Re: [PATCH] mm: slub: Ensure that slab_unlock() is atomic

From: Peter Zijlstra
Date: Wed Mar 09 2016 - 05:14:12 EST

On Wed, Mar 09, 2016 at 12:13:16PM +0530, Vineet Gupta wrote:
> +CC linux-arch, parisc folks, PeterZ
> On Wednesday 09 March 2016 02:10 AM, Christoph Lameter wrote:
> > On Tue, 8 Mar 2016, Vineet Gupta wrote:
> >
> >> # set the bit
> >> 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)
> >
> > Duh. Guess you need to take the spinlock also in the arch specific
> > implementation of __bit_spin_unlock(). This is certainly not the only case
> > in which we use the __ op to unlock.
> __bit_spin_lock() by definition is *not* required to be atomic, bit_spin_lock() is
> - so I don't think we need a spinlock there.

Agreed. The double underscore prefixed instructions are not required to
be atomic in any way shape or form.

> There is clearly a problem in slub code that it is pairing a test_and_set_bit()
> with a __clear_bit(). Latter can obviously clobber former if they are not a single
> instruction each unlike x86 or they use llock/scond kind of instructions where the
> interim store from other core is detected and causes a retry of whole llock/scond
> sequence.

Yes, test_and_set_bit() + __clear_bit() is broken.

> > If you take the lock in __bit_spin_unlock
> > then the race cannot happen.
> Of course it won't but that means we penalize all non atomic callers of the API
> with a superfluous spinlock which is not require din first place given the
> definition of API.

Quite. _However_, your arch is still broken, but not by your fault. Its
the generic-asm code that is wrong.

The thing is that __bit_spin_unlock() uses __clear_bit_unlock(), which
defaults to __clear_bit(). Which is wrong.

Subject: 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.

Reported-by: Vineet Gupta <Vineet.Gupta1@xxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
include/asm-generic/bitops/lock.h | 14 +++++++-------
1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/include/asm-generic/bitops/lock.h b/include/asm-generic/bitops/lock.h
index c30266e94806..8ef0ccbf8167 100644
--- a/include/asm-generic/bitops/lock.h
+++ b/include/asm-generic/bitops/lock.h
@@ -29,16 +29,16 @@ do { \
* @nr: the bit to set
* @addr: the address to start counting from
- * This operation is like clear_bit_unlock, however it is not atomic.
- * It does provide release barrier semantics so it can be used to unlock
- * a bit lock, however it would only be used if no other CPU can modify
- * any bits in the memory until the lock is released (a good example is
- * if the bit lock itself protects access to the other bits in the word).
+ * A weaker form of clear_bit_unlock() as used by __bit_lock_unlock(). If all
+ * the bits in the word are protected by this lock some archs can use weaker
+ * ops to safely unlock.
+ *
+ * See for example x86's implementation.
#define __clear_bit_unlock(nr, addr) \
do { \
- smp_mb(); \
- __clear_bit(nr, addr); \
+ smp_mb__before_atomic(); \
+ clear_bit(nr, addr); \
} while (0)