On 12/29/2017 02:02 PM, bingjingc wrote:
Guoqing Jiang 於 2017-12-28 15:59 寫到:
On 12/27/2017 01:47 PM, bingjingc wrote:
There is a potential deadlock if mount/umount happens when
raid5_finish_reshape() tries to grow the size of emulated disk.
How the deadlock happens?
1) The raid5 resync thread finished reshape (expanding array).
2) The mount or umount thread holds VFS sb->s_umount lock and tries to
write through critical data into raid5 emulated block device. So it
waits for raid5 kernel thread handling stripes in order to finish it
I/Os.
3) In the routine of raid5 kernel thread, md_check_recovery() will be
called first in order to reap the raid5 resync thread. That is,
raid5_finish_reshape() will be called. In this function, it will try
to update conf and call VFS revalidate_disk() to grow the raid5
emulated block device. It will try to acquire VFS sb->s_umount lock.
The raid5 kernel thread cannot continue, so no one can handle mount/
umount I/Os (stripes). Once the write-through I/Os cannot be finished,
mount/umount will not release sb->s_umount lock. The deadlock happens.
The raid5 kernel thread is an emulated block device. It is
responible to
handle I/Os (stripes) from upper layers. The emulated block device
should not request any I/Os on itself. That is, it should not call VFS
layer functions. (If it did, it will try to acquire VFS locks to
guarantee the I/Os sequence.) So we have the resync thread to send
resync I/O requests and to wait for the results.
For solving this potential deadlock, we can put the size growth of the
emulated block device as the final step of reshape thread.
Maybe another way to avoid the deadlock is to ensure the reshaping array
can't be mount at all. One reason is, if the reshaping is finished,
but fs still
don't know about the new array size since metadata is not refreshed.
Just
my $0.02.
It may take hours or couples of days to expand an array. So I bet that
it's a
feature that an expanding array can still serve fs requests. Once it
finished,
the new array size for metadata can be passed to fs layer via
revalidate_disk().
From then, fs is able to be expanded, too. There are no needs to
restart that
array. Based on this point, I don't want to break this feature.
Sorry, the metadata which I mentioned before is the metadata for file
system.
How fs know about the new size without update fs's metadata? Pls see the
wiki:
https://raid.wiki.kernel.org/index.php/Growing#Extending_the_filesystem
Thanks,
Guoqing
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