On Mon, 2012-04-09 at 03:38 +0400, Anton Vorontsov wrote:
> 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@xxxxxxxxxx>
What guarantees that there's only one thread writing to struct
vmevent_attr::value in vmevent_sample() now that the mutex is gone?
> ---
> mm/vmevent.c | 35 ++++++++++++-----------------------
> 1 files changed, 12 insertions(+), 23 deletions(-)
>
> diff --git a/mm/vmevent.c b/mm/vmevent.c
> index 1847b56..a56174f 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>
> @@ -23,7 +24,7 @@ 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.
> @@ -103,20 +104,18 @@ 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 +124,7 @@ 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 +147,9 @@ 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 +166,13 @@ 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 +188,10 @@ 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;
> }
>
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