> > > > I chased it further to: > > > > commit 8375ad98cc1defc36adf4a77d9ea1e71db51a371 > > Author: Paul E. McKenney <paulmck@xxxxxxxxxx> > > Date: Mon Apr 29 15:06:13 2013 -0700 > > > > vm: adjust ifdef for TINY_RCU > > There is an ifdef in page_cache_get_speculative() that checks for !SMP > > and TREE_RCU, which has been an impossible combination since the advent > > of TINY_RCU. The ifdef enables a fastpath that is valid when preemption > > is disabled by rcu_read_lock() in UP systems, which is the case when > > TINY_RCU is enabled. This commit therefore adjusts the ifdef to > > generate the fastpath when TINY_RCU is enabled. > > > > > > Where Paul explicitly restored that fastpath for TINY_RCU instead of removing that code. > > > > So maybe Paul can comment if that is still worth having. CCing him. > > It is currently an atomic operation either way, though the folio_ref_add() > avoids full ordering, but that is immaterial on x86. Some say that it is > in the noise on server-class ARMv8 as well, though they have also said > a great many things in the past. But if that is true, the big benefit > of the TINY_RCU check is that folio_ref_try_add_rcu() is guaranted not > to fail in that case (single CPU with preemption disabled). Except that > everyone has to check the return value anyway, right? > > So the usual advice, unsatisfying though it might be, is to remove that > #ifdef and see if anyone notices. > > After all, both 2013 and 2008 were quite some time ago. ;-) Thanks, Paul. I will submit a patch to remove the #ifdef as the fix for the bug report. And do the clean up in a separate patch which is preferred by David. > > Thanx, Paul
