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Subject[PATCH 07/30] aio: add delayed cancel support
Date
The upcoming aio poll support would like to be able to complete the
iocb inline from the cancellation context, but that would cause
a lock order reversal. Add support for optionally moving the cancelation
outside the context lock to avoid this reversal.

To make this safe aio_complete needs to check if this call should complete
the iocb. If it didn't the callers must not release any other resources.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
fs/aio.c | 81 ++++++++++++++++++++++++++++++++++++++++++++++------------------
1 file changed, 59 insertions(+), 22 deletions(-)

diff --git a/fs/aio.c b/fs/aio.c
index c36eec8b0879..232dd84fc897 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -177,6 +177,11 @@ struct aio_kiocb {
struct list_head ki_list; /* the aio core uses this
* for cancellation */

+ unsigned int flags; /* protected by ctx->ctx_lock */
+#define AIO_IOCB_CAN_CANCEL (1 << 0)
+#define AIO_IOCB_DELAYED_CANCEL (1 << 1)
+#define AIO_IOCB_CANCELLED (1 << 2)
+
/*
* If the aio_resfd field of the userspace iocb is not zero,
* this is the underlying eventfd context to deliver events to.
@@ -543,9 +548,9 @@ static int aio_setup_ring(struct kioctx *ctx, unsigned int nr_events)
#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 *iocb, kiocb_cancel_fn *cancel)
+static void __kiocb_set_cancel_fn(struct aio_kiocb *req,
+ kiocb_cancel_fn *cancel, unsigned int iocb_flags)
{
- struct aio_kiocb *req = container_of(iocb, struct aio_kiocb, rw);
struct kioctx *ctx = req->ki_ctx;
unsigned long flags;

@@ -555,8 +560,15 @@ void kiocb_set_cancel_fn(struct kiocb *iocb, kiocb_cancel_fn *cancel)
spin_lock_irqsave(&ctx->ctx_lock, flags);
list_add_tail(&req->ki_list, &ctx->active_reqs);
req->ki_cancel = cancel;
+ req->flags |= (AIO_IOCB_CAN_CANCEL | iocb_flags);
spin_unlock_irqrestore(&ctx->ctx_lock, flags);
}
+
+void kiocb_set_cancel_fn(struct kiocb *iocb, kiocb_cancel_fn *cancel)
+{
+ return __kiocb_set_cancel_fn(container_of(iocb, struct aio_kiocb, rw),
+ cancel, 0);
+}
EXPORT_SYMBOL(kiocb_set_cancel_fn);

static int kiocb_cancel(struct aio_kiocb *kiocb)
@@ -599,17 +611,26 @@ static void free_ioctx_users(struct percpu_ref *ref)
{
struct kioctx *ctx = container_of(ref, struct kioctx, users);
struct aio_kiocb *req;
+ LIST_HEAD(list);

spin_lock_irq(&ctx->ctx_lock);
-
while (!list_empty(&ctx->active_reqs)) {
req = list_first_entry(&ctx->active_reqs,
struct aio_kiocb, ki_list);
- kiocb_cancel(req);
+ if (req->flags & AIO_IOCB_DELAYED_CANCEL) {
+ req->flags |= AIO_IOCB_CANCELLED;
+ list_move_tail(&req->ki_list, &list);
+ } else {
+ kiocb_cancel(req);
+ }
}
-
spin_unlock_irq(&ctx->ctx_lock);

+ while (!list_empty(&list)) {
+ req = list_first_entry(&list, struct aio_kiocb, ki_list);
+ kiocb_cancel(req);
+ }
+
percpu_ref_kill(&ctx->reqs);
percpu_ref_put(&ctx->reqs);
}
@@ -1045,22 +1066,30 @@ static struct kioctx *lookup_ioctx(unsigned long ctx_id)
return ret;
}

+#define AIO_COMPLETE_CANCEL (1 << 0)
+
/* aio_complete
* Called when the io request on the given iocb is complete.
*/
-static void aio_complete(struct aio_kiocb *iocb, long res, long res2)
+static bool aio_complete(struct aio_kiocb *iocb, long res, long res2,
+ unsigned complete_flags)
{
struct kioctx *ctx = iocb->ki_ctx;
struct aio_ring *ring;
struct io_event *ev_page, *event;
unsigned tail, pos, head;
- unsigned long flags;
-
- if (!list_empty_careful(&iocb->ki_list)) {
- unsigned long flags;
+ unsigned long flags;

+ if (iocb->flags & AIO_IOCB_CAN_CANCEL) {
spin_lock_irqsave(&ctx->ctx_lock, flags);
- list_del(&iocb->ki_list);
+ if (!(complete_flags & AIO_COMPLETE_CANCEL) &&
+ (iocb->flags & AIO_IOCB_CANCELLED)) {
+ spin_unlock_irqrestore(&ctx->ctx_lock, flags);
+ return false;
+ }
+
+ if (!list_empty(&iocb->ki_list))
+ list_del(&iocb->ki_list);
spin_unlock_irqrestore(&ctx->ctx_lock, flags);
}

@@ -1136,6 +1165,7 @@ static void aio_complete(struct aio_kiocb *iocb, long res, long res2)
wake_up(&ctx->wait);

percpu_ref_put(&ctx->reqs);
+ return true;
}

/* aio_read_events_ring
@@ -1384,6 +1414,7 @@ SYSCALL_DEFINE1(io_destroy, aio_context_t, ctx)
static void aio_complete_rw(struct kiocb *kiocb, long res, long res2)
{
struct aio_kiocb *iocb = container_of(kiocb, struct aio_kiocb, rw);
+ struct file *file = kiocb->ki_filp;

if (kiocb->ki_flags & IOCB_WRITE) {
struct inode *inode = file_inode(kiocb->ki_filp);
@@ -1397,8 +1428,8 @@ static void aio_complete_rw(struct kiocb *kiocb, long res, long res2)
file_end_write(kiocb->ki_filp);
}

- fput(kiocb->ki_filp);
- aio_complete(iocb, res, res2);
+ if (aio_complete(iocb, res, res2, 0))
+ fput(file);
}

static int aio_prep_rw(struct kiocb *req, struct iocb *iocb)
@@ -1541,11 +1572,13 @@ static ssize_t aio_write(struct kiocb *req, struct iocb *iocb, bool vectored,
static void aio_fsync_work(struct work_struct *work)
{
struct fsync_iocb *req = container_of(work, struct fsync_iocb, work);
+ struct aio_kiocb *iocb = container_of(req, struct aio_kiocb, fsync);
+ struct file *file = req->file;
int ret;

ret = vfs_fsync(req->file, req->datasync);
- fput(req->file);
- aio_complete(container_of(req, struct aio_kiocb, fsync), ret, 0);
+ if (aio_complete(iocb, ret, 0, 0))
+ fput(file);
}

static int aio_fsync(struct fsync_iocb *req, struct iocb *iocb, bool datasync)
@@ -1807,8 +1840,8 @@ SYSCALL_DEFINE3(io_cancel, aio_context_t, ctx_id, struct iocb __user *, iocb,
{
struct kioctx *ctx;
struct aio_kiocb *kiocb;
+ int ret = -EINVAL;
u32 key;
- int ret;

ret = get_user(key, &iocb->aio_key);
if (unlikely(ret))
@@ -1819,15 +1852,19 @@ SYSCALL_DEFINE3(io_cancel, aio_context_t, ctx_id, struct iocb __user *, iocb,
return -EINVAL;

spin_lock_irq(&ctx->ctx_lock);
-
kiocb = lookup_kiocb(ctx, iocb, key);
- if (kiocb)
- ret = kiocb_cancel(kiocb);
- else
- ret = -EINVAL;
-
+ if (kiocb) {
+ if (kiocb->flags & AIO_IOCB_DELAYED_CANCEL) {
+ kiocb->flags |= AIO_IOCB_CANCELLED;
+ } else {
+ ret = kiocb_cancel(kiocb);
+ kiocb = NULL;
+ }
+ }
spin_unlock_irq(&ctx->ctx_lock);

+ if (kiocb)
+ ret = kiocb_cancel(kiocb);
if (!ret) {
/*
* The result argument is no longer used - the io_event is
--
2.14.2
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