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prlimit: do not grab the tasklist_lock

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Unnecessarily grabbing the tasklist_lock can be a scalability bottleneck
for workloads that also must grab the tasklist_lock for waiting,
killing, and cloning.

The tasklist_lock was grabbed to protect tsk->sighand from disappearing
(becoming NULL).  tsk->signal was already protected by holding a
reference to tsk.

update_rlimit_cpu() assumed tsk->sighand != NULL.  With this commit, it
attempts to lock_task_sighand().  However, this means that
update_rlimit_cpu() can fail.  This only happens when a task is exiting.
Note that during exec, sighand may *change*, but it will not be NULL.

Prior to this commit, the do_prlimit() ensured that update_rlimit_cpu()
would not fail by read locking the tasklist_lock and checking tsk->sighand
!= NULL.

If update_rlimit_cpu() fails, there may be other tasks that are not
exiting that share tsk->signal.  However, the group_leader is the last
task to be released, so if we cannot update_rlimit_cpu(group_leader),
then the entire process is exiting.

The only other caller of update_rlimit_cpu() is
selinux_bprm_committing_creds().  It has tsk == current, so
update_rlimit_cpu() cannot fail (current->sighand cannot disappear
until current exits).

This change resulted in a 14% speedup on a microbenchmark where parents
kill and wait on their children, and children getpriority, setpriority,
and getrlimit.

Signed-off-by: Barret Rhoden <brho@google.com>
Link: https://lkml.kernel.org/r/20220106172041.522167-4-brho@google.com
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
This commit is contained in:
Barret Rhoden
2022-01-06 12:20:41 -05:00
committed by Eric W. Biederman
parent c57bef0287
commit 18c91bb2d8
3 changed files with 24 additions and 15 deletions
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25
kernel/sys.c
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@@ -1441,13 +1441,7 @@ static int do_prlimit(struct task_struct *tsk, unsigned int resource,
return -EPERM;
}
/* protect tsk->signal and tsk->sighand from disappearing */
read_lock(&tasklist_lock);
if (!tsk->sighand) {
retval = -ESRCH;
goto out;
}
/* Holding a refcount on tsk protects tsk->signal from disappearing. */
rlim = tsk->signal->rlim + resource;
task_lock(tsk->group_leader);
if (new_rlim) {
@@ -1476,10 +1470,19 @@ static int do_prlimit(struct task_struct *tsk, unsigned int resource,
*/
if (!retval && new_rlim && resource == RLIMIT_CPU &&
new_rlim->rlim_cur != RLIM_INFINITY &&
IS_ENABLED(CONFIG_POSIX_TIMERS))
update_rlimit_cpu(tsk, new_rlim->rlim_cur);
out:
read_unlock(&tasklist_lock);
IS_ENABLED(CONFIG_POSIX_TIMERS)) {
/*
* update_rlimit_cpu can fail if the task is exiting, but there
* may be other tasks in the thread group that are not exiting,
* and they need their cpu timers adjusted.
*
* The group_leader is the last task to be released, so if we
* cannot update_rlimit_cpu on it, then the entire process is
* exiting and we do not need to update at all.
*/
update_rlimit_cpu(tsk->group_leader, new_rlim->rlim_cur);
}
return retval;
}