On Fri, Aug 31, 2018 at 12:29:27AM +0000, Vineet Gupta wrote:

On 08/30/2018 02:44 AM, Peter Zijlstra wrote:

quoted

quoted

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
    atomics.

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':

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

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.