fbdev: bugfix for multiprocess defio (f31ad92f) · Commits · 戴 / test

This patch is a bugfix for how defio handles multiple processes manipulating the same framebuffer. Thanks to Bernard Blackham for identifying this bug. It occurs when two applications mmap the same framebuffer and concurrently write to the same page. Normally, this doesn't occur since only a single process mmaps the framebuffer. The symptom of the bug is that the mapping applications will hang. The cause is that defio incorrectly tries to add the same page twice to the pagelist. The solution I have is to walk the pagelist and check for a duplicate before adding. Since I needed to walk the pagelist, I now also keep the pagelist in sorted order. Signed-off-by:

Jaya Kumar <jayakumar.lkml@gmail.com> Cc: Bernard Blackham <bernard@largestprime.net> Cc: <stable@kernel.org> Signed-off-by:

Andrew Morton <akpm@linux-foundation.org> Signed-off-by:

Linus Torvalds <torvalds@linux-foundation.org>

drivers/video/fb_defio.c

+19 −1

Original line number	Diff line number	Diff line
		@@ -74,6 +74,7 @@ static int fb_deferred_io_mkwrite(struct vm_area_struct *vma,
		{
		struct fb_info *info = vma->vm_private_data;
		struct fb_deferred_io *fbdefio = info->fbdefio;
		struct page *cur;

		/* this is a callback we get when userspace first tries to
		write to the page. we schedule a workqueue. that workqueue
		@@ -83,7 +84,24 @@ static int fb_deferred_io_mkwrite(struct vm_area_struct *vma,

		/* protect against the workqueue changing the page list */
		mutex_lock(&fbdefio->lock);
		list_add(&page->lru, &fbdefio->pagelist);

		/* we loop through the pagelist before adding in order
		to keep the pagelist sorted */
		list_for_each_entry(cur, &fbdefio->pagelist, lru) {
		/* this check is to catch the case where a new
		process could start writing to the same page
		through a new pte. this new access can cause the
		mkwrite even when the original ps's pte is marked
		writable */
		if (unlikely(cur == page))
		goto page_already_added;
		else if (cur->index > page->index)
		break;
		}

		list_add_tail(&page->lru, &cur->lru);

		page_already_added:
		mutex_unlock(&fbdefio->lock);

		/* come back after delay to process the deferred IO */

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