On 06/23, Paul E. McKenney wrote: > > On Mon, Jun 23, 2014 at 06:43:21PM +0200, Oleg Nesterov wrote: > > > > should equally work, or ACCESS_ONCE() can't be used to RMW ? > > It can be, but Linus doesn't like it to be. I recently changed all of > the RMW ACCESS_ONCE() calls as a result. One of the reasons for avoiding > RMW ACCESS_ONCE() is that language features that might one day replace > ACCESS_ONCE() do not support RMW use.
OK, thanks. > > Or even INIT_LIST_HEAD_RCU(). The comment in list_splice_init_rcu() says: > > > > /* > > * "first" and "last" tracking list, so initialize it. RCU readers > > * have access to this list, so we must use INIT_LIST_HEAD_RCU() > > * instead of INIT_LIST_HEAD(). > > */ > > > > INIT_LIST_HEAD_RCU(list); > > > > but we are going to call synchronize_rcu() or something similar, this should > > act as compiler barrier too? > > Indeed, synchronize_rcu() enforces a barrier on each CPU between > any prior and subsequent accesses to RCU-protected data by that CPU. > (Which means that CPUs that would otherwise sleep through the entire > grace period can continue sleeping, given that it is not accessing > any RCU-protected data while sleeping.) I would guess load-tearing > or store-tearing concerns. But the kernel depends on the fact that "long" should be updated atomically, and the concurent reader should see the old-or-new value without any tricks. Perhaps we should add ACCESS_ONCE_PARANOID_FOR_COMPILER(). Otherwise when you read the code it is not always clear why it is uses ACCESS_ONCE(), and sometimes this look as if you simply do not understand it. Or at least a /* not really needed but gcc can have bugs */ could help in these cases. Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
