linux-kernel - Re: [RFC PATCH] fs: elide the smp_rmb fence in fd

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <a9b7f0a0-bd15-4990-b67b-48986c2eb31d@paulmck-laptop>
Date: Thu, 5 Dec 2024 10:41:17 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: Al Viro <viro@...iv.linux.org.uk>, brauner@...nel.org, jack@...e.cz,
	linux-fsdevel@...r.kernel.org, torvalds@...ux-foundation.org,
	edumazet@...gle.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] fs: elide the smp_rmb fence in fd_install()

On Thu, Dec 05, 2024 at 03:43:41PM +0100, Mateusz Guzik wrote:
> On Thu, Dec 5, 2024 at 3:18 PM Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > On Thu, Dec 05, 2024 at 01:03:32PM +0100, Mateusz Guzik wrote:
> > >  void fd_install(unsigned int fd, struct file *file)
> > >  {
> > > -     struct files_struct *files = current->files;
> > > +     struct files_struct *files;
> > >       struct fdtable *fdt;
> > >
> > >       if (WARN_ON_ONCE(unlikely(file->f_mode & FMODE_BACKING)))
> > >               return;
> > >
> > > +     /*
> > > +      * Synchronized with expand_fdtable(), see that routine for an
> > > +      * explanation.
> > > +      */
> > >       rcu_read_lock_sched();
> > > +     files = READ_ONCE(current->files);
> >
> > What are you trying to do with that READ_ONCE()?  current->files
> > itself is *not* changed by any of that code; current->files->fdtab is.
> 
> To my understanding this is the idiomatic way of spelling out the
> non-existent in Linux smp_consume_load, for the resize_in_progress
> flag.

In Linus, "smp_consume_load()" is named rcu_dereference().

> Anyway to elaborate I'm gunning for a setup where the code is
> semantically equivalent to having a lock around the work.

Except that rcu_read_lock_sched() provides mutual-exclusion guarantees
only with later RCU grace periods, such as those implemented by
synchronize_rcu().

> Pretend ->resize_lock exists, then:
> fd_install:
> files = current->files;
> read_lock(files->resize_lock);
> fdt = rcu_dereference_sched(files->fdt);
> rcu_assign_pointer(fdt->fd[fd], file);
> read_unlock(files->resize_lock);
> 
> expand_fdtable:
> write_lock(files->resize_lock);
> [snip]
> rcu_assign_pointer(files->fdt, new_fdt);
> write_unlock(files->resize_lock);
> 
> Except rcu_read_lock_sched + appropriately fenced resize_in_progress +
> synchronize_rcu do it.

OK, good, you did get the grace-period part of the puzzle.

Howver, please keep in mind that synchronize_rcu() has significant
latency by design.  There is a tradeoff between CPU consumption and
latency, and synchronize_rcu() therefore has latencies ranging upwards of
several milliseconds (not microseconds or nanoseconds).  I would be very
surprised if expand_fdtable() users would be happy with such a long delay.
Or are you using some trick to hide this delay?