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Re: [PATCH] xfs_file_last_byte() needs to acquire ilock

To: Lachlan McIlroy <lmcilroy@xxxxxxxxxx>
Subject: Re: [PATCH] xfs_file_last_byte() needs to acquire ilock
From: Felix Blyakher <felixb@xxxxxxx>
Date: Tue, 28 Apr 2009 00:03:19 -0500
Cc: Eric Sandeen <sandeen@xxxxxxxxxxx>, xfs@xxxxxxxxxxx
In-reply-to: <860553965.233621240891896139.JavaMail.root@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
References: <860553965.233621240891896139.JavaMail.root@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>

On Apr 27, 2009, at 11:11 PM, Lachlan McIlroy wrote:

----- "Felix Blyakher" <felixb@xxxxxxx> wrote:

On Apr 23, 2009, at 10:46 PM, Lachlan McIlroy wrote:


----- "Eric Sandeen" <sandeen@xxxxxxxxxxx> wrote:

Lachlan McIlroy wrote:
We had some systems crash with this stack:

[<a00000010000cb20>] ia64_leave_kernel+0x0/0x280
[<a00000021291ca00>] xfs_bmbt_get_startoff+0x0/0x20 [xfs]
[<a0000002129080b0>] xfs_bmap_last_offset+0x210/0x280 [xfs]
[<a00000021295b010>] xfs_file_last_byte+0x70/0x1a0 [xfs]
[<a00000021295b200>] xfs_itruncate_start+0xc0/0x1a0 [xfs]
[<a0000002129935f0>] xfs_inactive_free_eofblocks+0x290/0x460
[xfs]
[<a000000212998fb0>] xfs_release+0x1b0/0x240 [xfs]
[<a0000002129ad930>] xfs_file_release+0x70/0xa0 [xfs]
[<a000000100162ea0>] __fput+0x1a0/0x420
[<a000000100163160>] fput+0x40/0x60

The problem here is that xfs_file_last_byte() does not acquire
the
inode lock and can therefore race with another thread that is
modifying
the extext list.  While xfs_bmap_last_offset() is trying to
lookup
what was the last extent some extents were merged and the extent
list
shrunk so the index we lookup is now beyond the end of the extent
list
and potentially in a freed buffer.

diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
index e7ae08d..cf62d9d 100644
--- a/fs/xfs/xfs_inode.c
+++ b/fs/xfs/xfs_inode.c
@@ -1258,8 +1258,10 @@ xfs_file_last_byte(

      /*
       * Only check for blocks beyond the EOF if the extents have
       * been read in.  This eliminates the need for the inode
lock,
       * and it also saves us from looking when it really isn't
         * necessary.
         */

I suppose that comment should be modified too, and maybe the
commit
log
should say why, exactly, it was wrong? :)
Ha, I didn't even read the comment!  It's still kind of correct in
that we wont have to get the inode lock if the extents have not
been

read in.

I'd still think the comments could be made less confusing
if we're adding the inode lock here.
The more I read the comment the more it makes sense and it seems to
make more sense now with the change because it is clear how we can
avoid the inode lock if the extents are not read in.

OK, now after your explanation and reading the comments the Nth time,
I think, I know what you mean.

I think, the original comment intention was the following:

        if (ip->i_df.if_flags & XFS_IFEXTENTS) {
                // extents have been read in. This (the fact that the extents
                // have been read in) eliminates the need for the inode lock, as
                // we are not going to read them in through xfs_iread_extents().
                error = xfs_bmap_last_offset(NULL, ip, &last_block,
                        XFS_DATA_FORK);
                if (error) {
                        last_block = 0;
                }
        } else {
                last_block = 0;
        }

while in the patched version it'll become:

        if (ip->i_df.if_flags & XFS_IFEXTENTS) {
                // extents have been read in ...
                xfs_ilock(ip, XFS_ILOCK_SHARED);
                error = xfs_bmap_last_offset(NULL, ip, &last_block,
                        XFS_DATA_FORK);
                xfs_iunlock(ip, XFS_ILOCK_SHARED);
                if (error) {
                        last_block = 0;
                }
        } else {
                // this (the fact that the extents have _NOT_ been read in)
                // eliminates the need for the inode lock.
                // Doh, obvious.
                last_block = 0;
        }

Is that how you see the comment now?

Was the assumption in the original comment about not needing the ilock
simply incorrect?

How would you prefer the comment reads?

I'd just leave the first sentence from the original comment.

         * Only check for blocks beyond the EOF if the extents have
         * been read in.

The mentioning about the ilock is too confusing now, imho.

Felix




Felix




-Eric

        if (ip->i_df.if_flags & XFS_IFEXTENTS) {
+               xfs_ilock(ip, XFS_ILOCK_SHARED);
                error = xfs_bmap_last_offset(NULL, ip, &last_block,
                        XFS_DATA_FORK);
+               xfs_iunlock(ip, XFS_ILOCK_SHARED);
                if (error) {
                        last_block = 0;
                }

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