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Subject[PATCH 16/32] aio: Use cancellation list lazily
Date
Cancelling kiocbs requires adding them to a per kioctx linked list,
which is one of the few things we need to take the kioctx lock for in
the fast path. But most kiocbs can't be cancelled - so if we just do
this lazily, we can avoid quite a bit of locking overhead.

While we're at it, instead of using a flag bit switch to using ki_cancel
itself to indicate that a kiocb has been cancelled/completed. This lets
us get rid of ki_flags entirely.

Signed-off-by: Kent Overstreet <koverstreet@google.com>
---
drivers/usb/gadget/inode.c | 3 +-
fs/aio.c | 95 +++++++++++++++++++++++++---------------------
include/linux/aio.h | 16 ++++----
3 files changed, 59 insertions(+), 55 deletions(-)

diff --git a/drivers/usb/gadget/inode.c b/drivers/usb/gadget/inode.c
index 7640e01..3bf0c35 100644
--- a/drivers/usb/gadget/inode.c
+++ b/drivers/usb/gadget/inode.c
@@ -534,7 +534,6 @@ static int ep_aio_cancel(struct kiocb *iocb, struct io_event *e)
local_irq_disable();
epdata = priv->epdata;
// spin_lock(&epdata->dev->lock);
- kiocbSetCancelled(iocb);
if (likely(epdata && epdata->ep && priv->req))
value = usb_ep_dequeue (epdata->ep, priv->req);
else
@@ -664,7 +663,7 @@ fail:
goto fail;
}

- iocb->ki_cancel = ep_aio_cancel;
+ kiocb_set_cancel_fn(iocb, ep_aio_cancel);
get_ep(epdata);
priv->epdata = epdata;
priv->actual = 0;
diff --git a/fs/aio.c b/fs/aio.c
index c1047c8..276c6ea 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -97,6 +97,8 @@ struct kioctx {

struct aio_ring_info ring_info;

+ spinlock_t completion_lock;
+
struct rcu_head rcu_head;
struct work_struct rcu_work;
};
@@ -217,25 +219,40 @@ static int aio_setup_ring(struct kioctx *ctx)
#define AIO_EVENTS_FIRST_PAGE ((PAGE_SIZE - sizeof(struct aio_ring)) / sizeof(struct io_event))
#define AIO_EVENTS_OFFSET (AIO_EVENTS_PER_PAGE - AIO_EVENTS_FIRST_PAGE)

+void kiocb_set_cancel_fn(struct kiocb *req, kiocb_cancel_fn *cancel)
+{
+ if (!req->ki_list.next) {
+ struct kioctx *ctx = req->ki_ctx;
+ unsigned long flags;
+
+ spin_lock_irqsave(&ctx->ctx_lock, flags);
+ list_add(&req->ki_list, &ctx->active_reqs);
+ spin_unlock_irqrestore(&ctx->ctx_lock, flags);
+ }
+
+ req->ki_cancel = cancel;
+}
+EXPORT_SYMBOL(kiocb_set_cancel_fn);
+
static int kiocb_cancel(struct kioctx *ctx, struct kiocb *kiocb,
struct io_event *res)
{
- int (*cancel)(struct kiocb *, struct io_event *);
+ kiocb_cancel_fn *cancel;
int ret = -EINVAL;

- cancel = kiocb->ki_cancel;
- kiocbSetCancelled(kiocb);
- if (cancel) {
- atomic_inc(&kiocb->ki_users);
- spin_unlock_irq(&ctx->ctx_lock);
+ cancel = xchg(&kiocb->ki_cancel, KIOCB_CANCELLED);
+ if (!cancel || cancel == KIOCB_CANCELLED)
+ return ret;
+
+ atomic_inc(&kiocb->ki_users);
+ spin_unlock_irq(&ctx->ctx_lock);

- memset(res, 0, sizeof(*res));
- res->obj = (u64) kiocb->ki_obj.user;
- res->data = kiocb->ki_user_data;
- ret = cancel(kiocb, res);
+ memset(res, 0, sizeof(*res));
+ res->obj = (u64) kiocb->ki_obj.user;
+ res->data = kiocb->ki_user_data;
+ ret = cancel(kiocb, res);

- spin_lock_irq(&ctx->ctx_lock);
- }
+ spin_lock_irq(&ctx->ctx_lock);

return ret;
}
@@ -323,6 +340,7 @@ static struct kioctx *ioctx_alloc(unsigned nr_events)
atomic_set(&ctx->users, 2);
atomic_set(&ctx->dead, 0);
spin_lock_init(&ctx->ctx_lock);
+ spin_lock_init(&ctx->completion_lock);
mutex_init(&ctx->ring_info.ring_lock);
init_waitqueue_head(&ctx->wait);

@@ -465,20 +483,12 @@ static struct kiocb *__aio_get_req(struct kioctx *ctx)
{
struct kiocb *req = NULL;

- req = kmem_cache_alloc(kiocb_cachep, GFP_KERNEL);
+ req = kmem_cache_alloc(kiocb_cachep, GFP_KERNEL|__GFP_ZERO);
if (unlikely(!req))
return NULL;

- req->ki_flags = 0;
atomic_set(&req->ki_users, 2);
- req->ki_key = 0;
req->ki_ctx = ctx;
- req->ki_cancel = NULL;
- req->ki_retry = NULL;
- req->ki_dtor = NULL;
- req->private = NULL;
- req->ki_iovec = NULL;
- req->ki_eventfd = NULL;

return req;
}
@@ -509,7 +519,6 @@ static void kiocb_batch_free(struct kioctx *ctx, struct kiocb_batch *batch)
spin_lock_irq(&ctx->ctx_lock);
list_for_each_entry_safe(req, n, &batch->head, ki_batch) {
list_del(&req->ki_batch);
- list_del(&req->ki_list);
kmem_cache_free(kiocb_cachep, req);
atomic_dec(&ctx->reqs_active);
}
@@ -555,10 +564,7 @@ static int kiocb_batch_refill(struct kioctx *ctx, struct kiocb_batch *batch)
}

batch->count -= allocated;
- list_for_each_entry(req, &batch->head, ki_batch) {
- list_add(&req->ki_list, &ctx->active_reqs);
- atomic_inc(&ctx->reqs_active);
- }
+ atomic_add(allocated, &ctx->reqs_active);

kunmap_atomic(ring);
spin_unlock_irq(&ctx->ctx_lock);
@@ -649,25 +655,34 @@ void aio_complete(struct kiocb *iocb, long res, long res2)
info = &ctx->ring_info;

/*
- * Add a completion event to the ring buffer. Must be done holding
- * ctx->ctx_lock to prevent other code from messing with the tail
- * pointer since we might be called from irq context.
- *
* Take rcu_read_lock() in case the kioctx is being destroyed, as we
* need to issue a wakeup after decrementing reqs_active.
*/
rcu_read_lock();
- spin_lock_irqsave(&ctx->ctx_lock, flags);

- list_del(&iocb->ki_list); /* remove from active_reqs */
+ if (iocb->ki_list.next) {
+ unsigned long flags;
+
+ spin_lock_irqsave(&ctx->ctx_lock, flags);
+ list_del(&iocb->ki_list);
+ spin_unlock_irqrestore(&ctx->ctx_lock, flags);
+ }

/*
* cancelled requests don't get events, userland was given one
* when the event got cancelled.
*/
- if (kiocbIsCancelled(iocb))
+ if (unlikely(xchg(&iocb->ki_cancel,
+ KIOCB_CANCELLED) == KIOCB_CANCELLED))
goto put_rq;

+ /*
+ * Add a completion event to the ring buffer. Must be done holding
+ * ctx->ctx_lock to prevent other code from messing with the tail
+ * pointer since we might be called from irq context.
+ */
+ spin_lock_irqsave(&ctx->completion_lock, flags);
+
tail = info->tail;
pos = tail + AIO_EVENTS_OFFSET;

@@ -701,6 +716,8 @@ void aio_complete(struct kiocb *iocb, long res, long res2)
kunmap_atomic(ring);
flush_dcache_page(info->ring_pages[0]);

+ spin_unlock_irqrestore(&ctx->completion_lock, flags);
+
pr_debug("added to ring %p at [%u]\n", iocb, tail);

/*
@@ -727,7 +744,6 @@ put_rq:
if (waitqueue_active(&ctx->wait))
wake_up(&ctx->wait);

- spin_unlock_irqrestore(&ctx->ctx_lock, flags);
rcu_read_unlock();
}
EXPORT_SYMBOL(aio_complete);
@@ -1196,15 +1212,10 @@ static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
req->ki_opcode = iocb->aio_lio_opcode;

ret = aio_setup_iocb(req, compat);
-
if (ret)
goto out_put_req;

- if (unlikely(kiocbIsCancelled(req))) {
- ret = -EINTR;
- } else {
- ret = req->ki_retry(req);
- }
+ ret = req->ki_retry(req);
if (ret != -EIOCBQUEUED) {
/*
* There's no easy way to restart the syscall since other AIO's
@@ -1220,10 +1231,6 @@ static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
return 0;

out_put_req:
- spin_lock_irq(&ctx->ctx_lock);
- list_del(&req->ki_list);
- spin_unlock_irq(&ctx->ctx_lock);
-
atomic_dec(&ctx->reqs_active);
aio_put_req(req); /* drop extra ref to req */
aio_put_req(req); /* drop i/o ref to req */
diff --git a/include/linux/aio.h b/include/linux/aio.h
index 1e728f0..fc3c467 100644
--- a/include/linux/aio.h
+++ b/include/linux/aio.h
@@ -10,17 +10,13 @@
#include <linux/atomic.h>

struct kioctx;
+struct kiocb;

#define KIOCB_SYNC_KEY (~0U)

-/* ki_flags bits */
-#define KIF_CANCELLED 2
+#define KIOCB_CANCELLED ((void *) (~0ULL))

-#define kiocbSetCancelled(iocb) set_bit(KIF_CANCELLED, &(iocb)->ki_flags)
-
-#define kiocbClearCancelled(iocb) clear_bit(KIF_CANCELLED, &(iocb)->ki_flags)
-
-#define kiocbIsCancelled(iocb) test_bit(KIF_CANCELLED, &(iocb)->ki_flags)
+typedef int (kiocb_cancel_fn)(struct kiocb *, struct io_event *);

/* is there a better place to document function pointer methods? */
/**
@@ -48,13 +44,12 @@ struct kioctx;
* calls may result in undefined behaviour.
*/
struct kiocb {
- unsigned long ki_flags;
atomic_t ki_users;
unsigned ki_key; /* id of this request */

struct file *ki_filp;
struct kioctx *ki_ctx; /* may be NULL for sync ops */
- int (*ki_cancel)(struct kiocb *, struct io_event *);
+ kiocb_cancel_fn *ki_cancel;
ssize_t (*ki_retry)(struct kiocb *);
void (*ki_dtor)(struct kiocb *);

@@ -112,6 +107,7 @@ struct mm_struct;
extern void exit_aio(struct mm_struct *mm);
extern long do_io_submit(aio_context_t ctx_id, long nr,
struct iocb __user *__user *iocbpp, bool compat);
+void kiocb_set_cancel_fn(struct kiocb *req, kiocb_cancel_fn *cancel);
#else
static inline ssize_t wait_on_sync_kiocb(struct kiocb *iocb) { return 0; }
static inline void aio_put_req(struct kiocb *iocb) { }
@@ -121,6 +117,8 @@ static inline void exit_aio(struct mm_struct *mm) { }
static inline long do_io_submit(aio_context_t ctx_id, long nr,
struct iocb __user * __user *iocbpp,
bool compat) { return 0; }
+static inline void kiocb_set_cancel_fn(struct kiocb *req,
+ kiocb_cancel_fn *cancel) { }
#endif /* CONFIG_AIO */

static inline struct kiocb *list_kiocb(struct list_head *h)
--
1.7.12


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