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ucount: use RCU for ucounts lookups

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The ucounts element is looked up under ucounts_lock.  This can be
optimized by using RCU for a lockless lookup and return and element if the
reference can be obtained.

Replace hlist_head with hlist_nulls_head which is RCU compatible.  Let
find_ucounts() search for the required item within a RCU section and
return the item if a reference could be obtained.  This means
alloc_ucounts() will always return an element (unless the memory
allocation failed).  Let put_ucounts() RCU free the element if the
reference counter dropped to zero.

Link: https://lkml.kernel.org/r/20250203150525.456525-4-bigeasy@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Lai jiangshan <jiangshanlai@gmail.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Mengen Sun <mengensun@tencent.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Cc: YueHong Wu <yuehongwu@tencent.com>
Cc: Zqiang <qiang.zhang1211@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Sebastian Andrzej Siewior
2025-02-03 16:05:24 +01:00
committed by Andrew Morton
parent 328152e677
commit 5f01a22c5b
2 changed files with 43 additions and 36 deletions
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4
include/linux/user_namespace.h
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@@ -5,6 +5,7 @@
#include <linux/kref.h>
#include <linux/nsproxy.h>
#include <linux/ns_common.h>
#include <linux/rculist_nulls.h>
#include <linux/sched.h>
#include <linux/workqueue.h>
#include <linux/rwsem.h>
@@ -115,9 +116,10 @@ struct user_namespace {
} __randomize_layout;
struct ucounts {
struct hlist_node node;
struct hlist_nulls_node node;
struct user_namespace *ns;
kuid_t uid;
struct rcu_head rcu;
atomic_t count;
atomic_long_t ucount[UCOUNT_COUNTS];
atomic_long_t rlimit[UCOUNT_RLIMIT_COUNTS];