On Friday 12 December 2008 09:40, Eric Dumazet wrote:
> From: Christoph Lameter <cl@xxxxxxxxxxxxxxxxxxxx>
>
> [PATCH] fs: struct file move from call_rcu() to SLAB_DESTROY_BY_RCU
>
> Currently we schedule RCU frees for each file we free separately. That has
> several drawbacks against the earlier file handling (in 2.6.5 f.e.), which
> did not require RCU callbacks:
>
> 1. Excessive number of RCU callbacks can be generated causing long RCU
> queues that in turn cause long latencies. We hit SLUB page allocation
> more often than necessary.
>
> 2. The cache hot object is not preserved between free and realloc. A close
> followed by another open is very fast with the RCUless approach because
> the last freed object is returned by the slab allocator that is
> still cache hot. RCU free means that the object is not immediately
> available again. The new object is cache cold and therefore open/close
> performance tests show a significant degradation with the RCU
> implementation.
>
> One solution to this problem is to move the RCU freeing into the Slab
> allocator by specifying SLAB_DESTROY_BY_RCU as an option at slab creation
> time. The slab allocator will do RCU frees only when it is necessary
> to dispose of slabs of objects (rare). So with that approach we can cut
> out the RCU overhead significantly.
>
> However, the slab allocator may return the object for another use even
> before the RCU period has expired under SLAB_DESTROY_BY_RCU. This means
> there is the (unlikely) possibility that the object is going to be
> switched under us in sections protected by rcu_read_lock() and
> rcu_read_unlock(). So we need to verify that we have acquired the correct
> object after establishing a stable object reference (incrementing the
> refcounter does that).
>
>
> Signed-off-by: Christoph Lameter <cl@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Eric Dumazet <dada1@xxxxxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> ---
> Documentation/filesystems/files.txt | 21 ++++++++++++++--
> fs/file_table.c | 33 ++++++++++++++++++--------
> include/linux/fs.h | 5 ---
> 3 files changed, 42 insertions(+), 17 deletions(-)
>
> diff --git a/Documentation/filesystems/files.txt
> b/Documentation/filesystems/files.txt index ac2facc..6916baa 100644
> --- a/Documentation/filesystems/files.txt
> +++ b/Documentation/filesystems/files.txt
> @@ -78,13 +78,28 @@ the fdtable structure -
> that look-up may race with the last put() operation on the
> file structure. This is avoided using atomic_long_inc_not_zero()
> on ->f_count :
> + As file structures are allocated with SLAB_DESTROY_BY_RCU,
> + they can also be freed before a RCU grace period, and reused,
> + but still as a struct file.
> + It is necessary to check again after getting
> + a stable reference (ie after atomic_long_inc_not_zero()),
> + that fcheck_files(files, fd) points to the same file.
>
> rcu_read_lock();
> file = fcheck_files(files, fd);
> if (file) {
> - if (atomic_long_inc_not_zero(&file->f_count))
> + if (atomic_long_inc_not_zero(&file->f_count)) {
> *fput_needed = 1;
> - else
> + /*
> + * Now we have a stable reference to an object.
> + * Check if other threads freed file and reallocated it.
> + */
> + if (file != fcheck_files(files, fd)) {
> + *fput_needed = 0;
> + put_filp(file);
> + file = NULL;
> + }
> + } else
> /* Didn't get the reference, someone's freed */
> file = NULL;
> }
> @@ -95,6 +110,8 @@ the fdtable structure -
> atomic_long_inc_not_zero() detects if refcounts is already zero or
> goes to zero during increment. If it does, we fail
> fget()/fget_light().
> + The second call to fcheck_files(files, fd) checks that this filp
> + was not freed, then reused by an other thread.
>
> 6. Since both fdtable and file structures can be looked up
> lock-free, they must be installed using rcu_assign_pointer()
> diff --git a/fs/file_table.c b/fs/file_table.c
> index a46e880..3e9259d 100644
> --- a/fs/file_table.c
> +++ b/fs/file_table.c
> @@ -37,17 +37,11 @@ static struct kmem_cache *filp_cachep __read_mostly;
>
> static struct percpu_counter nr_files __cacheline_aligned_in_smp;
>
> -static inline void file_free_rcu(struct rcu_head *head)
> -{
> - struct file *f = container_of(head, struct file, f_u.fu_rcuhead);
> - kmem_cache_free(filp_cachep, f);
> -}
> -
> static inline void file_free(struct file *f)
> {
> percpu_counter_dec(&nr_files);
> file_check_state(f);
> - call_rcu(&f->f_u.fu_rcuhead, file_free_rcu);
> + kmem_cache_free(filp_cachep, f);
> }
>
> /*
> @@ -306,6 +300,14 @@ struct file *fget(unsigned int fd)
> rcu_read_unlock();
> return NULL;
> }
> + /*
> + * Now we have a stable reference to an object.
> + * Check if other threads freed file and re-allocated it.
> + */
> + if (unlikely(file != fcheck_files(files, fd))) {
> + put_filp(file);
> + file = NULL;
> + }
This is a non-trivial change, because that put_filp may drop the last
reference to the file. So now we have the case where we free the file
from a context in which it had never been allocated.
>From a quick glance though the callchains, I can't seen an obvious
problem. But it needs to have documentation in put_filp, or at least
a mention in the changelog, and also cc'ed to the security lists.
Also, it adds code and cost to the get/put path in return for
improvement in the free path. get/put is the more common path, but
it is a small loss for a big improvement. So it might be worth it. But
it is not justified by your microbenchmark. Do we have a more useful
case that it helps?
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