From mboxrd@z Thu Jan  1 00:00:00 1970
From: Nick Piggin <npiggin@gmail.com>
Subject: Re: [patch] fs: aio fix rcu lookup
Date: Sat, 15 Jan 2011 02:00:25 +1100
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Cc: Andrew Morton <akpm@linux-foundation.org>,
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To: Jeff Moyer <jmoyer@redhat.com>
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On Sat, Jan 15, 2011 at 1:52 AM, Jeff Moyer <jmoyer@redhat.com> wrote:
> Nick Piggin <npiggin@gmail.com> writes:
>
>> Hi,
>>
>> While hunting down a bug in NFS's AIO, I believe I found this
>> buggy code...
>>
>> fs: aio fix rcu ioctx lookup
>>
>> aio-dio-invalidate-failure GPFs in aio_put_req from io_submit.
>>
>> lookup_ioctx doesn't implement the rcu lookup pattern properly.
>> rcu_read_lock does not prevent refcount going to zero, so we
>> might take a refcount on a zero count ioctx.
>
> So, does this patch fix the problem? =A0You didn't actually say....

No, it seemd to be an NFS AIO problem, although it was a
slightly older kernel so I'll re test after -rc1 if I haven't heard
back about it.

Do you agree with the theoretical problem? I didn't try to
write a racer to break it yet. Inserting a delay before the
get_ioctx might do the trick.


>> Signed-off-by: Nick Piggin <npiggin@kernel.dk>
>>
>> Index: linux-2.6/fs/aio.c
>> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>> --- linux-2.6.orig/fs/aio.c =A0 2011-01-14 00:29:00.000000000 +1100
>> +++ linux-2.6/fs/aio.c =A0 =A0 =A0 =A02011-01-14 11:31:47.000000000 =
+1100
>> @@ -239,15 +239,23 @@ static void __put_ioctx(struct kioctx *c
>> =A0 =A0 =A0 call_rcu(&ctx->rcu_head, ctx_rcu_free);
>> =A0}
>>
>> -#define get_ioctx(kioctx) do { =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =
=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 \
>> - =A0 =A0 BUG_ON(atomic_read(&(kioctx)->users) <=3D 0); =A0 =A0 =A0 =
=A0 =A0 =A0 =A0 =A0 =A0 =A0 \
>> - =A0 =A0 atomic_inc(&(kioctx)->users); =A0 =A0 =A0 =A0 =A0 =A0 =A0 =
=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 \
>> -} while (0)
>> -#define put_ioctx(kioctx) do { =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =
=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 \
>> - =A0 =A0 BUG_ON(atomic_read(&(kioctx)->users) <=3D 0); =A0 =A0 =A0 =
=A0 =A0 =A0 =A0 =A0 =A0 =A0 \
>> - =A0 =A0 if (unlikely(atomic_dec_and_test(&(kioctx)->users))) =A0 =A0=
 =A0 =A0 =A0 =A0\
>> - =A0 =A0 =A0 =A0 =A0 =A0 __put_ioctx(kioctx); =A0 =A0 =A0 =A0 =A0 =A0=
 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0\
>> -} while (0)
>> +static inline void get_ioctx(struct kioctx *kioctx)
>> +{
>> + =A0 =A0 BUG_ON(atomic_read(&kioctx->users) <=3D 0);
>> + =A0 =A0 atomic_inc(&kioctx->users);
>> +}
>> +
>> +static inline int try_get_ioctx(struct kioctx *kioctx)
>> +{
>> + =A0 =A0 return atomic_inc_not_zero(&kioctx->users);
>> +}
>> +
>> +static inline void put_ioctx(struct kioctx *kioctx)
>> +{
>> + =A0 =A0 BUG_ON(atomic_read(&kioctx->users) <=3D 0);
>> + =A0 =A0 if (unlikely(atomic_dec_and_test(&kioctx->users)))
>> + =A0 =A0 =A0 =A0 =A0 =A0 __put_ioctx(kioctx);
>> +}
>
> Why did you switch from macros? =A0Personal preference? =A0Can you at=
 least
> mention it in the changelog?

Yeah, I couldn't bring myself to add another macro :) I can mention
it, sure.
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