Re: [PATCH] fuse: hold i_mutex in fuse_file_fallocate()

From: Anand Avati
Date: Wed Jun 12 2013 - 14:55:29 EST

On 6/12/13 4:40 AM, Brian Foster wrote:
On 06/11/2013 06:59 AM, Maxim Patlasov wrote:
Changing size of a file on server and local update (fuse_write_update_size)
should be always protected by inode->i_mutex. Otherwise a race like this is

1. Process 'A' calls fallocate(2) to extend file (~FALLOC_FL_KEEP_SIZE).
fuse_file_fallocate() sends FUSE_FALLOCATE request to the server.
2. Process 'B' performs ordinary buffered write(2) with a length big enough
to extend the file beyond "offset + length" of fallocate call.
3. Process 'A' resumes execution of fuse_file_fallocate() and calls
fuse_write_update_size(inode, offset + length). But 'offset + length' was
obsoleted by write from previous step.

Hi Maxim,

Doesn't fuse_write_update_size() already handle this particular case by
only ever extending the size?

As you say, fuse_write_update_size() does seem to protect against the case Maxim writes in the commit log.

However, there is still an issue with with truncate(shrinking_offset) and fallocate(growing_offset,~FALLOC_FL_KEEP_SIZE) racing, and changing inode size in opposing order between file server and in core ->i_size. Therefore, grabbing i_mutex is making fallocate and truncate atomic against each other.

I guess we just need an updated commit log, and same code change?


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