Messages in this thread | | | Subject | Re: [RFC PATCH] fs: use a sequence counter instead of file_lock in fd_install | From | Eric Dumazet <> | Date | Fri, 17 Apr 2015 14:46:56 -0700 |
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On Thu, 2015-04-16 at 14:16 +0200, Mateusz Guzik wrote: > Hi, > > Currently obtaining a new file descriptor results in locking fdtable > twice - once in order to reserve a slot and second time to fill it
...
> void __fd_install(struct files_struct *files, unsigned int fd, > struct file *file) > { > + unsigned long seq;
unsigned int seq;
> struct fdtable *fdt; > - spin_lock(&files->file_lock); > - fdt = files_fdtable(files); > - BUG_ON(fdt->fd[fd] != NULL); > - rcu_assign_pointer(fdt->fd[fd], file); > - spin_unlock(&files->file_lock); > + > + rcu_read_lock(); > + do { > + seq = read_seqcount_begin(&files->fdt_seqcount); > + fdt = files_fdtable_seq(files); > + /* > + * Entry in the table can already be equal to file if we > + * had to restart and copy_fdtable picked up our update. > + */ > + BUG_ON(!(fdt->fd[fd] == NULL || fdt->fd[fd] == file)); > + rcu_assign_pointer(fdt->fd[fd], file); > + smp_mb(); > + } while (__read_seqcount_retry(&files->fdt_seqcount, seq)); > + rcu_read_unlock(); > } >
So one problem here is :
As soon as rcu_assign_pointer(fdt->fd[fd], file) is done, and other cpu does one expand_fdtable() and releases files->file_lock, another cpu can close(fd).
Then another cpu can reuse the [fd] now empty slot and install a new file in it.
Then this cpu will crash here :
BUG_ON(!(fdt->fd[fd] == NULL || fdt->fd[fd] == file));
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