linux-kernel - [PATCH] rlimits: do not grab tasklist_lock for do

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Message-Id: <20211213220401.1039578-1-brho@google.com>
Date:   Mon, 13 Dec 2021 17:04:01 -0500
From:   Barret Rhoden <brho@...gle.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexey Gladkov <legion@...nel.org>,
        William Cohen <wcohen@...hat.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Chris Hyser <chris.hyser@...cle.com>,
        Peter Collingbourne <pcc@...gle.com>,
        Xiaofeng Cao <caoxiaofeng@...ong.com>,
        David Hildenbrand <david@...hat.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] rlimits: do not grab tasklist_lock for do_prlimit on current

The tasklist_lock can be a scalability bottleneck.  For current tasks,
we don't need the tasklist_lock to protect tsk->sighand or tsk->signal.
If non-current callers become a bottleneck, we could use
lock_task_sighand().

Signed-off-by: Barret Rhoden <brho@...gle.com>
---
 kernel/sys.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/sys.c b/kernel/sys.c
index 8fdac0d90504..e56d1ae910af 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1576,7 +1576,8 @@ int do_prlimit(struct task_struct *tsk, unsigned int resource,
 	}
 
 	/* protect tsk->signal and tsk->sighand from disappearing */
-	read_lock(&tasklist_lock);
+	if (tsk != current)
+		read_lock(&tasklist_lock);
 	if (!tsk->sighand) {
 		retval = -ESRCH;
 		goto out;
@@ -1611,7 +1612,8 @@ int do_prlimit(struct task_struct *tsk, unsigned int resource,
 	     IS_ENABLED(CONFIG_POSIX_TIMERS))
 		update_rlimit_cpu(tsk, new_rlim->rlim_cur);
 out:
-	read_unlock(&tasklist_lock);
+	if (tsk != current)
+		read_unlock(&tasklist_lock);
 	return retval;
 }
 
-- 
2.34.1.173.g76aa8bc2d0-goog