Re: [PATCH v3 6/7] fs: struct file move from call_rcu() to SLAB_DESTROY_BY_RCU

Christoph Lameter a écrit :

On Fri, 12 Dec 2008, Eric Dumazet wrote:

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a truly allocated file. At this point the file is
a truly allocated file but not anymore ours.

Its a valid file. Does ownership matter here?

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Reading again this mail I realise we call put_filp(file), while this should
be fput(file) or put_filp(file), we dont know.

Damned, this patch is wrong as is.

Christoph, Paul, do you see the problem ?

Yes.

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In fget()/fget_light() we dont know if the other thread (the one who re-allocated the file,
and tried to close it while we got a reference on file) had to call put_filp() or fput()
to release its own reference. So we call atomic_long_dec_and_test() and cannot
take the appropriate action (calling the full __fput() version or the small one,
that some systems use to 'close' an not really opened file.

The difference is mainly that fput() does full processing whereas
put_filp() is used when we know that the file was not fully operational.
If the checks in __fput are able to handle the put_filp() situation by not
releasing resources that were not allocated then we should be fine.

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I believe put_filp() is only called on slowpath (error cases).

Looks like it. It seems to assume that no dentry is associated.

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Should we just zap it and always call fput() ?

Only if fput() can handle partially setup files.

It can do that if we add a check for NULL dentry in __fput(), so put_filp() can disappear.

But there is a remaining point where we do an atomic_long_dec_and_test(&...->f_count),
in fs/aio.c, function __aio_put_req(). This one is tricky :(