Re: 3.11.4: kernel BUG at fs/buffer.c:1268

On Tue 10-12-13 04:35:28, George Spelvin wrote:

One of those additional WARN_ON tests tripped, hooray!
And it turned out to be in the ext4 metadata checksumming.  To be
precise, ext4_block_bitmap_csum_set() returned with irqs disabled,
and kaboom.

  Ha, great. Thanks for the persistence in testing.

Since I have this experimental feature turned on and most people don't,
this explains why I'm finding it and World+Dog aren't.

I appear to be the designated finder of ext4 metadata_csum bugs, so tytso
notified on general principles.  I dropped the generic linux-fsdevel
list from the Cc: list.

But looking at the code, it just calls into the linux-crypto layer and
Tim Chen's SSE CRC32C implementation which uses kernel_fpu_begin()
and kernel_fpu_end() if the block is large enough.

  Yup, that code was also my last hope but I can't say I see any problem in
there either.

I was going to add and Herbert Xu and Tim Chen and all those mailing
lists, but looking at the code, it sure *looks* like they're Doing The
right Thing, so I'm holding off for a bit.

I'm not sure quite where to pass th buck on this one.

Relevant platform info:
- Intel i7-2700K processor, with SSE4.2 and thus the CRC32C instruction.
- CONFIG_PREEMPT_VOLUNTARY=y
- # CONFIG_PREEMPT_NONE is not set
- CONFIG_PREEMPT_VOLUNTARY=y
- # CONFIG_PREEMPT is not set
- CONFIG_PREEMPT_COUNT=y
- CONFIG_DEBUG_ATOMIC_SLEEP=y
- CONFIG_DEBUG_BUGVERBOSE=y

...

=== Discussion ===
desc.shash.tfm is filled in from sbi->s_chksum_driver, which is filled in at
ext4_fill_super() time by crypto_alloc_shash("crc32c", 0, 0).

Thus, shash->update should turn into a call to crypto/crc32c.c:chksum_update(),
which calls lib/crc32.c:__crc32c_le().

Now, I happen to be running an i7-2700k which has sse4_2, and thus calls
into the x86 specific code, and apparently for large blocks it uses PCLMULQDQ,
which requires kernel_fpu_begin/end.

At least that makes some degree of sense.  The low level code, though
uses the functions in a very simple way that I can't see how it could fail
to unlock at the end.

  Hum, can you try disabling the HW support of CRC32C implementation
(CRYPTO_CRC32C_INTEL)? If the problem disappears, we know there's some
problem in the HW support code...

								Honza
-- 
Jan Kara [off-list ref]
SUSE Labs, CR