From c7e25e6e0b0486492c5faaf6312b37413642c48e Mon Sep 17 00:00:00 2001 From: Jan Kara Date: Thu, 23 Jun 2011 22:51:47 +0200 Subject: [PATCH] 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 Signed-off-by: Joel Becker --- fs/ocfs2/aops.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/fs/ocfs2/aops.c b/fs/ocfs2/aops.c index 4c1ec8f2d8c1..ba3ca1e63b51 100644 --- a/fs/ocfs2/aops.c +++ b/fs/ocfs2/aops.c @@ -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; } -- 2.20.1