Messages in this thread |  | | Subject | Re: [RFC PATCH] fs: use a sequence counter instead of file_lock in fd_install | From | Eric Dumazet <> | Date | Tue, 21 Apr 2015 13:57:40 -0700 |
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On Tue, 2015-04-21 at 22:06 +0200, Mateusz Guzik wrote: > On Tue, Apr 21, 2015 at 11:05:43AM -0700, Eric Dumazet wrote:
> > 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).
Not really. The prior code was fine, but the addition of synchronize_sched() made the overhead much bigger in case multiple threads do this at the same time.
> > As I read up about synchronize_sched and rcu_read_lock_sched, the code > should be correct.
> > 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.
My intent is to remove completely this code, but I left this WARN_ON_ONCE() for my tests, just to make sure my theory was right ;)
> > > } > > + /* 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/ ?
You are right, thanks !
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