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bpf/task_iter: In task_file_seq_get_next use task_lookup_next_fd_rcu

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When discussing[1] exec and posix file locks it was realized that none
of the callers of get_files_struct fundamentally needed to call
get_files_struct, and that by switching them to helper functions
instead it will both simplify their code and remove unnecessary
increments of files_struct.count.  Those unnecessary increments can
result in exec unnecessarily unsharing files_struct which breaking
posix locks, and it can result in fget_light having to fallback to
fget reducing system performance.

Using task_lookup_next_fd_rcu simplifies task_file_seq_get_next, by
moving the checking for the maximum file descritor into the generic
code, and by remvoing the need for capturing and releasing a reference
on files_struct.  As the reference count of files_struct no longer
needs to be maintained bpf_iter_seq_task_file_info can have it's files
member removed and task_file_seq_get_next no longer needs it's fstruct
argument.

The curr_fd local variable does need to become unsigned to be used
with fnext_task.  As curr_fd is assigned from and assigned a u32
making curr_fd an unsigned int won't cause problems and might prevent
them.

[1] https://lkml.kernel.org/r/20180915160423.GA31461@redhat.com
Suggested-by: Oleg Nesterov <oleg@redhat.com>
v1: https://lkml.kernel.org/r/20200817220425.9389-11-ebiederm@xmission.com
Link: https://lkml.kernel.org/r/20201120231441.29911-16-ebiederm@xmission.com
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
This commit is contained in:
Eric W. Biederman 2020-11-20 17:14:33 -06:00
parent 5b17b61870
commit 66ed594409
1 changed files with 10 additions and 34 deletions
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44
kernel/bpf/task_iter.c
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@ -130,45 +130,33 @@ struct bpf_iter_seq_task_file_info {
*/
struct bpf_iter_seq_task_common common;
struct task_struct *task;
struct files_struct *files;
u32 tid;
u32 fd;
};
static struct file *
task_file_seq_get_next(struct bpf_iter_seq_task_file_info *info,
struct task_struct **task, struct files_struct **fstruct)
struct task_struct **task)
{
struct pid_namespace *ns = info->common.ns;
u32 curr_tid = info->tid, max_fds;
struct files_struct *curr_files;
u32 curr_tid = info->tid;
struct task_struct *curr_task;
int curr_fd = info->fd;
unsigned int curr_fd = info->fd;
/* If this function returns a non-NULL file object,
* it held a reference to the task/files_struct/file.
* it held a reference to the task/file.
* Otherwise, it does not hold any reference.
*/
again:
if (*task) {
curr_task = *task;
curr_files = *fstruct;
curr_fd = info->fd;
} else {
curr_task = task_seq_get_next(ns, &curr_tid, true);
if (!curr_task)
return NULL;
curr_files = get_files_struct(curr_task);
if (!curr_files) {
put_task_struct(curr_task);
curr_tid = ++(info->tid);
info->fd = 0;
goto again;
}
/* set *fstruct, *task and info->tid */
*fstruct = curr_files;
/* set *task and info->tid */
*task = curr_task;
if (curr_tid == info->tid) {
curr_fd = info->fd;
@ -179,13 +167,11 @@ again:
}
rcu_read_lock();
max_fds = files_fdtable(curr_files)->max_fds;
for (; curr_fd < max_fds; curr_fd++) {
for (;; curr_fd++) {
struct file *f;
f = files_lookup_fd_rcu(curr_files, curr_fd);
f = task_lookup_next_fd_rcu(curr_task, &curr_fd);
if (!f)
continue;
break;
if (!get_file_rcu(f))
continue;
@ -197,10 +183,8 @@ again:
/* the current task is done, go to the next task */
rcu_read_unlock();
put_files_struct(curr_files);
put_task_struct(curr_task);
*task = NULL;
*fstruct = NULL;
info->fd = 0;
curr_tid = ++(info->tid);
goto again;
@ -209,13 +193,11 @@ again:
static void *task_file_seq_start(struct seq_file *seq, loff_t *pos)
{
struct bpf_iter_seq_task_file_info *info = seq->private;
struct files_struct *files = NULL;
struct task_struct *task = NULL;
struct file *file;
file = task_file_seq_get_next(info, &task, &files);
file = task_file_seq_get_next(info, &task);
if (!file) {
info->files = NULL;
info->task = NULL;
return NULL;
}
@ -223,7 +205,6 @@ static void *task_file_seq_start(struct seq_file *seq, loff_t *pos)
if (*pos == 0)
++*pos;
info->task = task;
info->files = files;
return file;
}
@ -231,22 +212,19 @@ static void *task_file_seq_start(struct seq_file *seq, loff_t *pos)
static void *task_file_seq_next(struct seq_file *seq, void *v, loff_t *pos)
{
struct bpf_iter_seq_task_file_info *info = seq->private;
struct files_struct *files = info->files;
struct task_struct *task = info->task;
struct file *file;
++*pos;
++info->fd;
fput((struct file *)v);
file = task_file_seq_get_next(info, &task, &files);
file = task_file_seq_get_next(info, &task);
if (!file) {
info->files = NULL;
info->task = NULL;
return NULL;
}
info->task = task;
info->files = files;
return file;
}
@ -295,9 +273,7 @@ static void task_file_seq_stop(struct seq_file *seq, void *v)
(void)__task_file_seq_show(seq, v, true);
} else {
fput((struct file *)v);
put_files_struct(info->files);
put_task_struct(info->task);
info->files = NULL;
info->task = NULL;
}
}