[PATCH v2] io_uring: refactor file register/unregister/update codes

Xiaoguang Wang
2020-03-24T09:37:21Z

While diving into iouring fileset resigster/unregister/update codes,
we found one bug in fileset update codes. Iouring fileset update codes
use a percpu_ref variable to check whether can put previous registered
file, only when the refcnt of the perfcpu_ref variable reachs zero, can
we safely put these files, but this do not work well. If applications
always issue requests continually, this perfcpu_ref will never have an
chance to reach zero, and it'll always be in atomic mode, also will
defeat the gains introduced by fileset register/unresiger/update feature,
which are used to reduce the atomic operation overhead of fput/fget.

To fix this issue, while applications do IORING_REGISTER_FILES or
IORING_REGISTER_FILES_UPDATE operations, we allocate a new percpu_ref
and kill the old percpu_ref, new requests will use the new percpu_ref.
Once all previous old requests complete, old percpu_refs will be dropped
and registered files will be put safely.

Link: https://lore.kernel.org/io-uring/5a8dac33-4ca2-4847-b091-f7dcd3ad0ff3@linux.alibaba.com/T/#t
Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
---
fs/io_uring.c | 213 ++++++++++++++++++++++++++++++--------------------
1 file changed, 127 insertions(+), 86 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 3affd96a98ba..34525a237f99 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -183,14 +183,23 @@ struct fixed_file_table {
struct file **files;
};

+struct fixed_file_ref_node {
+ struct percpu_ref refs;
+ struct list_head node;
+ struct list_head file_list;
+ struct fixed_file_data *file_data;
+ struct work_struct work;
+};
+
struct fixed_file_data {
struct fixed_file_table *table;
struct io_ring_ctx *ctx;

+ struct percpu_ref *cur_refs;
struct percpu_ref refs;
- struct llist_head put_llist;
- struct work_struct ref_work;
struct completion done;
+ struct list_head ref_list;
+ spinlock_t lock;
};

struct io_ring_ctx {
@@ -572,6 +581,7 @@ struct io_kiocb {
struct list_head inflight_entry;

struct io_wq_work work;
+ struct percpu_ref *node_ref;
};

#define IO_PLUG_THRESHOLD 2
@@ -759,7 +769,6 @@ static int __io_sqe_files_update(struct io_ring_ctx *ctx,
struct io_uring_files_update *ip,
unsigned nr_args);
static int io_grab_files(struct io_kiocb *req);
-static void io_ring_file_ref_flush(struct fixed_file_data *data);
static void io_cleanup_req(struct io_kiocb *req);

static struct kmem_cache *req_cachep;
@@ -1256,15 +1265,13 @@ static void __io_req_do_free(struct io_kiocb *req)

static void __io_req_aux_free(struct io_kiocb *req)
{
- struct io_ring_ctx *ctx = req->ctx;
-
if (req->flags & REQ_F_NEED_CLEANUP)
io_cleanup_req(req);

kfree(req->io);
if (req->file) {
if (req->flags & REQ_F_FIXED_FILE)
- percpu_ref_put(&ctx->file_data->refs);
+ percpu_ref_put(req->node_ref);
else
fput(req->file);
}
@@ -1299,21 +1306,18 @@ struct req_batch {

static void io_free_req_many(struct io_ring_ctx *ctx, struct req_batch *rb)
{
- int fixed_refs = rb->to_free;
-
if (!rb->to_free)
return;
if (rb->need_iter) {
int i, inflight = 0;
unsigned long flags;

- fixed_refs = 0;
for (i = 0; i < rb->to_free; i++) {
struct io_kiocb *req = rb->reqs[i];

if (req->flags & REQ_F_FIXED_FILE) {
req->file = NULL;
- fixed_refs++;
+ percpu_ref_put(req->node_ref);
}
if (req->flags & REQ_F_INFLIGHT)
inflight++;
@@ -1339,8 +1343,6 @@ static void io_free_req_many(struct io_ring_ctx *ctx, struct req_batch *rb)
}
do_free:
kmem_cache_free_bulk(req_cachep, rb->to_free, rb->reqs);
- if (fixed_refs)
- percpu_ref_put_many(&ctx->file_data->refs, fixed_refs);
percpu_ref_put_many(&ctx->refs, rb->to_free);
rb->to_free = rb->need_iter = 0;
}
@@ -4600,7 +4602,8 @@ static int io_req_set_file(struct io_submit_state *state, struct io_kiocb *req,
if (!req->file)
return -EBADF;
req->flags |= REQ_F_FIXED_FILE;
- percpu_ref_get(&ctx->file_data->refs);
+ req->node_ref = ctx->file_data->cur_refs;
+ percpu_ref_get(req->node_ref);
} else {
if (req->needs_fixed_file)
return -EBADF;
@@ -5344,43 +5347,35 @@ static void io_file_ref_kill(struct percpu_ref *ref)
complete(&data->done);
}

-static void io_file_ref_exit_and_free(struct work_struct *work)
-{
- struct fixed_file_data *data;
-
- data = container_of(work, struct fixed_file_data, ref_work);
-
- /*
- * Ensure any percpu-ref atomic switch callback has run, it could have
- * been in progress when the files were being unregistered. Once
- * that's done, we can safely exit and free the ref and containing
- * data structure.
- */
- rcu_barrier();
- percpu_ref_exit(&data->refs);
- kfree(data);
-}
-
static int io_sqe_files_unregister(struct io_ring_ctx *ctx)
{
struct fixed_file_data *data = ctx->file_data;
+ struct fixed_file_ref_node *ref_node = NULL;
unsigned nr_tables, i;
+ unsigned long flags;

if (!data)
return -ENXIO;

- percpu_ref_kill_and_confirm(&data->refs, io_file_ref_kill);
- flush_work(&data->ref_work);
+ spin_lock_irqsave(&data->lock, flags);
+ if (!list_empty(&data->ref_list))
+ ref_node = list_first_entry(&data->ref_list,
+ struct fixed_file_ref_node, node);
+ spin_unlock_irqrestore(&data->lock, flags);
+ if (ref_node)
+ percpu_ref_kill(&ref_node->refs);
+
+ percpu_ref_kill(&data->refs);
+
+ /* wait for all refs nodes to complete */
wait_for_completion(&data->done);
- io_ring_file_ref_flush(data);

__io_sqe_files_unregister(ctx);
nr_tables = DIV_ROUND_UP(ctx->nr_user_files, IORING_MAX_FILES_TABLE);
for (i = 0; i < nr_tables; i++)
kfree(data->table[i].files);
kfree(data->table);
- INIT_WORK(&data->ref_work, io_file_ref_exit_and_free);
- queue_work(system_wq, &data->ref_work);
+ kfree(data);
ctx->file_data = NULL;
ctx->nr_user_files = 0;
return 0;
@@ -5605,50 +5600,74 @@ static void io_ring_file_put(struct io_ring_ctx *ctx, struct file *file)
}

struct io_file_put {
- struct llist_node llist;
+ struct list_head list;
struct file *file;
struct completion *done;
};

-static void io_ring_file_ref_flush(struct fixed_file_data *data)
+static void io_file_put_work(struct work_struct *work)
{
+ struct fixed_file_ref_node *ref_node;
+ struct fixed_file_data *file_data;
+ struct io_ring_ctx *ctx;
struct io_file_put *pfile, *tmp;
- struct llist_node *node;
+ unsigned long flags;

- while ((node = llist_del_all(&data->put_llist)) != NULL) {
- llist_for_each_entry_safe(pfile, tmp, node, llist) {
- io_ring_file_put(data->ctx, pfile->file);
- if (pfile->done)
- complete(pfile->done);
- else
- kfree(pfile);
- }
+ ref_node = container_of(work, struct fixed_file_ref_node, work);
+ file_data = ref_node->file_data;
+ ctx = file_data->ctx;
+
+ list_for_each_entry_safe(pfile, tmp, &ref_node->file_list, list) {
+ list_del_init(&pfile->list);
+ io_ring_file_put(ctx, pfile->file);
+ if (pfile->done)
+ complete(pfile->done);
+ else
+ kfree(pfile);
}
+
+ spin_lock_irqsave(&file_data->lock, flags);
+ list_del_init(&ref_node->node);
+ spin_unlock_irqrestore(&file_data->lock, flags);
+
+ percpu_ref_exit(&ref_node->refs);
+ kfree(ref_node);
+ percpu_ref_put(&file_data->refs);
}

-static void io_ring_file_ref_switch(struct work_struct *work)
+static void io_file_data_ref_zero(struct percpu_ref *ref)
{
- struct fixed_file_data *data;
+ struct fixed_file_ref_node *ref_node;

- data = container_of(work, struct fixed_file_data, ref_work);
- io_ring_file_ref_flush(data);
- percpu_ref_switch_to_percpu(&data->refs);
+ ref_node = container_of(ref, struct fixed_file_ref_node, refs);
+
+ queue_work(system_wq, &ref_node->work);
}

-static void io_file_data_ref_zero(struct percpu_ref *ref)
+static struct fixed_file_ref_node *alloc_fixed_file_ref_node(void)
{
- struct fixed_file_data *data;
+ struct fixed_file_ref_node *ref_node;

- data = container_of(ref, struct fixed_file_data, refs);
+ ref_node = kzalloc(sizeof(*ref_node), GFP_KERNEL);
+ if (!ref_node)
+ return ERR_PTR(-ENOMEM);
+
+ if (percpu_ref_init(&ref_node->refs, io_file_data_ref_zero,
+ 0, GFP_KERNEL)) {
+ kfree(ref_node);
+ return ERR_PTR(-ENOMEM);
+ }
+ INIT_LIST_HEAD(&ref_node->node);
+ INIT_LIST_HEAD(&ref_node->file_list);
+ INIT_WORK(&ref_node->work, io_file_put_work);
+ return ref_node;

- /*
- * We can't safely switch from inside this context, punt to wq. If
- * the table ref is going away, the table is being unregistered.
- * Don't queue up the async work for that case, the caller will
- * handle it.
- */
- if (!percpu_ref_is_dying(&data->refs))
- queue_work(system_wq, &data->ref_work);
+}
+
+static void destroy_fixed_file_ref_node(struct fixed_file_ref_node *ref_node)
+{
+ percpu_ref_exit(&ref_node->refs);
+ kfree(ref_node);
}

static int io_sqe_files_register(struct io_ring_ctx *ctx, void __user *arg,
@@ -5659,6 +5678,8 @@ static int io_sqe_files_register(struct io_ring_ctx *ctx, void __user *arg,
struct file *file;
int fd, ret = 0;
unsigned i;
+ struct fixed_file_ref_node *ref_node;
+ unsigned long flags;

if (ctx->file_data)
return -EBUSY;
@@ -5672,6 +5693,7 @@ static int io_sqe_files_register(struct io_ring_ctx *ctx, void __user *arg,
return -ENOMEM;
ctx->file_data->ctx = ctx;
init_completion(&ctx->file_data->done);
+ INIT_LIST_HEAD(&ctx->file_data->ref_list);

nr_tables = DIV_ROUND_UP(nr_args, IORING_MAX_FILES_TABLE);
ctx->file_data->table = kcalloc(nr_tables,
@@ -5683,15 +5705,13 @@ static int io_sqe_files_register(struct io_ring_ctx *ctx, void __user *arg,
return -ENOMEM;
}

- if (percpu_ref_init(&ctx->file_data->refs, io_file_data_ref_zero,
+ if (percpu_ref_init(&ctx->file_data->refs, io_file_ref_kill,
PERCPU_REF_ALLOW_REINIT, GFP_KERNEL)) {
kfree(ctx->file_data->table);
kfree(ctx->file_data);
ctx->file_data = NULL;
return -ENOMEM;
}
- ctx->file_data->put_llist.first = NULL;
- INIT_WORK(&ctx->file_data->ref_work, io_ring_file_ref_switch);

if (io_sqe_alloc_file_tables(ctx, nr_tables, nr_args)) {
percpu_ref_exit(&ctx->file_data->refs);
@@ -5757,6 +5777,18 @@ static int io_sqe_files_register(struct io_ring_ctx *ctx, void __user *arg,
if (ret)
io_sqe_files_unregister(ctx);

+ ref_node = alloc_fixed_file_ref_node();
+ if (IS_ERR(ref_node)) {
+ io_sqe_files_unregister(ctx);
+ return PTR_ERR(ref_node);
+ }
+ ref_node->file_data = ctx->file_data;
+
+ ctx->file_data->cur_refs = &ref_node->refs;
+ spin_lock_irqsave(&ctx->file_data->lock, flags);
+ list_add(&ref_node->node, &ctx->file_data->ref_list);
+ spin_unlock_irqrestore(&ctx->file_data->lock, flags);
+ percpu_ref_get(&ctx->file_data->refs);
return ret;
}

@@ -5803,25 +5835,15 @@ static int io_sqe_file_register(struct io_ring_ctx *ctx, struct file *file,
#endif
}

-static void io_atomic_switch(struct percpu_ref *ref)
-{
- struct fixed_file_data *data;
-
- /*
- * Juggle reference to ensure we hit zero, if needed, so we can
- * switch back to percpu mode
- */
- data = container_of(ref, struct fixed_file_data, refs);
- percpu_ref_put(&data->refs);
- percpu_ref_get(&data->refs);
-}
-
static bool io_queue_file_removal(struct fixed_file_data *data,
struct file *file)
{
struct io_file_put *pfile, pfile_stack;
+ struct percpu_ref *refs = data->cur_refs;
+ struct fixed_file_ref_node *ref_node;
DECLARE_COMPLETION_ONSTACK(done);

+ ref_node = container_of(refs, struct fixed_file_ref_node, refs);
/*
* If we fail allocating the struct we need for doing async reomval
* of this file, just punt to sync and wait for it.
@@ -5833,12 +5855,11 @@ static bool io_queue_file_removal(struct fixed_file_data *data,
}

pfile->file = file;
- llist_add(&pfile->llist, &data->put_llist);
+ list_add(&pfile->list, &ref_node->file_list);

if (pfile == &pfile_stack) {
- percpu_ref_switch_to_atomic(&data->refs, io_atomic_switch);
+ percpu_ref_kill(&ref_node->refs);
wait_for_completion(&done);
- flush_work(&data->ref_work);
return false;
}

@@ -5850,17 +5871,25 @@ static int __io_sqe_files_update(struct io_ring_ctx *ctx,
unsigned nr_args)
{
struct fixed_file_data *data = ctx->file_data;
- bool ref_switch = false;
+ struct fixed_file_ref_node *ref_node;
struct file *file;
__s32 __user *fds;
int fd, i, err;
__u32 done;
+ unsigned long flags;
+ bool ref_killed = false;
+ bool needs_switch = false;

if (check_add_overflow(up->offset, nr_args, &done))
return -EOVERFLOW;
if (done > ctx->nr_user_files)
return -EINVAL;

+ ref_node = alloc_fixed_file_ref_node();
+ if (IS_ERR(ref_node))
+ return PTR_ERR(ref_node);
+ ref_node->file_data = ctx->file_data;
+
done = 0;
fds = u64_to_user_ptr(up->fds);
while (nr_args) {
@@ -5878,8 +5907,9 @@ static int __io_sqe_files_update(struct io_ring_ctx *ctx,
if (table->files[index]) {
file = io_file_from_index(ctx, index);
table->files[index] = NULL;
- if (io_queue_file_removal(data, file))
- ref_switch = true;
+ needs_switch = true;
+ if (!io_queue_file_removal(data, file))
+ ref_killed = true;
}
if (fd != -1) {
file = fget(fd);
@@ -5910,11 +5940,22 @@ static int __io_sqe_files_update(struct io_ring_ctx *ctx,
up->offset++;
}

- if (ref_switch)
- percpu_ref_switch_to_atomic(&data->refs, io_atomic_switch);
+ if (needs_switch) {
+ if (!ref_killed)
+ percpu_ref_kill(data->cur_refs);
+
+ spin_lock_irqsave(&data->lock, flags);
+ list_add(&ref_node->node, &data->ref_list);
+ data->cur_refs = &ref_node->refs;
+ spin_unlock_irqrestore(&data->lock, flags);
+ percpu_ref_get(&ctx->file_data->refs);
+ } else
+ destroy_fixed_file_ref_node(ref_node);
+

return done ? done : err;
}
+
static int io_sqe_files_update(struct io_ring_ctx *ctx, void __user *arg,
unsigned nr_args)
{
--
2.17.2