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    SubjectRe: linux-next: build failure after merge of the vfs tree
    On Wed 04-01-12 13:50:20, Dave Chinner wrote:
    > On Wed, Jan 04, 2012 at 02:17:54AM +0000, Al Viro wrote:
    > > I'm still not
    > > sure about ->statfs(), BTW - any input on that would be welcome. Can
    > > it end up blocked on a frozen fs until said fs is thawed?
    > I don't see why this should ever happen - ->statfs has to work on
    > read-only filesystems so shoul dnot be modifying state, and hence
    > should never need to care about the frozen state of the superblock.
    Well, I'm also not aware of a filesystem where ->statfs would wait on
    frozen filesystem. Just note that e.g. for stat(2) frozen filesystem and
    RO filesystem *are* different because of atime updates. So stat(2) can
    block on frozen fs because of atime update while on RO filesystem it is
    just fine.

    > So from a ->statfs POV, a frozen filesystem should look just like a
    > read-only filesystem. If frozen filesystems are holding locks that
    > ->statfs can block on until the filesystem us thawed, then I'd
    > consider that a bug in the filesystem freeze implementation....
    In an ideal world yes. Practically, current freeze code has races
    (vfs_check_frozen() is a totally racy check) which can leave processes
    waiting for frozen fs with filesystem locks held. I believe we need
    something like mnt_want_write()/mnt_drop_write() for freezing code in
    ->page_mkwrite() and ->write_begin/->write_end. I'm now looking into how
    to do that in the best way.

    > > to convert ustat(2) to "wait for thaw" semantics (should be interruptible,
    > > BTW) or document that ->statfs() is not allowed to wait for thawing.
    > > It's far too subtle to leave undocumented...
    > The latter, IMO.

    Jan Kara <>
    SUSE Labs, CR

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