Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932871AbbDUUGq (ORCPT ); Tue, 21 Apr 2015 16:06:46 -0400 Received: from mx1.redhat.com ([209.132.183.28]:35977 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932109AbbDUUGo (ORCPT ); Tue, 21 Apr 2015 16:06:44 -0400 Date: Tue, 21 Apr 2015 22:06:25 +0200 From: Mateusz Guzik To: Eric Dumazet Cc: Al Viro , Andrew Morton , "Paul E. McKenney" , Yann Droneaud , Konstantin Khlebnikov , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH] fs: use a sequence counter instead of file_lock in fd_install Message-ID: <20150421200624.GA16097@mguzik> References: <20150416121628.GA20615@mguzik> <1429307216.7346.255.camel@edumazet-glaptop2.roam.corp.google.com> <20150417221646.GA15589@mguzik> <20150417230252.GE889@ZenIV.linux.org.uk> <20150420130633.GA2513@mguzik> <20150420134326.GC2513@mguzik> <20150420151054.GD2513@mguzik> <1429550126.7346.268.camel@edumazet-glaptop2.roam.corp.google.com> <1429562991.7346.290.camel@edumazet-glaptop2.roam.corp.google.com> <1429639543.7346.329.camel@edumazet-glaptop2.roam.corp.google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <1429639543.7346.329.camel@edumazet-glaptop2.roam.corp.google.com> User-Agent: Mutt/1.5.23.1-rc1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5665 Lines: 184 On Tue, Apr 21, 2015 at 11:05:43AM -0700, Eric Dumazet wrote: > On Mon, 2015-04-20 at 13:49 -0700, Eric Dumazet wrote: > > On Mon, 2015-04-20 at 10:15 -0700, Eric Dumazet wrote: > > > On Mon, 2015-04-20 at 17:10 +0200, Mateusz Guzik wrote: > > > > > > > Sorry for spam but I came up with another hack. :) > > > > > > > > The idea is that we can have a variable which would signify the that > > > > given thread is playing with fd table in fd_install (kind of a lock > > > > embedded into task_struct). We would also have a flag in files struct > > > > indicating that a thread would like to resize it. > > > > > > > > expand_fdtable would set the flag and iterate over all threads waiting > > > > for all of them to have the var set to 0. > > > > > > The opposite : you have to block them in some way and add a rcu_sched() > > > or something. > > What I described would block them, although it was a crappy approach (iterating threads vs cpus). I was wondering if RCU could be abused for this feature and apparently it can. > > Here is the patch I cooked here but not yet tested. > > In following version : > > 1) I replaced the yield() hack by a proper wait queue. > > 2) I do not invoke synchronize_sched() for mono threaded programs. > > 3) I avoid multiple threads doing a resize and then only one wins the > deal. > One could argue this last bit could be committed separately (a different logical change). As I read up about synchronize_sched and rcu_read_lock_sched, the code should be correct. Also see nits below. > (copying/clearing big amount of memory only to discover another guy did > the same is a big waste of resources) > > > This seems to run properly on my hosts. > > Your comments/tests are most welcomed, thanks ! > > fs/file.c | 41 +++++++++++++++++++++++++++++++++----- > include/linux/fdtable.h | 3 ++ > 2 files changed, 39 insertions(+), 5 deletions(-) > > diff --git a/fs/file.c b/fs/file.c > index 93c5f89c248b..e0e113a56444 100644 > --- a/fs/file.c > +++ b/fs/file.c > @@ -147,6 +147,9 @@ static int expand_fdtable(struct files_struct *files, int nr) > > spin_unlock(&files->file_lock); > new_fdt = alloc_fdtable(nr); > + /* make sure no __fd_install() are still updating fdt */ > + if (atomic_read(&files->count) > 1) > + synchronize_sched(); > spin_lock(&files->file_lock); > if (!new_fdt) > return -ENOMEM; > @@ -170,9 +173,12 @@ static int expand_fdtable(struct files_struct *files, int nr) > if (cur_fdt != &files->fdtab) > call_rcu(&cur_fdt->rcu, free_fdtable_rcu); > } else { > + WARN_ON_ONCE(1); > /* Somebody else expanded, so undo our attempt */ > __free_fdtable(new_fdt); The reader may be left confused why there is a warning while the comment does not indicate anything is wrong. > } > + /* coupled with smp_rmb() in __fd_install() */ > + smp_wmb(); > return 1; > } > > @@ -187,19 +193,33 @@ static int expand_fdtable(struct files_struct *files, int nr) > static int expand_files(struct files_struct *files, int nr) > { > struct fdtable *fdt; > + int expanded = 0; > > +begin: > fdt = files_fdtable(files); > > /* Do we need to expand? */ > if (nr < fdt->max_fds) > - return 0; > + return expanded; > > /* Can we expand? */ > if (nr >= sysctl_nr_open) > return -EMFILE; > > + while (unlikely(files->resize_in_progress)) { > + spin_unlock(&files->file_lock); > + expanded = 1; > + wait_event(files->resize_wait, !files->resize_in_progress); > + spin_lock(&files->file_lock); > + goto begin; > + } This does not loop anymore, so s/while/if/ ? > + > /* All good, so we try */ > - return expand_fdtable(files, nr); > + files->resize_in_progress = true; > + expanded = expand_fdtable(files, nr); > + files->resize_in_progress = false; > + wake_up_all(&files->resize_wait); > + return expanded; > } > > static inline void __set_close_on_exec(int fd, struct fdtable *fdt) > @@ -256,6 +276,8 @@ struct files_struct *dup_fd(struct files_struct *oldf, int *errorp) > atomic_set(&newf->count, 1); > > spin_lock_init(&newf->file_lock); > + newf->resize_in_progress = 0; > + init_waitqueue_head(&newf->resize_wait); > newf->next_fd = 0; > new_fdt = &newf->fdtab; > new_fdt->max_fds = NR_OPEN_DEFAULT; > @@ -553,11 +575,20 @@ void __fd_install(struct files_struct *files, unsigned int fd, > struct file *file) > { > struct fdtable *fdt; > - spin_lock(&files->file_lock); > - fdt = files_fdtable(files); > + > + rcu_read_lock_sched(); > + > + while (unlikely(files->resize_in_progress)) { > + rcu_read_unlock_sched(); > + wait_event(files->resize_wait, !files->resize_in_progress); > + rcu_read_lock_sched(); > + } > + /* coupled with smp_wmb() in expand_fdtable() */ > + smp_rmb(); > + fdt = READ_ONCE(files->fdt); > BUG_ON(fdt->fd[fd] != NULL); > rcu_assign_pointer(fdt->fd[fd], file); > - spin_unlock(&files->file_lock); > + rcu_read_unlock_sched(); > } > > void fd_install(unsigned int fd, struct file *file) > diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h > index 230f87bdf5ad..fbb88740634a 100644 > --- a/include/linux/fdtable.h > +++ b/include/linux/fdtable.h > @@ -47,6 +47,9 @@ struct files_struct { > * read mostly part > */ > atomic_t count; > + bool resize_in_progress; > + wait_queue_head_t resize_wait; > + > struct fdtable __rcu *fdt; > struct fdtable fdtab; > /* > > > > -- Mateusz Guzik -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/