Commit c7e25e6e authored Jun 23, 2011 by Jan Kara Committed by Joel Becker Jul 28, 2011

ocfs2: Avoid livelock in ocfs2_readpage()



When someone writes to an inode, readers accessing the same inode via
ocfs2_readpage() just busyloop trying to get ip_alloc_sem because
do_generic_file_read() looks up the page again and retries ->readpage()
when previous attempt failed with AOP_TRUNCATED_PAGE. When there are enough
readers, they can occupy all CPUs and in non-preempt kernel the system is
deadlocked because writer holding ip_alloc_sem is never run to release the
semaphore. Fix the problem by making reader block on ip_alloc_sem to break
the busy loop.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Joel Becker <jlbec@evilplan.org>

parent a11f7e63

fs/ocfs2/aops.c

+8 −0

Original line number	Diff line number	Diff line
		@@ -290,7 +290,15 @@ static int ocfs2_readpage(struct file file, struct page page)
		}

		if (down_read_trylock(&oi->ip_alloc_sem) == 0) {
		/*
		* Unlock the page and cycle ip_alloc_sem so that we don't
		* busyloop waiting for ip_alloc_sem to unlock
		*/
		ret = AOP_TRUNCATED_PAGE;
		unlock_page(page);
		unlock = 0;
		down_read(&oi->ip_alloc_sem);
		up_read(&oi->ip_alloc_sem);
		goto out_inode_unlock;
		}