[Xen-devel] Re: [PATCH 13/14] x86/ticketlock: add slowpath logic

[Srivatsa Vaddagiri <vatsa@xxxxxxxxxxxxxxxxxx>]

On Mon, Jan 17, 2011 at 08:52:22PM +0530, Srivatsa Vaddagiri wrote:
> I think this is still racy ..
> 
> Unlocker                              Locker
> 
>                               
> test slowpath
>       -> false
>               
>                               set slowpath flag
>                               test for lock pickup
>                                       -> fail
>                               block
> 
> 
> unlock
> 
> unlock needs to happen first before testing slowpath? I have made that change
> for my KVM guest and it seems to be working well with that change .. Will
> cleanup and post my patches shortly

Patch below fixes the race described above. You can fold this to your patch
13/14 if you agree this is in right direction.


Signed-off-by: Srivatsa Vaddagiri <vatsa@xxxxxxxxxxxxxxxxxx>

---
 arch/x86/include/asm/spinlock.h      |    7 +++----
 arch/x86/kernel/paravirt-spinlocks.c |   22 +++++-----------------
 2 files changed, 8 insertions(+), 21 deletions(-)

Index: linux-2.6.37/arch/x86/include/asm/spinlock.h
===================================================================
--- linux-2.6.37.orig/arch/x86/include/asm/spinlock.h
+++ linux-2.6.37/arch/x86/include/asm/spinlock.h
@@ -55,7 +55,7 @@ static __always_inline void __ticket_unl
 
 /* 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
 
@@ -166,10 +166,9 @@ static __always_inline int arch_spin_try
 static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)
 {
        barrier();              /* prevent reordering out of locked region */
+       __ticket_unlock_release(lock);
        if (unlikely(__ticket_in_slowpath(lock)))
-               __ticket_unlock_release_slowpath(lock);
-       else
-               __ticket_unlock_release(lock);
+               __ticket_unlock_slowpath(lock);
        barrier();              /* prevent reordering into locked region */
 }
 
Index: linux-2.6.37/arch/x86/kernel/paravirt-spinlocks.c
===================================================================
--- linux-2.6.37.orig/arch/x86/kernel/paravirt-spinlocks.c
+++ linux-2.6.37/arch/x86/kernel/paravirt-spinlocks.c
@@ -22,33 +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);
-       }
-
+       /* Wake up an appropriate waiter */
        __ticket_unlock_kick(lock, new.tickets.head);
 }
-EXPORT_SYMBOL(__ticket_unlock_release_slowpath);
+EXPORT_SYMBOL(__ticket_unlock_slowpath);


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