Re: Patch "asm-generic/bitops/lock.h: Rewrite using atomic_fetch_" causes kernel crash

[Vineet Gupta]

On 08/29/2018 11:33 AM, Eugeniy Paltsev wrote:
> Hi Guys,
> Since v4.19-rc1 we are getting a serious regression on platforms with ARC architecture.
> The kernel have become unstable and spontaneously crashes on LTP tests execution / IO tests or
> even on boot.
>
> I don't know exactly what breaks but bisect clearly assign the blame to this commit:
> 84c6591103db ("locking/atomics, asm-generic/bitops/lock.h: Rewrite using atomic_fetch_*()")
> https://github.com/torvalds/linux/commit/84c6591103dbeaf393a092a3fc7b09510825f6b9
>
> Reverting the commit solves this problem.
>
> I tested v4.19-rc1 on ARM (wandboard, i.mx6, 32bit, quard core, ARMv7) which uses same
> generic bitops implementation and it works fine.
>
> Do you have any ideas what went wrong?

Back in 2016, Peter had fixed this file due to a problem I reported on ARC. See
commit f75d48644c56a ("bitops: Do not default to __clear_bit() for
__clear_bit_unlock()")
That made __clear_bit_unlock() use the atomic clear_bit() vs. non-atomic
__clear_bit(), effectively making clear_bit_unlock() and __clear_bit_unlock() same.

This patch undoes that which could explain the issues you see. @Peter, @Will ?

-Vineet