[RFC PATCH 3/5] asm-generic/bitops/atomic.h: Rewrite using atomic_fetch_*

Hi Peter,

On Fri, Feb 16, 2018 at 11:21:00AM +0100, Peter Zijlstra wrote:

On Thu, Feb 15, 2018 at 06:20:49PM +0000, Will Deacon wrote:

quoted

On Thu, Feb 15, 2018 at 06:08:47PM +0100, Peter Zijlstra wrote:

quoted

On Thu, Feb 15, 2018 at 03:29:33PM +0000, Will Deacon wrote:

quoted

+static inline void __clear_bit_unlock(unsigned int nr,
+				      volatile unsigned long *p)
+{
+	unsigned long old;
 
+	p += BIT_WORD(nr);
+	old = READ_ONCE(*p);
+	old &= ~BIT_MASK(nr);
+	smp_store_release(p, old);

This should be atomic_set_release() I think, for the special case where
atomics are implemented with spinlocks, see the 'fun' case in
Documentation/atomic_t.txt.

My understanding of __clear_bit_unlock is that there is guaranteed to be
no concurrent accesses to the same word, so why would it matter whether
locks are used to implement atomics?

commit f75d48644c56a31731d17fa693c8175328957e1d
Author: Peter Zijlstra [off-list ref]
Date:   Wed Mar 9 12:40:54 2016 +0100

    bitops: Do not default to __clear_bit() for __clear_bit_unlock()
    
    __clear_bit_unlock() is a special little snowflake. While it carries the
    non-atomic '__' prefix, it is specifically documented to pair with
    test_and_set_bit() and therefore should be 'somewhat' atomic.
    
    Therefore the generic implementation of __clear_bit_unlock() cannot use
    the fully non-atomic __clear_bit() as a default.
    
    If an arch is able to do better; is must provide an implementation of
    __clear_bit_unlock() itself.
    
    Specifically, this came up as a result of hackbench livelock'ing in
    slab_lock() on ARC with SMP + SLUB + !LLSC.
    
    The issue was incorrect pairing of atomic ops.
    
     slab_lock() -> bit_spin_lock() -> test_and_set_bit()
     slab_unlock() -> __bit_spin_unlock() -> __clear_bit()
    
    The non serializing __clear_bit() was getting "lost"
    
     80543b8e:      ld_s       r2,[r13,0] <--- (A) Finds PG_locked is set
     80543b90:      or         r3,r2,1    <--- (B) other core unlocks right here
     80543b94:      st_s       r3,[r13,0] <--- (C) sets PG_locked (overwrites unlock)

Ah, so it's problematic for the case where atomics are built using locks.
Got it. I'll err on the side of caution here and have the asm-generic header
(which should be bitops/lock.h not bitops/atomic.h) conditionally define
__clear_bit_unlock as clear_bit_lock unless the architecture has provided
its own implementation.

Thanks,

Will