ip_alloc_sem, page lock and cluster lock

[renzhengeek]

commit e9dfc0b2bc42761410e8db6c252c6c5889e178b8
Author: Mark Fasheh <mark.fasheh@oracle.com>
Date:   Mon May 14 11:38:51 2007 -0700

    ocfs2: trylock in ocfs2_readpage()

    Similarly to the page lock / cluster lock inversion in ocfs2_readpage, we
    can deadlock on ip_alloc_sem. We can down_read_trylock() instead and just
    return AOP_TRUNCATED_PAGE if the operation fails.

    Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>

diff --git a/fs/ocfs2/aops.c b/fs/ocfs2/aops.c
index 8e7cafb..3030670 100644
--- a/fs/ocfs2/aops.c
+++ b/fs/ocfs2/aops.c
@@ -222,7 +222,10 @@ static int ocfs2_readpage(struct file *file, struct page *page)
                goto out;
        }

-       down_read(&OCFS2_I(inode)->ip_alloc_sem);
+       if (down_read_trylock(&OCFS2_I(inode)->ip_alloc_sem) == 0) {
+               ret = AOP_TRUNCATED_PAGE;
+               goto out_meta_unlock;
+       }

        /*
         * i_size might have just been updated as we grabed the meta lock.  We
@@ -258,6 +261,7 @@ static int ocfs2_readpage(struct file *file, struct page *page)
        ocfs2_data_unlock(inode, 0);
 out_alloc:
        up_read(&OCFS2_I(inode)->ip_alloc_sem);
+out_meta_unlock:
        ocfs2_meta_unlock(inode, 0);
 out:
        if (unlock)

[renzhengeek]

commit c7e25e6e0b0486492c5faaf6312b37413642c48e
Author: Jan Kara <jack@suse.cz>
Date:   Thu Jun 23 22:51:47 2011 +0200

    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>

diff --git a/fs/ocfs2/aops.c b/fs/ocfs2/aops.c
index 4c1ec8f..ba3ca1e 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;
        }