Re: __clear_bit_lock to use atomic clear_bit (was Re: Patch "asm-generic/bitops/lock.h)

From: Peter Zijlstra
Date: Fri Aug 31 2018 - 03:25:07 EST

On Fri, Aug 31, 2018 at 12:29:27AM +0000, Vineet Gupta wrote:
> On 08/30/2018 02:44 AM, Peter Zijlstra wrote:
> >> 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 ?
> > Right, so the thinking is that on platforms that suffer that issue,
> > atomic_set*() should DTRT. And if you look at your spinlock based atomic
> > implementation, you'll note that atomic_set() does indeed do the right
> > thing.
> >
> > arch/arc/include/asm/atomic.h:108
> For !LLSC atomics, ARC has always had atomic_set() DTRT even in the git revision
> of 2016. The problem was not in atomics, but the asymmetric way slub bit lock etc
> worked (haven't checked if this changed), i.e.
> slab_lock() -> bit_spin_lock() -> test_and_set_bit() # atomic
> slab_unlock() -> __bit_spin_unlock() -> __clear_bit() # non-atomic
> And with v4.19-rc1, we have essentially reverted f75d48644c56a due to 84c6591103db
> ("locking/atomics, asm-generic/bitops/lock.h: Rewrite using atomic_fetch_*()")
> So what we have with 4.19-rc1 is
> static inline void __clear_bit_unlock(unsigned int nr, volatile unsigned long *p)
> {
> unsigned long old;
> p += ((nr) / 32);
> old = // some typecheck magic on *p
> old &= ~(1UL << ((nr) % 32));
> atomic_long_set_release((atomic_long_t *)p, old);
> }
> So @p is being r-m-w non atomically. The lock variant uses atomic op...
> int test_and_set_bit_lock(unsigned int nr, volatile unsigned long *p)
> {
> ...
> old = atomic_long_fetch_or_acquire(mask, (atomic_long_t *)p);
> ....
> }
> Now I don't know why we don't see the issue with LLSC atomics, perhaps race window
> reduces due to less verbose code itself etc..
> Am I missing something still ?

Yes :-) So there are 2 things to consider:

1) this whole test_and_set_bit() + __clear_bit() combo only works if we
have the guarantee that no other bit will change while we have our
'lock' bit set.

This means that @old is invariant.

2) atomic ops and stores work as 'expected' -- which is true for all
hardware LL/SC or CAS implementations, but not for spinlock based

The bug in f75d48644c56a was the atomic test_and_set loosing the
__clear_bit() store.

With LL/SC this cannot happen, because the competing store (__clear_bit)
will cause the SC to fail, then we'll retry, the second LL observes the
new value.

So the main point is that test_and_set must not loose a store.
atomic_fetch_or() vs atomic_set() ensures this.

NOTE: another possible solution for spinlock based bitops is making
test_and_set 'smarter':

val = READ_ONCE(word);
if (!(val & bit)) {
val |= bit;
WRITE_ONCE(word, val);

But that is not something that works in generic (the other atomic ops),
and therefore atomic_set() is required to take the spinlock too, which
also cures the problem.