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proc/fd: In proc_readfd_common use task_lookup_next_fd_rcu 

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 proc_readfd_common, 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 task_lookup_fd_rcu may update the fd ctx->pos has been changed
to be the fd +2 after task_lookup_fd_rcu returns.

[1] https://lkml.kernel.org/r/20180915160423.GA31461@redhat.com
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Tested-by: Andy Lavr <andy.lavr@gmail.com>
v1: https://lkml.kernel.org/r/20200817220425.9389-10-ebiederm@xmission.com
Link: https://lkml.kernel.org/r/20201120231441.29911-15-ebiederm@xmission.com
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
This commit is contained in:
Eric W. Biederman 2020-11-20 17:14:32 -06:00
parent e9a53aeb5e
commit 5b17b61870
1 changed files with 5 additions and 12 deletions
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17
fs/proc/fd.c
View File

@ -217,7 +217,6 @@ static int proc_readfd_common(struct file *file, struct dir_context *ctx,
instantiate_t instantiate)
{
struct task_struct *p = get_proc_task(file_inode(file));
struct files_struct *files;
unsigned int fd;
if (!p)
@ -225,22 +224,18 @@ static int proc_readfd_common(struct file *file, struct dir_context *ctx,
if (!dir_emit_dots(file, ctx))
goto out;
files = get_files_struct(p);
if (!files)
goto out;
rcu_read_lock();
for (fd = ctx->pos - 2;
fd < files_fdtable(files)->max_fds;
fd++, ctx->pos++) {
for (fd = ctx->pos - 2;; fd++) {
struct file *f;
struct fd_data data;
char name[10 + 1];
unsigned int len;
f = files_lookup_fd_rcu(files, fd);
f = task_lookup_next_fd_rcu(p, &fd);
ctx->pos = fd + 2LL;
if (!f)
continue;
break;
data.mode = f->f_mode;
rcu_read_unlock();
data.fd = fd;
@ -249,13 +244,11 @@ static int proc_readfd_common(struct file *file, struct dir_context *ctx,
if (!proc_fill_cache(file, ctx,
name, len, instantiate, p,
&data))
goto out_fd_loop;
goto out;
cond_resched();
rcu_read_lock();
}
rcu_read_unlock();
out_fd_loop:
put_files_struct(files);
out:
put_task_struct(p);
return 0;