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aio: make aio_put_req() lockless
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Freeing a kiocb needed to touch the kioctx for three things:

 * Pull it off the reqs_active list
 * Decrementing reqs_active
 * Issuing a wakeup, if the kioctx was in the process of being freed.

This patch moves these to aio_complete(), for a couple reasons:

 * aio_complete() already has to issue the wakeup, so if we drop the
   kioctx refcount before aio_complete does its wakeup we don't have to
   do it twice.
 * aio_complete currently has to take the kioctx lock, so it makes sense
   for it to pull the kiocb off the reqs_active list too.
 * A later patch is going to change reqs_active to include unreaped
   completions - this will mean allocating a kiocb doesn't have to look
   at the ringbuffer. So taking the decrement of reqs_active out of
   kiocb_free() is useful prep work for that patch.

This doesn't really affect cancellation, since existing (usb) code that
implements a cancel function still calls aio_complete() - we just have
to make sure that aio_complete does the necessary teardown for cancelled
kiocbs.

It does affect code paths where we free kiocbs that were never
submitted; they need to decrement reqs_active and pull the kiocb off the
reqs_active list.  This occurs in two places: kiocb_batch_free(), which
is going away in a later patch, and the error path in io_submit_one.

[[email protected]: coding-style fixes]
Signed-off-by: Kent Overstreet <[email protected]>
Cc: Zach Brown <[email protected]>
Cc: Felipe Balbi <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Mark Fasheh <[email protected]>
Cc: Joel Becker <[email protected]>
Cc: Rusty Russell <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Asai Thambi S P <[email protected]>
Cc: Selvan Mani <[email protected]>
Cc: Sam Bradshaw <[email protected]>
Acked-by: Jeff Moyer <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Benjamin LaHaise <[email protected]>
Reviewed-by: "Theodore Ts'o" <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
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@torvalds
Kent Overstreet authored and torvalds committed May 8, 2013
1 parent 1d98ebf commit 11599eb
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Showing 2 changed files with 36 additions and 54 deletions.
86 changes: 34 additions & 52 deletions fs/aio.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -89,7 +89,7 @@ struct kioctx {

spinlock_t ctx_lock;

int reqs_active;
atomic_t reqs_active;
struct list_head active_reqs; /* used for cancellation */

/* sys_io_setup currently limits this to an unsigned int */
Expand Down Expand Up @@ -250,7 +250,7 @@ static void ctx_rcu_free(struct rcu_head *head)
static void __put_ioctx(struct kioctx *ctx)
{
unsigned nr_events = ctx->max_reqs;
BUG_ON(ctx->reqs_active);
BUG_ON(atomic_read(&ctx->reqs_active));

aio_free_ring(ctx);
if (nr_events) {
Expand Down Expand Up @@ -284,7 +284,7 @@ static int kiocb_cancel(struct kioctx *ctx, struct kiocb *kiocb,
cancel = kiocb->ki_cancel;
kiocbSetCancelled(kiocb);
if (cancel) {
kiocb->ki_users++;
atomic_inc(&kiocb->ki_users);
spin_unlock_irq(&ctx->ctx_lock);

memset(res, 0, sizeof(*res));
Expand Down Expand Up @@ -383,12 +383,12 @@ static void kill_ctx(struct kioctx *ctx)
kiocb_cancel(ctx, req, &res);
}

if (!ctx->reqs_active)
if (!atomic_read(&ctx->reqs_active))
goto out;

add_wait_queue(&ctx->wait, &wait);
set_task_state(tsk, TASK_UNINTERRUPTIBLE);
while (ctx->reqs_active) {
while (atomic_read(&ctx->reqs_active)) {
spin_unlock_irq(&ctx->ctx_lock);
io_schedule();
set_task_state(tsk, TASK_UNINTERRUPTIBLE);
Expand All @@ -406,9 +406,9 @@ static void kill_ctx(struct kioctx *ctx)
*/
ssize_t wait_on_sync_kiocb(struct kiocb *iocb)
{
while (iocb->ki_users) {
while (atomic_read(&iocb->ki_users)) {
set_current_state(TASK_UNINTERRUPTIBLE);
if (!iocb->ki_users)
if (!atomic_read(&iocb->ki_users))
break;
io_schedule();
}
Expand Down Expand Up @@ -438,7 +438,7 @@ void exit_aio(struct mm_struct *mm)
printk(KERN_DEBUG
"exit_aio:ioctx still alive: %d %d %d\n",
atomic_read(&ctx->users), ctx->dead,
ctx->reqs_active);
atomic_read(&ctx->reqs_active));
/*
* We don't need to bother with munmap() here -
* exit_mmap(mm) is coming and it'll unmap everything.
Expand All @@ -453,11 +453,11 @@ void exit_aio(struct mm_struct *mm)
}

/* aio_get_req
* Allocate a slot for an aio request. Increments the users count
* Allocate a slot for an aio request. Increments the ki_users count
* of the kioctx so that the kioctx stays around until all requests are
* complete. Returns NULL if no requests are free.
*
* Returns with kiocb->users set to 2. The io submit code path holds
* Returns with kiocb->ki_users set to 2. The io submit code path holds
* an extra reference while submitting the i/o.
* This prevents races between the aio code path referencing the
* req (after submitting it) and aio_complete() freeing the req.
Expand All @@ -471,7 +471,7 @@ static struct kiocb *__aio_get_req(struct kioctx *ctx)
return NULL;

req->ki_flags = 0;
req->ki_users = 2;
atomic_set(&req->ki_users, 2);
req->ki_key = 0;
req->ki_ctx = ctx;
req->ki_cancel = NULL;
Expand Down Expand Up @@ -512,9 +512,9 @@ static void kiocb_batch_free(struct kioctx *ctx, struct kiocb_batch *batch)
list_del(&req->ki_batch);
list_del(&req->ki_list);
kmem_cache_free(kiocb_cachep, req);
ctx->reqs_active--;
atomic_dec(&ctx->reqs_active);
}
if (unlikely(!ctx->reqs_active && ctx->dead))
if (unlikely(!atomic_read(&ctx->reqs_active) && ctx->dead))
wake_up_all(&ctx->wait);
spin_unlock_irq(&ctx->ctx_lock);
}
Expand Down Expand Up @@ -545,7 +545,8 @@ static int kiocb_batch_refill(struct kioctx *ctx, struct kiocb_batch *batch)
spin_lock_irq(&ctx->ctx_lock);
ring = kmap_atomic(ctx->ring_info.ring_pages[0]);

avail = aio_ring_avail(&ctx->ring_info, ring) - ctx->reqs_active;
avail = aio_ring_avail(&ctx->ring_info, ring) -
atomic_read(&ctx->reqs_active);
BUG_ON(avail < 0);
if (avail < allocated) {
/* Trim back the number of requests. */
Expand All @@ -560,7 +561,7 @@ 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);
ctx->reqs_active++;
atomic_inc(&ctx->reqs_active);
}

kunmap_atomic(ring);
Expand All @@ -583,10 +584,8 @@ static inline struct kiocb *aio_get_req(struct kioctx *ctx,
return req;
}

static inline void really_put_req(struct kioctx *ctx, struct kiocb *req)
static void kiocb_free(struct kiocb *req)
{
assert_spin_locked(&ctx->ctx_lock);

if (req->ki_filp)
fput(req->ki_filp);
if (req->ki_eventfd != NULL)
Expand All @@ -596,40 +595,12 @@ static inline void really_put_req(struct kioctx *ctx, struct kiocb *req)
if (req->ki_iovec != &req->ki_inline_vec)
kfree(req->ki_iovec);
kmem_cache_free(kiocb_cachep, req);
ctx->reqs_active--;

if (unlikely(!ctx->reqs_active && ctx->dead))
wake_up_all(&ctx->wait);
}

/* __aio_put_req
* Returns true if this put was the last user of the request.
*/
static void __aio_put_req(struct kioctx *ctx, struct kiocb *req)
{
assert_spin_locked(&ctx->ctx_lock);

req->ki_users--;
BUG_ON(req->ki_users < 0);
if (likely(req->ki_users))
return;
list_del(&req->ki_list); /* remove from active_reqs */
req->ki_cancel = NULL;
req->ki_retry = NULL;

really_put_req(ctx, req);
}

/* aio_put_req
* Returns true if this put was the last user of the kiocb,
* false if the request is still in use.
*/
void aio_put_req(struct kiocb *req)
{
struct kioctx *ctx = req->ki_ctx;
spin_lock_irq(&ctx->ctx_lock);
__aio_put_req(ctx, req);
spin_unlock_irq(&ctx->ctx_lock);
if (atomic_dec_and_test(&req->ki_users))
kiocb_free(req);
}
EXPORT_SYMBOL(aio_put_req);

Expand Down Expand Up @@ -677,9 +648,9 @@ void aio_complete(struct kiocb *iocb, long res, long res2)
* - the sync task helpfully left a reference to itself in the iocb
*/
if (is_sync_kiocb(iocb)) {
BUG_ON(iocb->ki_users != 1);
BUG_ON(atomic_read(&iocb->ki_users) != 1);
iocb->ki_user_data = res;
iocb->ki_users = 0;
atomic_set(&iocb->ki_users, 0);
wake_up_process(iocb->ki_obj.tsk);
return;
}
Expand All @@ -694,6 +665,8 @@ void aio_complete(struct kiocb *iocb, long res, long res2)
*/
spin_lock_irqsave(&ctx->ctx_lock, flags);

list_del(&iocb->ki_list); /* remove from active_reqs */

/*
* cancelled requests don't get events, userland was given one
* when the event got cancelled.
Expand Down Expand Up @@ -740,7 +713,8 @@ void aio_complete(struct kiocb *iocb, long res, long res2)

put_rq:
/* everything turned out well, dispose of the aiocb. */
__aio_put_req(ctx, iocb);
aio_put_req(iocb);
atomic_dec(&ctx->reqs_active);

/*
* We have to order our ring_info tail store above and test
Expand Down Expand Up @@ -905,7 +879,7 @@ static int read_events(struct kioctx *ctx,
break;
/* Try to only show up in io wait if there are ops
* in flight */
if (ctx->reqs_active)
if (atomic_read(&ctx->reqs_active))
io_schedule();
else
schedule();
Expand Down Expand Up @@ -1369,6 +1343,14 @@ 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);
if (unlikely(!atomic_read(&ctx->reqs_active) && ctx->dead))
wake_up_all(&ctx->wait);

aio_put_req(req); /* drop extra ref to req */
aio_put_req(req); /* drop i/o ref to req */
return ret;
Expand Down
4 changes: 2 additions & 2 deletions include/linux/aio.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -49,7 +49,7 @@ struct kioctx;
*/
struct kiocb {
unsigned long ki_flags;
int ki_users;
atomic_t ki_users;
unsigned ki_key; /* id of this request */

struct file *ki_filp;
Expand Down Expand Up @@ -96,7 +96,7 @@ static inline bool is_sync_kiocb(struct kiocb *kiocb)
static inline void init_sync_kiocb(struct kiocb *kiocb, struct file *filp)
{
*kiocb = (struct kiocb) {
.ki_users = 1,
.ki_users = ATOMIC_INIT(1),
.ki_key = KIOCB_SYNC_KEY,
.ki_filp = filp,
.ki_obj.tsk = current,
Expand Down

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