| Message ID | 1401727810.7440.34.camel@j-VirtualBox (mailing list archive) |
|---|---|
| State | Not Applicable |
| Headers | show |
On Mon, Jun 02, 2014 at 09:50:10AM -0700, Jason Low wrote: > On Mon, 2014-06-02 at 12:00 -0400, Mikulas Patocka wrote: > > If you write to some variable with ACCESS_ONCE and use cmpxchg or xchg at > > the same time, you break it. ACCESS_ONCE doesn't take the hashed spinlock, > > so, in this case, cmpxchg or xchg isn't really atomic at all. > > So if the problem is using ACCESS_ONCE writes with cmpxchg and xchg at > the same time, would the below change address this problem? And one could use cmpxchg() or atomic_add_return(..., 0) to read a value out. Probably at the cost of some performance impact, though. Thanx, Paul > ----- > diff --git a/kernel/locking/mcs_spinlock.c b/kernel/locking/mcs_spinlock.c > index 838dc9e..8396721 100644 > --- a/kernel/locking/mcs_spinlock.c > +++ b/kernel/locking/mcs_spinlock.c > @@ -71,7 +71,7 @@ bool osq_lock(struct optimistic_spin_queue **lock) > if (likely(prev == NULL)) > return true; > > - ACCESS_ONCE(prev->next) = node; > + xchg(&prev->next, node); > > /* > * Normally @prev is untouchable after the above store; because at that > @@ -144,7 +144,7 @@ unqueue: > */ > > ACCESS_ONCE(next->prev) = prev; > - ACCESS_ONCE(prev->next) = next; > + xchg(&prev->next, next); > > return false; > } > > -- To unsubscribe from this list: send the line "unsubscribe linux-parisc" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On 06/02/2014 12:50 PM, Jason Low wrote: > On Mon, 2014-06-02 at 12:00 -0400, Mikulas Patocka wrote: >> If you write to some variable with ACCESS_ONCE and use cmpxchg or xchg at >> the same time, you break it. ACCESS_ONCE doesn't take the hashed spinlock, >> so, in this case, cmpxchg or xchg isn't really atomic at all. > So if the problem is using ACCESS_ONCE writes with cmpxchg and xchg at > the same time, would the below change address this problem? > > ----- > diff --git a/kernel/locking/mcs_spinlock.c b/kernel/locking/mcs_spinlock.c > index 838dc9e..8396721 100644 > --- a/kernel/locking/mcs_spinlock.c > +++ b/kernel/locking/mcs_spinlock.c > @@ -71,7 +71,7 @@ bool osq_lock(struct optimistic_spin_queue **lock) > if (likely(prev == NULL)) > return true; > > - ACCESS_ONCE(prev->next) = node; > + xchg(&prev->next, node); > > /* > * Normally @prev is untouchable after the above store; because at that > @@ -144,7 +144,7 @@ unqueue: > */ > > ACCESS_ONCE(next->prev) = prev; > - ACCESS_ONCE(prev->next) = next; > + xchg(&prev->next, next); > > return false; > } > > Doing an xchg is a very expensive operation compared with ACCESS_ONCE. I will not suggest doing that to make it right for PA-RISC at the expense of performance in other architectures. -Longman -- To unsubscribe from this list: send the line "unsubscribe linux-parisc" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On 06/02/2014 10:25 AM, Waiman Long wrote: > > Doing an xchg is a very expensive operation compared with ACCESS_ONCE. I > will not suggest doing that to make it right for PA-RISC at the expense > of performance in other architectures. > And of course, this gets into the toxic question: what are reasonable minimum requirements for Linux? How far do we need to stretch to support niche architectures which have very small (Linux) userbases? -hpa -- To unsubscribe from this list: send the line "unsubscribe linux-parisc" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
diff --git a/kernel/locking/mcs_spinlock.c b/kernel/locking/mcs_spinlock.c index 838dc9e..8396721 100644 --- a/kernel/locking/mcs_spinlock.c +++ b/kernel/locking/mcs_spinlock.c @@ -71,7 +71,7 @@ bool osq_lock(struct optimistic_spin_queue **lock) if (likely(prev == NULL)) return true; - ACCESS_ONCE(prev->next) = node; + xchg(&prev->next, node); /* * Normally @prev is untouchable after the above store; because at that @@ -144,7 +144,7 @@ unqueue: */ ACCESS_ONCE(next->prev) = prev; - ACCESS_ONCE(prev->next) = next; + xchg(&prev->next, next); return false; }