Hello,
On 2014-04-28 20:09, Davidlohr Bueso wrote:
Performing vma lookups without taking the mm->mmap_sem is asking
for trouble. While doing the search, the vma in question can
be modified or even removed before returning to the caller.
Take the lock in order to avoid races while iterating through
the vmacache and/or rbtree.
Also do some very minor cleanup changes.
This patch is only compile tested.
NACK.
mm->mmap_sem is taken by videobuf2-core to avoid AB-BA deadlock with v4l2 core lock. For more information, please check videobuf2-core.c. However you are right that this is a bit confusing and we need more comments about the place where mmap_sem is taken. Here is some background for this decision:
https://www.mail-archive.com/linux-media@xxxxxxxxxxxxxxx/msg38599.html
http://www.spinics.net/lists/linux-media/msg40225.html
Signed-off-by: Davidlohr Bueso <davidlohr@xxxxxx>
Cc: Pawel Osciak <pawel@xxxxxxxxxx>
Cc: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
Cc: Kyungmin Park <kyungmin.park@xxxxxxxxxxx>
Cc: Mauro Carvalho Chehab <m.chehab@xxxxxxxxxxx>
Cc: linux-media@xxxxxxxxxxxxxxx
---
It would seem this is the last offending user.
v4l2 is a maze but I believe that this is needed as I don't
see the mmap_sem being taken by any callers of vb2_dma_sg_get_userptr().
drivers/media/v4l2-core/videobuf2-dma-sg.c | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)
diff --git a/drivers/media/v4l2-core/videobuf2-dma-sg.c b/drivers/media/v4l2-core/videobuf2-dma-sg.c
index c779f21..2a21100 100644
--- a/drivers/media/v4l2-core/videobuf2-dma-sg.c
+++ b/drivers/media/v4l2-core/videobuf2-dma-sg.c
@@ -168,8 +168,9 @@ static void *vb2_dma_sg_get_userptr(void *alloc_ctx, unsigned long vaddr,
unsigned long first, last;
int num_pages_from_user;
struct vm_area_struct *vma;
+ struct mm_struct *mm = current->mm;
- buf = kzalloc(sizeof *buf, GFP_KERNEL);
+ buf = kzalloc(sizeof(*buf), GFP_KERNEL);
if (!buf)
return NULL;
@@ -178,7 +179,7 @@ static void *vb2_dma_sg_get_userptr(void *alloc_ctx, unsigned long vaddr,
buf->offset = vaddr & ~PAGE_MASK;
buf->size = size;
- first = (vaddr & PAGE_MASK) >> PAGE_SHIFT;
+ first = (vaddr & PAGE_MASK) >> PAGE_SHIFT;
last = ((vaddr + size - 1) & PAGE_MASK) >> PAGE_SHIFT;
buf->num_pages = last - first + 1;
@@ -187,7 +188,8 @@ static void *vb2_dma_sg_get_userptr(void *alloc_ctx, unsigned long vaddr,
if (!buf->pages)
goto userptr_fail_alloc_pages;
- vma = find_vma(current->mm, vaddr);
+ down_write(&mm->mmap_sem);
+ vma = find_vma(mm, vaddr);
if (!vma) {
dprintk(1, "no vma for address %lu\n", vaddr);
goto userptr_fail_find_vma;
@@ -218,7 +220,7 @@ static void *vb2_dma_sg_get_userptr(void *alloc_ctx, unsigned long vaddr,
buf->pages[num_pages_from_user] = pfn_to_page(pfn);
}
} else
- num_pages_from_user = get_user_pages(current, current->mm,
+ num_pages_from_user = get_user_pages(current, mm,
vaddr & PAGE_MASK,
buf->num_pages,
write,
@@ -233,6 +235,7 @@ static void *vb2_dma_sg_get_userptr(void *alloc_ctx, unsigned long vaddr,
buf->num_pages, buf->offset, size, 0))
goto userptr_fail_alloc_table_from_pages;
+ up_write(&mm->mmap_sem);
return buf;
userptr_fail_alloc_table_from_pages:
@@ -244,6 +247,7 @@ userptr_fail_get_user_pages:
put_page(buf->pages[num_pages_from_user]);
vb2_put_vma(buf->vma);
userptr_fail_find_vma:
+ up_write(&mm->mmap_sem);
kfree(buf->pages);
userptr_fail_alloc_pages:
kfree(buf);
Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland
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