Hello, Oleg. Oleg Nesterov wrote: > Hello Tejun, > > On 05/16, Tejun Heo wrote: >>>> lock is read arrier, unlock is write barrier. >> Let's say there's a shared data structure protected by a spinlock and >> two threads are accessing it. >> >> 1. thr1 locks spin >> 2. thr1 updates data structure >> 3. thr1 unlocks spin >> 4. thr2 locks spin >> 5. thr2 accesses data structure >> 6. thr2 unlocks spin >> >> If spin_unlock is not a write barrier and spin_lock is not a read >> barrier, nothing guarantees memory accesses from step#5 will see the >> changes made in step#2. Memory fetch can occur during updates in step#2 >> or even before that. > > Ah, but this is something different. Both lock/unlock are full barriers, > but they protect only one direction. A memory op must not leak out of the > critical section, but it may leak in. > > A = B; // 1 > lock(); // 2 > C = D; // 3 > > this can be re-ordered to > > lock(); // 2 > C = D; // 3 > A = B; // 1 > > but 2 and 3 must not be re-ordered.
OIC. Right, barriers with directionality would do that. > To be sure, I contacted Paul E. McKenney privately, and his reply is > > > No. See for example IA64 in file include/asm-ia64/spinlock.h, > > line 34 for spin_lock() and line 92 for spin_unlock(). The > > spin_lock() case uses a ,acq completer, which will allow preceding > > reads to be reordered into the critical section. The spin_unlock() > > uses the ,rel completer, which will allow subsequent writes to be > > reordered into the critical section. The locking primitives are > > guaranteed to keep accesses bound within the critical section, but > > are free to let outside accesses be reordered into the critical > > section. > > > > Download the Itanium Volume 2 manual: > > > > http://developer.intel.com/design/itanium/manuals/245318.htm > > > > Table 2.3 on page 2:489 (physical page 509) shows an example of how > > the rel and acq completers work. And, there actually is such a beast. Thanks for the enlightenment. Care to document these? >>> Could you also look at >>> http://marc.info/?t=116275561700001&r=1 >>> >>> and, in particular, >>> http://marc.info/?l=linux-kernel&m=116281136122456 >> This is because spin_lock() isn't a write barrier, right? I totally >> agree with you there. > > Yes, but in fact I think wake_up() needs a full mb() semantics (which we > don't have _in theory_), because try_to_wake_up() first checks task->state > and does nothing if it is TASK_RUNNING. > > That is why I think that smp_mb__before_spinlock() may be useful not only > for workqueue.c Yeap, I agree. -- tejun - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
