[Xen-devel] [PATCH 11/13] x86/ticketlock: only do kick after doing unlock

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

From: Srivatsa Vaddagiri <vatsa@xxxxxxxxxxxxxxxxxx>

We must release the lock before checking to see if the lock is in
slowpath or else there's a potential race where the lock enters the
slow path after the unlocker has checked the slowpath flag, but before
it has actually unlocked.

Signed-off-by: Srivatsa Vaddagiri <vatsa@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
---
 arch/x86/include/asm/spinlock.h      |    7 +++----
 arch/x86/kernel/paravirt-spinlocks.c |   23 ++++++-----------------
 2 files changed, 9 insertions(+), 21 deletions(-)

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index 365d787..bf6dff4 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -43,7 +43,7 @@
 
 /* Only defined when CONFIG_PARAVIRT_SPINLOCKS defined, but may as
  * well leave the prototype always visible.  */
-extern void __ticket_unlock_release_slowpath(struct arch_spinlock *lock);
+extern void __ticket_unlock_slowpath(struct arch_spinlock *lock);
 
 #ifdef CONFIG_PARAVIRT_SPINLOCKS
 
@@ -154,10 +154,9 @@ static __always_inline void 
__ticket_unlock_release(arch_spinlock_t *lock)
 static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)
 {
 
+       __ticket_unlock_release(lock);
        if (unlikely(__ticket_in_slowpath(lock)))
-               __ticket_unlock_release_slowpath(lock);
-       else
-               __ticket_unlock_release(lock);
+               __ticket_unlock_slowpath(lock);
 }
 
 static inline int arch_spin_is_locked(arch_spinlock_t *lock)
diff --git a/arch/x86/kernel/paravirt-spinlocks.c 
b/arch/x86/kernel/paravirt-spinlocks.c
index 71b8557..674718f 100644
--- a/arch/x86/kernel/paravirt-spinlocks.c
+++ b/arch/x86/kernel/paravirt-spinlocks.c
@@ -22,32 +22,21 @@ EXPORT_SYMBOL(pv_lock_ops);
  * bits.  However, we need to be careful about this because someone
  * may just be entering as we leave, and enter the slowpath.
  */
-void __ticket_unlock_release_slowpath(struct arch_spinlock *lock)
+void __ticket_unlock_slowpath(struct arch_spinlock *lock)
 {
        struct arch_spinlock old, new;
 
        BUILD_BUG_ON(((__ticket_t)NR_CPUS) != NR_CPUS);
 
        old = ACCESS_ONCE(*lock);
-
        new = old;
-       new.tickets.head += TICKET_LOCK_INC;
 
        /* Clear the slowpath flag */
        new.tickets.tail &= ~TICKET_SLOWPATH_FLAG;
+       if (new.tickets.head == new.tickets.tail)
+               cmpxchg(&lock->head_tail, old.head_tail, new.head_tail);
 
-       /*
-        * If there's currently people waiting or someone snuck in
-        * since we read the lock above, then do a normal unlock and
-        * kick.  If we managed to unlock with no queued waiters, then
-        * we can clear the slowpath flag.
-        */
-       if (new.tickets.head != new.tickets.tail ||
-           cmpxchg(&lock->head_tail,
-                   old.head_tail, new.head_tail) != old.head_tail) {
-               /* still people waiting */
-               __ticket_unlock_release(lock);
-               __ticket_unlock_kick(lock, new.tickets.head);
-       }
+       /* Wake up an appropriate waiter */
+       __ticket_unlock_kick(lock, new.tickets.head);
 }
-EXPORT_SYMBOL(__ticket_unlock_release_slowpath);
+EXPORT_SYMBOL(__ticket_unlock_slowpath);
-- 
1.7.6


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