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authorJan Kara2011-06-23 22:51:47 +0200
committerJoel Becker2011-07-28 02:07:19 -0700
commitc7e25e6e0b0486492c5faaf6312b37413642c48e (patch)
treec097a213a8eb2ad336fb2ef8046d2e71f0fa1f4a
parenta11f7e63c59810a81494d4c4b028af707d4c7ca4 (diff)
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>
-rw-r--r--fs/ocfs2/aops.c8
1 files changed, 8 insertions, 0 deletions
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;
}