(2011/02/16 23:56), Jan Kara wrote:
On Wed 16-02-11 08:17:46, Toshiyuki Okajima wrote:
On Tue, 15 Feb 2011 18:29:54 +0100
Jan Kara<jack@xxxxxxx> wrote:
On Tue 15-02-11 12:03:52, Ted Ts'o wrote:
On Tue, Feb 15, 2011 at 05:06:30PM +0100, Jan Kara wrote:
Thanks for detailed analysis. Indeed this is a bug. Whenever we do IO
under s_umount semaphore, we are prone to deadlock like the one you
describe above.
One of the fundamental problems here is that the freeze and thaw
routines are using down_write(&sb->s_umount) for two purposes. The
first is to prevent the resume/thaw from racing with a umount (which
it could do just as well by taking a read lock), but the second is to
prevent the resume/thaw code from racing with itself. That's the core
fundamental problem here.
So I think we can solve this by introduce a new mutex, s_freeze, and
having the the resume/thaw first take the s_freeze mutex and then
second take a read lock on the s_umount.
Sadly this does not quite work because even down_read(&sb->s_umount)
in thaw_super() can block if there is another process that tries to acquire
s_umount for writing - a situation like:
TASK 1 (e.g. flusher) TASK 2 (e.g. remount) TASK 3 (unfreeze)
down_read(&sb->s_umount)
block on s_frozen
down_write(&sb->s_umount)
-blocked
down_read(&sb->s_umount)
-blocked
behind the write access...
The only working solution I see is to check for frozen filesystem before
taking s_umount semaphore which seems rather ugly (but might be bearable if
we did so in some well described wrapper).
I created the patch that you imagine yesterday.
I got a reproducer from Mizuma-san yesterday, and then I executed it on the kernel
without a fixed patch. After an hour, I confirmed that this deadlock happened.
However, on the kernel with a fixed patch, this deadlock doesn't still happen
after 12 hours passed.
The patch for linux-2.6.38-rc4 is as follows:
---
fs/fs-writeback.c | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 59c6e49..1c9a05e 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -456,7 +456,7 @@ static bool pin_sb_for_writeback(struct super_block *sb)
spin_unlock(&sb_lock);
if (down_read_trylock(&sb->s_umount)) {
- if (sb->s_root)
+ if (sb->s_frozen == SB_UNFROZEN&& sb->s_root)
return true;
up_read(&sb->s_umount);
So this is something along the lines I thought but it actually won't work
for example if sync(1) is run while the filesystem is frozen (that takes
s_umount semaphore in a different place). And generally, I'm not convinced
there are not other places that try to do IO while holding s_umount
semaphore...
OK. I understand.
This code only fixes the case for the following path:
writeback_inodes_wb
-> ext4_da_writepages
-> ext4_journal_start_sb
-> vfs_check_frozen
But, the code doesn't fix the other cases.
We must modify the local filesystem part in order to fix all cases...?
Regards,
Toshiyuki Okajima
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