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X-MS-Exchange-CrossTenant-Id: 4e2c6054-71cb-48f1-bd6c-3a9705aca71b X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: Yr0z9UDRVyHtAiGDrHtNVwK89QqMDR3b0qwCHWHuT31cTtnfAHhXHbErJnZ8aUBh2OktRAi6aQZJotE6PTmOqGB/Ij2o2h7TqmXHwxMFAfI= X-MS-Exchange-Transport-CrossTenantHeadersStamped: MN6PR10MB8046 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1057,Hydra:6.0.680,FMLib:17.12.68.34 definitions=2024-11-25_13,2024-11-25_01,2024-11-22_01 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 mlxscore=0 spamscore=0 suspectscore=0 mlxlogscore=999 phishscore=0 adultscore=0 malwarescore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2409260000 definitions=main-2411250180 X-Proofpoint-GUID: INfKzGPecmz213AiBppA9k0DNhcN0y-r X-Proofpoint-ORIG-GUID: INfKzGPecmz213AiBppA9k0DNhcN0y-r Frederic Weisbecker writes: > Le Wed, Nov 06, 2024 at 12:17:55PM -0800, Ankur Arora a =C3=A9crit : >> PREEMPT_LAZY can be enabled stand-alone or alongside PREEMPT_DYNAMIC >> which allows for dynamic switching of preemption models. >> >> The choice of PREEMPT_RCU or not, however, is fixed at compile time. >> >> Given that PREEMPT_RCU makes some trade-offs to optimize for latency >> as opposed to throughput, configurations with limited preemption >> might prefer the stronger forward-progress guarantees of PREEMPT_RCU=3Dn= . >> >> Accordingly, explicitly limit PREEMPT_RCU=3Dy to the latency oriented >> preemption models: PREEMPT, PREEMPT_RT, and the runtime configurable >> model PREEMPT_DYNAMIC. >> >> This means the throughput oriented models, PREEMPT_NONE, >> PREEMPT_VOLUNTARY and PREEMPT_LAZY will run with PREEMPT_RCU=3Dn. >> >> Cc: Paul E. McKenney >> Cc: Peter Zijlstra >> Signed-off-by: Ankur Arora >> --- >> kernel/rcu/Kconfig | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig >> index 5a7ff5e1cdcb..9d52f87fac27 100644 >> --- a/kernel/rcu/Kconfig >> +++ b/kernel/rcu/Kconfig >> @@ -18,7 +18,7 @@ config TREE_RCU >> >> config PREEMPT_RCU >> bool >> - default y if PREEMPTION >> + default y if (PREEMPT || PREEMPT_RT || PREEMPT_DYNAMIC) >> select TREE_RCU >> help >> This option selects the RCU implementation that is > > Reviewed-by: Frederic Weisbecker > > But looking at !CONFIG_PREEMPT_RCU code on tree_plugin.h, I see > some issues now that the code can be preemptible. Well I think > it has always been preemptible but PREEMPTION && !PREEMPT_RCU > has seldom been exerciced (or was it even possible?). > > For example rcu_read_unlock_strict() can be called with preemption > enabled so we need the following otherwise the rdp is unstable, the > norm value becomes racy (though automagically fixed in rcu_report_qs_rdp(= )) > and rcu_report_qs_rdp() might warn. > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > index 58d84c59f3dd..368f00267d4e 100644 > --- a/include/linux/rcupdate.h > +++ b/include/linux/rcupdate.h > @@ -95,9 +95,9 @@ static inline void __rcu_read_lock(void) > > static inline void __rcu_read_unlock(void) > { > - preempt_enable(); > if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD)) > rcu_read_unlock_strict(); > + preempt_enable(); > } > > static inline int rcu_preempt_depth(void) Based on the discussion on the thread, how about keeping this and changing the preempt_count check in rcu_read_unlock_strict() instead? diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 1c7cbd145d5e..8fc67639d3a7 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -831,8 +831,15 @@ dump_blkd_tasks(struct rcu_node *rnp, int ncheck) void rcu_read_unlock_strict(void) { struct rcu_data *rdp; + int pc =3D ((preempt_count() & PREEMPT_MASK) >> PREEMPT_SHIFT); - if (irqs_disabled() || preempt_count() || !rcu_state.gp_kthread) + /* + * rcu_report_qs_rdp() can only be invoked with a stable rdp and + * and from the local CPU. + * With CONFIG_PREEMPTION=3Dy, do this while holding the last + * preempt_count which gets dropped after __rcu_read_unlock(). + */ + if (irqs_disabled() || pc > 1 || !rcu_state.gp_kthread) return; rdp =3D this_cpu_ptr(&rcu_data); rdp->cpu_no_qs.b.norm =3D false; Thanks -- ankur