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Message-Id: <20071106165900.4AA6F6E1@kernel>
Date: Tue, 06 Nov 2007 08:59:00 -0800
From: Dave Hansen <haveblue@...ibm.com>
To: akpm@...l.org
Cc: linux-kernel@...r.kernel.org, hch@...radead.org, hugh@...itas.com,
Dave Hansen <haveblue@...ibm.com>
Subject: [PATCH] r/o bind mounts: fix buggy loop
The mnt->__mnt_writers can go negative for a time if
a pair of mnt_want_write()/mnt_drop_write() calls is
done on a different cpu, but for the same mount.
This part is expected.
The lock_and_coalesce..() function should make that
count positive (or at least 0). Hugh Dickins had
found a bug in the unionfs code which caused a
permanent imbalance in this code, and eventually
underflowed the atomic_t mnt->__mnt_writers.
It also locked up the while() loop that expects the
count to go up after it is coalesced.
The following patch won't fix such a unionfs bug, but
it will keep the loop from locking up. It will also
warn a lot earlier that something funky is going on.
Signed-off-by: Dave Hansen <haveblue@...ibm.com>
---
linux-2.6.git-dave/fs/namespace.c | 31 ++++++++++++++++++++++---------
linux-2.6.git-dave/include/linux/mount.h | 1 +
2 files changed, 23 insertions(+), 9 deletions(-)
diff -puN fs/namei.c~fix-naughty-loop fs/namei.c
diff -puN fs/namespace.c~fix-naughty-loop fs/namespace.c
--- linux-2.6.git/fs/namespace.c~fix-naughty-loop 2007-11-05 08:03:59.000000000 -0800
+++ linux-2.6.git-dave/fs/namespace.c 2007-11-05 08:35:06.000000000 -0800
@@ -225,16 +225,29 @@ static void lock_and_coalesce_cpu_mnt_wr
*/
static void handle_write_count_underflow(struct vfsmount *mnt)
{
- while (atomic_read(&mnt->__mnt_writers) <
- MNT_WRITER_UNDERFLOW_LIMIT) {
- /*
- * It isn't necessary to hold all of the locks
- * at the same time, but doing it this way makes
- * us share a lot more code.
- */
- lock_and_coalesce_cpu_mnt_writer_counts();
- mnt_unlock_cpus();
+ if (atomic_read(&mnt->__mnt_writers) >=
+ MNT_WRITER_UNDERFLOW_LIMIT)
+ return;
+ /*
+ * It isn't necessary to hold all of the locks
+ * at the same time, but doing it this way makes
+ * us share a lot more code.
+ */
+ lock_and_coalesce_cpu_mnt_writer_counts();
+ /*
+ * If coalescing the per-cpu writer counts did not
+ * get us back to a positive writer count, we have
+ * a bug.
+ */
+ if ((atomic_read(&mnt->__mnt_writers) < 0) &&
+ !(mnt->mnt_flags & MNT_IMBALANCED_WRITE_COUNT)) {
+ printk("leak detected on mount(%p) writers count: %d\n",
+ mnt, atomic_read(&mnt->__mnt_writers));
+ WARN_ON(1);
+ /* use the flag to keep the dmesg spam down */
+ mnt->mnt_flags |= MNT_IMBALANCED_WRITE_COUNT;
}
+ mnt_unlock_cpus();
}
/**
diff -puN include/linux/mount.h~fix-naughty-loop include/linux/mount.h
--- linux-2.6.git/include/linux/mount.h~fix-naughty-loop 2007-11-05 08:22:21.000000000 -0800
+++ linux-2.6.git-dave/include/linux/mount.h 2007-11-05 08:28:20.000000000 -0800
@@ -32,6 +32,7 @@ struct mnt_namespace;
#define MNT_READONLY 0x40 /* does the user want this to be r/o? */
#define MNT_SHRINKABLE 0x100
+#define MNT_IMBALANCED_WRITE_COUNT 0x200 /* just for debugging */
#define MNT_SHARED 0x1000 /* if the vfsmount is a shared mount */
#define MNT_UNBINDABLE 0x2000 /* if the vfsmount is a unbindable mount */
diff -puN include/linux/fs.h~fix-naughty-loop include/linux/fs.h
diff -puN fs/nfsd/nfs4state.c~fix-naughty-loop fs/nfsd/nfs4state.c
_
-
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