Btrfs: fix race when getting the eb out of page->private (b5bae261) · Commits · 戴 / test

We can race when checking wether PagePrivate is set on a page and we actually have an eb saved in the pages private pointer. We could have easily written out this page and released it in the time that we did the pagevec lookup and actually got around to looking at this page. So use mapping->private_lock to ensure we get a consistent view of the page->private pointer. This is inline with the alloc and releasepage paths which use private_lock when manipulating page->private. Thanks, Reported-by:

David Sterba <dave@jikos.cz> Signed-off-by:

Josef Bacik <jbacik@fusionio.com>

fs/btrfs/extent_io.c

+19 −4

Original line number	Diff line number	Diff line
		@@ -3256,19 +3256,34 @@ retry:
		break;
		}

		spin_lock(&mapping->private_lock);
		if (!PagePrivate(page)) {
		spin_unlock(&mapping->private_lock);
		continue;
		}

		eb = (struct extent_buffer *)page->private;

		/*
		* Shouldn't happen and normally this would be a BUG_ON
		* but no sense in crashing the users box for something
		* we can survive anyway.
		*/
		if (!eb) {
		spin_unlock(&mapping->private_lock);
		WARN_ON(1);
		continue;
		}

		if (eb == prev_eb)
		if (eb == prev_eb) {
		spin_unlock(&mapping->private_lock);
		continue;
		}

		if (!atomic_inc_not_zero(&eb->refs)) {
		WARN_ON(1);
		ret = atomic_inc_not_zero(&eb->refs);
		spin_unlock(&mapping->private_lock);
		if (!ret)
		continue;
		}

		prev_eb = eb;
		ret = lock_extent_buffer_for_io(eb, fs_info, &epd);

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