Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1459739

Re: spin_lock implicit/explicit memory barrier

Show all headers | View raw

On Wed, Aug 10, 2016 at 10:05:37AM +1000, Benjamin Herrenschmidt wrote:
> On Tue, 2016-08-09 at 20:52 +0200, Manfred Spraul wrote:
> > Hi Benjamin, Hi Michael,
> > 
> > regarding commit 51d7d5205d33 ("powerpc: Add smp_mb() to 
> > arch_spin_is_locked()"):
> > 
> > For the ipc/sem code, I would like to replace the spin_is_locked() with 
> > a smp_load_acquire(), see:
> > 
> > http://git.cmpxchg.org/cgit.cgi/linux-mmots.git/tree/ipc/sem.c#n367
> > 
> > http://www.ozlabs.org/~akpm/mmots/broken-out/ipc-semc-fix-complex_count-vs-simple-op-race.patch
> > 
> > To my understanding, I must now add a smp_mb(), otherwise it would be 
> > broken on PowerPC:
> > 
> > The approach that the memory barrier is added into spin_is_locked() 
> > doesn't work because the code doesn't use spin_is_locked().
> > 
> > Correct?
> 
> Right, otherwise you aren't properly ordered. The current powerpc locks provide
> good protection between what's inside vs. what's outside the lock but not vs.
> the lock *value* itself, so if, like you do in the sem code, use the lock
> value as something that is relevant in term of ordering, you probably need
> an explicit full barrier.
> 
> Adding Paul McKenney.

To amplify what Ben said...

Any CPU holding a given lock will see any previous accesses made under
the protection of that lock.

A CPU -not- holding the lock can see misordering.  As Ben noted, to
that non-lock-holding CPU it might appear that a write made under the
protection of that lock was made after the lock was released.  Similarly,
to that CPU it might appear that a load done under the protection of that
lock completed before the lock was acquired.  Finally, a CPU not holding
the lock might see a store by one CPU holding the lock as happening
after a load (from some other variable) by the next CPU holding that lock.

							Thanx, Paul

Back to linux.kernel | Previous | Next — Previous in thread | Find similar | Unroll thread

Thread

spin_lock implicit/explicit memory barrier Manfred Spraul <manfred@colorfullife.com> - 2016-08-09 21:00 +0200
  Re: spin_lock implicit/explicit memory barrier Benjamin Herrenschmidt <benh@kernel.crashing.org> - 2016-08-10 02:10 +0200
    Re: spin_lock implicit/explicit memory barrier Manfred Spraul <manfred@colorfullife.com> - 2016-08-10 20:30 +0200
      Re: spin_lock implicit/explicit memory barrier Davidlohr Bueso <dave@stgolabs.net> - 2016-08-10 21:30 +0200
        Re: spin_lock implicit/explicit memory barrier "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-08-10 23:10 +0200
          Re: spin_lock implicit/explicit memory barrier Manfred Spraul <manfred@colorfullife.com> - 2016-08-15 22:10 +0200
            Re: spin_lock implicit/explicit memory barrier "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-08-15 22:30 +0200
        Re: spin_lock implicit/explicit memory barrier Boqun Feng <boqun.feng@gmail.com> - 2016-08-12 04:50 +0200
          Re: spin_lock implicit/explicit memory barrier Manfred Spraul <manfred@colorfullife.com> - 2016-08-12 20:50 +0200
            Re: spin_lock implicit/explicit memory barrier Boqun Feng <boqun.feng@gmail.com> - 2016-08-22 11:20 +0200
      Re: spin_lock implicit/explicit memory barrier "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-08-10 23:00 +0200
        Re: spin_lock implicit/explicit memory barrier Davidlohr Bueso <dave@stgolabs.net> - 2016-08-11 00:30 +0200
          Re: spin_lock implicit/explicit memory barrier "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-08-11 01:00 +0200
            Re: spin_lock implicit/explicit memory barrier Davidlohr Bueso <dave@stgolabs.net> - 2016-08-11 01:40 +0200
              Re: spin_lock implicit/explicit memory barrier Peter Zijlstra <peterz@infradead.org> - 2016-08-11 10:20 +0200
                Re: spin_lock implicit/explicit memory barrier Davidlohr Bueso <dave@stgolabs.net> - 2016-08-11 20:40 +0200
                Re: spin_lock implicit/explicit memory barrier Boqun Feng <boqun.feng@gmail.com> - 2016-08-12 05:10 +0200
                Re: spin_lock implicit/explicit memory barrier Peter Zijlstra <peterz@infradead.org> - 2016-08-19 16:10 +0200
          Re: spin_lock implicit/explicit memory barrier Benjamin Herrenschmidt <benh@kernel.crashing.org> - 2016-08-11 02:10 +0200
    Re: spin_lock implicit/explicit memory barrier "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-08-10 22:00 +0200

csiph-web