NFS: Prevent a deadlock in the new writeback code

We have to unlock the nfs_page before we call nfs_end_page_writeback
to avoid races with functions that expect the page to be unlocked
when PG_locked and PG_writeback are not set.
The problem is that nfs_unlock_request also releases the nfs_page,
causing a deadlock if the release of the nfs_open_context
triggers an iput() while the PG_writeback flag is still set...

The solution is to separate the unlocking and release of the nfs_page,
so that we can do the former before nfs_end_page_writeback and the
latter after.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: Fred Isaman <iisaman@netapp.com>

diff --git a/fs/nfs/pagelist.c b/fs/nfs/pagelist.c
index 33a21ca9b84bf625a010b6cb88cbe450688ae097..69146f3869897037a96962fda9eabf5fc401e462 100644 (file)
--- a/fs/nfs/pagelist.c
+++ b/fs/nfs/pagelist.c
@@ -128,10 +128,10 @@ nfs_create_request(struct nfs_open_context *ctx, struct inode *inode,
 }
 
 /**
- * nfs_unlock_request - Unlock request and wake up sleepers.
+ * nfs_unlock_request_dont_release - Unlock request and wake up sleepers.
  * @req:
  */
-void nfs_unlock_request(struct nfs_page *req)
+void nfs_unlock_request_dont_release(struct nfs_page *req)
 {
        if (!NFS_WBACK_BUSY(req)) {
                printk(KERN_ERR "NFS: Invalid unlock attempted\n");
@@ -141,6 +141,14 @@ void nfs_unlock_request(struct nfs_page *req)
        clear_bit(PG_BUSY, &req->wb_flags);
        smp_mb__after_clear_bit();
        wake_up_bit(&req->wb_flags, PG_BUSY);
+}
+
+/**
+ * nfs_unlock_request - Unlock request and release the nfs_page
+ */
+void nfs_unlock_request(struct nfs_page *req)
+{
+       nfs_unlock_request_dont_release(req);
        nfs_release_request(req);
 }
 

diff --git a/fs/nfs/write.c b/fs/nfs/write.c
index 6f263daac7484ee71afdef981a931aa0fda21c7d..fd36b31ee72ea3cdb6db930b98741ed989c30437 100644 (file)
--- a/fs/nfs/write.c
+++ b/fs/nfs/write.c
@@ -628,8 +628,9 @@ static void nfs_write_completion(struct nfs_pgio_header *hdr)
 remove_req:
                nfs_inode_remove_request(req);
 next:
-               nfs_unlock_request(req);
+               nfs_unlock_request_dont_release(req);
                nfs_end_page_writeback(page);
+               nfs_release_request(req);
        }
 out:
        hdr->release(hdr);
@@ -1042,8 +1043,9 @@ static void nfs_redirty_request(struct nfs_page *req)
        struct page *page = req->wb_page;
 
        nfs_mark_request_dirty(req);
-       nfs_unlock_request(req);
+       nfs_unlock_request_dont_release(req);
        nfs_end_page_writeback(page);
+       nfs_release_request(req);
 }
 
 static void nfs_async_write_error(struct list_head *head)

diff --git a/include/linux/nfs_page.h b/include/linux/nfs_page.h
index f9ee9eba7f8821e786e16b2111cc8a6a0c95079c..ef7504215446e89bb69ebdc09847c22fc3d55b78 100644 (file)
--- a/include/linux/nfs_page.h
+++ b/include/linux/nfs_page.h
@@ -96,6 +96,7 @@ extern bool nfs_generic_pg_test(struct nfs_pageio_descriptor *desc,
                                struct nfs_page *req);
 extern  int nfs_wait_on_request(struct nfs_page *);
 extern void nfs_unlock_request(struct nfs_page *req);
+extern void nfs_unlock_request_dont_release(struct nfs_page *req);
 
 /*
  * Lock the page of an asynchronous request without getting a new reference