Re: [RFC][WIP] DIO simplification and AIO-DIO stability

[Wendy Cheng <wcheng@redhat.com>]

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Suparna Bhattacharya wrote:

>http://www.kernel.org/pub/linux/kernel/people/suparna/DIO-simplify.txt
>(also inlined below)
>  
>
Hi, Suparna,
                                                                                

It would be nice to ensure that the lock sequence will not cause issues 
for out-of-tree external kernel modules (e.g. cluster files System) that 
require extra locking for various purposes. We've found several 
deadlocks issues in Global File System (GFS) Direct IO path due to lock 
order enforced by VFS layer:
                                                                                

1) In sys_ftruncate()->do_truncate(), VFS layer grabs
  * i_sem
  * then i_alloc_sem (i_mutex)
  * then call filesystem's setattr().
                                                                                

2) In Direct IO read, VFS layer calls
  * filesystem's direct_IO()
  * grabs i_sem (i_mutex)
  * followed by i_alloc_sem.

In our case, both gfs_setattr() and gfs_direct_IO() need its own 
(global) locks to synchronize inter-nodes (and inter-processes) control 
structures access but gfs_direct_IO later ends up in 
__blockdev_direct_IO path that deadlocks with i_sem (i_mutex) and 
i_alloc_sem.
                                                                                

A new DIO flag is added into our distribution (2.6.9 based) to work 
around the problem by moving the inode semaphore acquiring within 
__blockdev_direct_IO() (patch attached) into GFS code path (so lock 
order can be re-arranged). The new lock granularity is not ideal but it 
gets us out of this deadlock.

We havn't had a chance to go thru your mail (and patch) in details yet 
but would like bring up this issue earlier before it gets messy.
                                                                                                                                                             

-- Wendy



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--- linux-2.6.9-22.EL/include/linux/fs.h	2005-12-07 12:43:55.000000000 -0500
+++ linux.truncate/include/linux/fs.h	2005-12-02 00:25:22.000000000 -0500
@@ -1509,7 +1509,8 @@ ssize_t __blockdev_direct_IO(int rw, str
 	int lock_type);
 
 enum {
-	DIO_LOCKING = 1, /* need locking between buffered and direct access */
+	DIO_CLUSTER_LOCKING = 0, /* allow (cluster) fs handle its own locking */
+	DIO_LOCKING,     /* need locking between buffered and direct access */
 	DIO_NO_LOCKING,  /* bdev; no locking at all between buffered/direct */
 	DIO_OWN_LOCKING, /* filesystem locks buffered and direct internally */
 };
@@ -1541,6 +1542,15 @@ static inline ssize_t blockdev_direct_IO
 				nr_segs, get_blocks, end_io, DIO_OWN_LOCKING);
 }
 
+static inline ssize_t blockdev_direct_IO_cluster_locking(int rw, struct kiocb *iocb,
+	struct inode *inode, struct block_device *bdev, const struct iovec *iov,
+	loff_t offset, unsigned long nr_segs, get_blocks_t get_blocks,
+	dio_iodone_t end_io)
+{
+	return __blockdev_direct_IO(rw, iocb, inode, bdev, iov, offset,
+			nr_segs, get_blocks, end_io, DIO_CLUSTER_LOCKING);
+}
+
 extern struct file_operations generic_ro_fops;
 
 #define special_file(m) (S_ISCHR(m)||S_ISBLK(m)||S_ISFIFO(m)||S_ISSOCK(m))
--- linux-2.6.9-22.EL/fs/direct-io.c	2005-11-09 17:26:02.000000000 -0500
+++ linux.truncate/fs/direct-io.c	2005-12-07 12:27:17.000000000 -0500
@@ -515,7 +515,7 @@ static int get_more_blocks(struct dio *d
 			fs_count++;
 
 		create = dio->rw == WRITE;
-		if (dio->lock_type == DIO_LOCKING) {
+		if ((dio->lock_type == DIO_LOCKING) || (dio->lock_type == DIO_CLUSTER_LOCKING)) {
 			if (dio->block_in_file < (i_size_read(dio->inode) >>
 							dio->blkbits))
 				create = 0;
@@ -1183,9 +1183,16 @@ __blockdev_direct_IO(int rw, struct kioc
 	 * For regular files using DIO_OWN_LOCKING,
 	 *	neither readers nor writers take any locks here
 	 *	(i_sem is already held and release for writers here)
+	 * The DIO_CLUSTER_LOCKING allows (cluster) filesystem manages its own
+	 *	locking (bypassing i_sem and i_alloc_sem handling within
+	 *	__blockdev_direct_IO()).
 	 */
+
 	dio->lock_type = dio_lock_type;
-	if (dio_lock_type != DIO_NO_LOCKING) {
+	if (dio_lock_type == DIO_CLUSTER_LOCKING)
+		goto cluster_skip_locking;
+
+	if (dio_lock_type != DIO_NO_LOCKING) { 
 		if (rw == READ) {
 			struct address_space *mapping;
 
@@ -1205,6 +1212,9 @@ __blockdev_direct_IO(int rw, struct kioc
 		if (dio_lock_type == DIO_LOCKING)
 			down_read(&inode->i_alloc_sem);
 	}
+
+cluster_skip_locking:
+
 	/*
 	 * For file extending writes updating i_size before data
 	 * writeouts complete can expose uninitialized blocks. So