On Fri, 28 May 2010, Jeremy Fitzhardinge wrote:
>
> I managed to catch a lockdep problem in gntdev, which may be the same as
> before:
>
> BUG: sleeping function called from invalid context at kernel/rwsem.c:21
> in_atomic(): 1, irqs_disabled(): 0, pid: 4091, name: qemu-dm
> 2 locks held by qemu-dm/4091:
> #0: (&mm->mmap_sem){++++++}, at: [<ffffffff810bb50f>] sys_munmap+0x33/0x58
> #1: (rcu_read_lock){.+.+..}, at: [<ffffffff810cd63a>]
> __mmu_notifier_invalidate_range_start+0x0/0xc7
> Pid: 4091, comm: qemu-dm Not tainted 2.6.32.13 #23
> Call Trace:
> [<ffffffff8106705b>] ? __debug_show_held_locks+0x22/0x24
> [<ffffffff81039522>] __might_sleep+0x123/0x127
> [<ffffffff810a8536>] ? release_pages+0xd2/0x1e7
> [<ffffffff81498849>] down_read+0x1f/0x57
> [<ffffffff81010142>] ? check_events+0x12/0x20
> [<ffffffff810a8536>] ? release_pages+0xd2/0x1e7
> [<ffffffff810cd63a>] ? __mmu_notifier_invalidate_range_start+0x0/0xc7
> [<ffffffff8123e069>] mn_invl_range_start+0x32/0x118
> [<ffffffff810cd69c>] __mmu_notifier_invalidate_range_start+0x62/0xc7
> [<ffffffff810cd63a>] ? __mmu_notifier_invalidate_range_start+0x0/0xc7
> [<ffffffff810b54bc>] unmap_vmas+0x8c/0x91a
> [<ffffffff810ba363>] unmap_region+0xda/0x178
> [<ffffffff810bb472>] do_munmap+0x2ae/0x318
> [<ffffffff810bb51d>] sys_munmap+0x41/0x58
> [<ffffffff81013b82>] system_call_fastpath+0x16/0x1b
>
>
> The problem is that mn_invl_range_start does a down_read(), but it is
> called from __mmu_notifier_invalidate_range_start(), which does an
> rcu_read_lock, which has the side-effect of disabling preemption.
>
> The mmu notifier code seems to have always used rcu_read_lock this way,
> so I guess this bug has always been there. It's not immediately obvious
> how to fix it.
>
> Thoughts?
What about turning the semaphore into a rwlock?
Performances shouldn't matter in this case.
Something like this:
---
diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index ddc59cc..91846b9 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -28,7 +28,7 @@
#include <linux/types.h>
#include <linux/uaccess.h>
#include <linux/sched.h>
-#include <linux/rwsem.h>
+#include <linux/spinlock_types.h>
#include <xen/xen.h>
#include <xen/grant_table.h>
@@ -51,7 +51,7 @@ struct gntdev_priv {
struct list_head maps;
uint32_t used;
uint32_t limit;
- struct rw_semaphore sem;
+ rwlock_t rwlock;
struct mm_struct *mm;
struct mmu_notifier mn;
};
@@ -277,7 +277,7 @@ static void mn_invl_range_start(struct mmu_notifier *mn,
unsigned long mstart, mend;
int err;
- down_read(&priv->sem);
+ read_lock(&priv->rwlock);
list_for_each_entry(map, &priv->maps, next) {
if (!map->vma)
continue;
@@ -299,7 +299,7 @@ static void mn_invl_range_start(struct mmu_notifier *mn,
(mend - mstart) >> PAGE_SHIFT);
WARN_ON(err);
}
- up_read(&priv->sem);
+ read_unlock(&priv->rwlock);
}
static void mn_invl_page(struct mmu_notifier *mn,
@@ -316,7 +316,7 @@ static void mn_release(struct mmu_notifier *mn,
struct grant_map *map;
int err;
- down_read(&priv->sem);
+ read_lock(&priv->rwlock);
list_for_each_entry(map, &priv->maps, next) {
if (!map->vma)
continue;
@@ -327,7 +327,7 @@ static void mn_release(struct mmu_notifier *mn,
err = unmap_grant_pages(map, 0, map->count);
WARN_ON(err);
}
- up_read(&priv->sem);
+ read_unlock(&priv->rwlock);
}
struct mmu_notifier_ops gntdev_mmu_ops = {
@@ -347,7 +347,7 @@ static int gntdev_open(struct inode *inode, struct file
*flip)
return -ENOMEM;
INIT_LIST_HEAD(&priv->maps);
- init_rwsem(&priv->sem);
+ priv->rwlock = RW_LOCK_UNLOCKED;
priv->limit = limit;
priv->mm = get_task_mm(current);
@@ -375,13 +375,13 @@ static int gntdev_release(struct inode *inode, struct
file *flip)
if (debug)
printk("%s: priv %p\n", __FUNCTION__, priv);
- down_write(&priv->sem);
+ write_lock(&priv->rwlock);
while (!list_empty(&priv->maps)) {
map = list_entry(priv->maps.next, struct grant_map, next);
err = gntdev_del_map(map);
WARN_ON(err);
}
- up_write(&priv->sem);
+ write_unlock(&priv->rwlock);
mmu_notifier_unregister(&priv->mn, priv->mm);
kfree(priv);
return 0;
@@ -404,7 +404,7 @@ static long gntdev_ioctl_map_grant_ref(struct gntdev_priv
*priv,
if (unlikely(op.count > priv->limit))
return -EINVAL;
- down_write(&priv->sem);
+ write_lock(&priv->rwlock);
err = -ENOMEM;
map = gntdev_add_map(priv, op.count);
if (!map)
@@ -417,13 +417,13 @@ static long gntdev_ioctl_map_grant_ref(struct gntdev_priv
*priv,
op.index = map->index << PAGE_SHIFT;
if (copy_to_user(u, &op, sizeof(op)) != 0)
goto err_free;
- up_write(&priv->sem);
+ write_unlock(&priv->rwlock);
return 0;
err_free:
gntdev_del_map(map);
err_unlock:
- up_write(&priv->sem);
+ write_unlock(&priv->rwlock);
return err;
}
@@ -440,11 +440,11 @@ static long gntdev_ioctl_unmap_grant_ref(struct
gntdev_priv *priv,
printk("%s: priv %p, del %d+%d\n", __FUNCTION__, priv,
(int)op.index, (int)op.count);
- down_write(&priv->sem);
+ write_lock(&priv->rwlock);
map = gntdev_find_map_index(priv, op.index >> PAGE_SHIFT, op.count);
if (map)
err = gntdev_del_map(map);
- up_write(&priv->sem);
+ write_unlock(&priv->rwlock);
return err;
}
@@ -460,16 +460,16 @@ static long gntdev_ioctl_get_offset_for_vaddr(struct
gntdev_priv *priv,
printk("%s: priv %p, offset for vaddr %lx\n", __FUNCTION__,
priv,
(unsigned long)op.vaddr);
- down_read(&priv->sem);
+ read_lock(&priv->rwlock);
map = gntdev_find_map_vaddr(priv, op.vaddr);
if (map == NULL ||
map->vma->vm_start != op.vaddr) {
- up_read(&priv->sem);
+ read_unlock(&priv->rwlock);
return -EINVAL;
}
op.offset = map->index << PAGE_SHIFT;
op.count = map->count;
- up_read(&priv->sem);
+ read_unlock(&priv->rwlock);
if (copy_to_user(u, &op, sizeof(op)) != 0)
return -EFAULT;
@@ -488,9 +488,9 @@ static long gntdev_ioctl_set_max_grants(struct gntdev_priv
*priv,
if (op.count > limit)
return -EINVAL;
- down_write(&priv->sem);
+ write_lock(&priv->rwlock);
priv->limit = op.count;
- up_write(&priv->sem);
+ write_unlock(&priv->rwlock);
return 0;
}
@@ -538,7 +538,7 @@ static int gntdev_mmap(struct file *flip, struct
vm_area_struct *vma)
printk("%s: map %d+%d at %lx (pgoff %lx)\n", __FUNCTION__,
index, count, vma->vm_start, vma->vm_pgoff);
- down_read(&priv->sem);
+ read_lock(&priv->rwlock);
map = gntdev_find_map_index(priv, index, count);
if (!map)
goto unlock_out;
@@ -580,7 +580,7 @@ static int gntdev_mmap(struct file *flip, struct
vm_area_struct *vma)
map->is_mapped = 1;
unlock_out:
- up_read(&priv->sem);
+ read_unlock(&priv->rwlock);
return err;
}
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