Re: [PATCH] x86 spinlock: Fix memory corruption on completing completions

From: Raghavendra K T
Date: Mon Feb 09 2015 - 04:32:36 EST

On 02/09/2015 02:44 AM, Jeremy Fitzhardinge wrote:
On 02/06/2015 06:49 AM, Raghavendra K T wrote:
[...]

Linus suggested that we should not do any writes to lock after unlock(),
and we can move slowpath clearing to fastpath lock.

Yep, that seems like a sound approach.

Current approach seem to be working now. (though we could not avoid read).
Related question: Do you think we could avoid SLOWPATH_FLAG itself by
checking head and tail difference. or is it costly because it may
result in unnecessary unlock_kicks?

However it brings additional case to be handled, viz., slowpath still
could be set when somebody does arch_trylock. Handle that too by ignoring
slowpath flag during lock availability check.

Reported-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>
---
arch/x86/include/asm/spinlock.h | 70 ++++++++++++++++++++---------------------
1 file changed, 34 insertions(+), 36 deletions(-)

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index 625660f..0829f86 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -49,6 +49,23 @@ static inline void __ticket_enter_slowpath(arch_spinlock_t *lock)
set_bit(0, (volatile unsigned long *)&lock->tickets.tail);
}

+static inline void __ticket_check_and_clear_slowpath(arch_spinlock_t *lock)
+{
+ arch_spinlock_t old, new;
+ __ticket_t diff;
+
+ old.tickets = READ_ONCE(lock->tickets);

Couldn't the caller pass in the lock state that it read rather than
re-reading it?


Yes we could. do you mean we could pass additional read value apart from lock, (because lock will be anyway needed for cmpxchg).


+static inline void __ticket_check_and_clear_slowpath(arch_spinlock_t *lock)
+{
+}
+
#endif /* CONFIG_PARAVIRT_SPINLOCKS */

static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
@@ -84,7 +105,7 @@ static __always_inline void arch_spin_lock(arch_spinlock_t *lock)
register struct __raw_tickets inc = { .tail = TICKET_LOCK_INC };

inc = xadd(&lock->tickets, inc);
- if (likely(inc.head == inc.tail))
+ if (likely(inc.head == (inc.tail & ~TICKET_SLOWPATH_FLAG)))


good point, we can get rid of this as well.

The intent of this conditional was to be the quickest possible path when
taking a fastpath lock, with the code below being used for all slowpath
locks (free or taken). So I don't think masking out SLOWPATH_FLAG is
necessary here.

goto out;

inc.tail &= ~TICKET_SLOWPATH_FLAG;
@@ -98,7 +119,10 @@ static __always_inline void arch_spin_lock(arch_spinlock_t *lock)
} while (--count);
__ticket_lock_spinning(lock, inc.tail);
}
-out: barrier(); /* make sure nothing creeps before the lock is taken */
+out:
+ __ticket_check_and_clear_slowpath(lock);
+
+ barrier(); /* make sure nothing creeps before the lock is taken */

Which means that if "goto out" path is only ever used for fastpath
locks, you can limit calling __ticket_check_and_clear_slowpath() to the
slowpath case.


Yes, I ll move that call up.

}

static __always_inline int arch_spin_trylock(arch_spinlock_t *lock)
@@ -115,47 +139,21 @@ static __always_inline int arch_spin_trylock(arch_spinlock_t *lock)
return cmpxchg(&lock->head_tail, old.head_tail, new.head_tail) == old.head_tail;
}

-static inline void __ticket_unlock_slowpath(arch_spinlock_t *lock,
- arch_spinlock_t old)
-{
- arch_spinlock_t new;
-
- BUILD_BUG_ON(((__ticket_t)NR_CPUS) != NR_CPUS);
-
- /* Perform the unlock on the "before" copy */
- old.tickets.head += TICKET_LOCK_INC;

NB (see below)

Thanks for pointing, this solved the hang issue. I
missed this exact addition.


-
- /* Clear the slowpath flag */
- new.head_tail = old.head_tail & ~(TICKET_SLOWPATH_FLAG << TICKET_SHIFT);
-
- /*
- * If the lock is uncontended, clear the flag - use cmpxchg in
- * case it changes behind our back though.
- */
- if (new.tickets.head != new.tickets.tail ||
- cmpxchg(&lock->head_tail, old.head_tail,
- new.head_tail) != old.head_tail) {
- /*
- * Lock still has someone queued for it, so wake up an
- * appropriate waiter.
- */
- __ticket_unlock_kick(lock, old.tickets.head);
- }
-}
-
static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)
{
if (TICKET_SLOWPATH_FLAG &&
- static_key_false(&paravirt_ticketlocks_enabled)) {
- arch_spinlock_t prev;
+ static_key_false(&paravirt_ticketlocks_enabled)) {
+ __ticket_t prev_head;

- prev = *lock;
+ prev_head = lock->tickets.head;
add_smp(&lock->tickets.head, TICKET_LOCK_INC);

/* add_smp() is a full mb() */

- if (unlikely(lock->tickets.tail & TICKET_SLOWPATH_FLAG))
- __ticket_unlock_slowpath(lock, prev);
+ if (unlikely(lock->tickets.tail & TICKET_SLOWPATH_FLAG)) {

So we're OK with still having a ("speculative"?) read-after-unlock here?
I guess the only way to avoid it is to make the add_smp an xadd, but
that's pretty expensive even compared to a locked add (at least last
time I checked, which was at least a couple of microarchitectures ago).
An unlocked add followed by lfence should also do the trick, but that
was also much worse in practice.

So we have 3 choices,
1. xadd
2. continue with current approach.
3. a read before unlock and also after that.


+ BUILD_BUG_ON(((__ticket_t)NR_CPUS) != NR_CPUS);
+ __ticket_unlock_kick(lock, prev_head);

Should be "prev_head + TICKET_LOCK_INC" to match the previous code,
otherwise it won't find the CPU waiting for the new head.

Yes it is :)


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