x1e0
[<ffffffff810bd990>] ? vmevent_match+0xe0/0xe0
[<ffffffff81038d90>] __do_softirq+0xb0/0x160
[<ffffffff8105eb0f>] ? tick_program_event+0x1f/0x30
[<ffffffff8132642c>] call_softirq+0x1c/0x26
[<ffffffff810036d5>] do_softirq+0x85/0xc0
This patch fixes the issue by removing the mutex and making the logic
lock-free.
Signed-off-by: Anton Vorontsov <anton.vorontsov@...aro.org>
---
mm/vmevent.c | 52 ++++++++++++++++++++++++++--------------------------
1 file changed, 26 insertions(+), 26 deletions(-)
diff --git a/mm/vmevent.c b/mm/vmevent.c
index 1847b56..9ed6aca 100644
--- a/mm/vmevent.c
+++ b/mm/vmevent.c
@@ -1,4 +1,5 @@
#include <linux/anon_inodes.h>
+#include <linux/atomic.h>
#include <linux/vmevent.h>
#include <linux/syscalls.h>
#include <linux/timer.h>
@@ -22,8 +23,7 @@ struct vmevent_watch_event {
struct vmevent_watch {
struct vmevent_config config;
- struct mutex mutex;
- bool pending;
+ atomic_t pending;
/*
* Attributes that are exported as part of delivered VM events.
@@ -99,24 +99,36 @@ static bool vmevent_match(struct vmevent_watch *watch)
return false;
}
+/*
+ * This function is called from the timer context, which has the same
+ * guaranties as an interrupt handler: it can have only one execution
+ * thread (unlike bare softirq handler), so we don't need to worry
+ * about racing w/ ourselves.
+ *
+ * We also don't need to worry about several instances of timers
+ * accessing the same vmevent_watch, as we allocate vmevent_watch
+ * together w/ the timer instance in vmevent_fd(), so there is always
+ * one timer per vmevent_watch.
+ *
+ * All the above makes it possible to implement the lock-free logic,
+ * using just the atomic watch->pending variable.
+ */
static void vmevent_sample(struct vmevent_watch *watch)
{
int i;
+ if (atomic_read(&watch->pending))
+ return;
if (!vmevent_match(watch))
return;
- mutex_lock(&watch->mutex);
-
- watch->pending = true;
-
for (i = 0; i < watch->nr_attrs; i++) {
struct vmevent_attr *attr = &watch->sample_attrs[i];
attr->value = vmevent_sample_attr(watch, attr);
}
- mutex_unlock(&watch->mutex);
+ atomic_set(&watch->pending, 1);
}
static void vmevent_timer_fn(unsigned long data)
@@ -125,7 +137,7 @@ static void vmevent_timer_fn(unsigned long data)
vmevent_sample(watch);
- if (watch->pending)
+ if (atomic_read(&watch->pending))
wake_up(&watch->waitq);
mod_timer(&watch->timer, jiffies +
nsecs_to_jiffies64(watch->config.sample_period_ns));
@@ -148,13 +160,9 @@ static unsigned int vmevent_poll(struct file *file, poll_table *wait)
poll_wait(file, &watch->waitq, wait);
- mutex_lock(&watch->mutex);
-
- if (watch->pending)
+ if (atomic_read(&watch->pending))
events |= POLLIN;
- mutex_unlock(&watch->mutex);
-
return events;
}
@@ -171,15 +179,13 @@ static ssize_t vmevent_read(struct file *file, char __user *buf, size_t count, l
if (count < size)
return -EINVAL;
- mutex_lock(&watch->mutex);
-
- if (!watch->pending)
- goto out_unlock;
+ if (!atomic_read(&watch->pending))
+ goto out;
event = kmalloc(size, GFP_KERNEL);
if (!event) {
ret = -ENOMEM;
- goto out_unlock;
+ goto out;
}
for (i = 0; i < watch->nr_attrs; i++) {
@@ -195,14 +201,10 @@ static ssize_t vmevent_read(struct file *file, char __user *buf, size_t count, l
ret = count;
- watch->pending = false;
-
+ atomic_set(&watch->pending, 0);
out_free:
kfree(event);
-
-out_unlock:
- mutex_unlock(&watch->mutex);
-
+out:
return ret;
}
@@ -231,8 +233,6 @@ static struct vmevent_watch *vmevent_watch_alloc(void)
if (!watch)
return NULL;
- mutex_init(&watch->mutex);
-
init_waitqueue_head(&watch->waitq);
return watch;
--
1.7.9.2
--
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