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proc/fd: In tid_fd_mode use task_lookup_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.

Instead of manually coding finding the files struct for a task and
then calling files_lookup_fd_rcu, use the helper task_lookup_fd_rcu
that combines those to steps.   Making the code simpler and removing
the need to get a reference on a files_struct.

[1] https://lkml.kernel.org/r/20180915160423.GA31461@redhat.com
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
v1: https://lkml.kernel.org/r/20200817220425.9389-7-ebiederm@xmission.com
Link: https://lkml.kernel.org/r/20201120231441.29911-12-ebiederm@xmission.com
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
This commit is contained in:
Eric W. Biederman 2020-11-20 17:14:29 -06:00
parent 3a879fb380
commit 64eb661fda
1 changed files with 1 additions and 6 deletions
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7
fs/proc/fd.c
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@ -83,18 +83,13 @@ static const struct file_operations proc_fdinfo_file_operations = {
static bool tid_fd_mode(struct task_struct *task, unsigned fd, fmode_t *mode)
{
struct files_struct *files = get_files_struct(task);
struct file *file;
if (!files)
return false;
rcu_read_lock();
file = files_lookup_fd_rcu(files, fd);
file = task_lookup_fd_rcu(task, fd);
if (file)
*mode = file->f_mode;
rcu_read_unlock();
put_files_struct(files);
return !!file;
}