Re: [PATCH] vfio-pci: Use fdget() rather than eventfd_fget()

[Al Viro]

On Tue, Aug 20, 2013 at 01:18:07PM -0600, Alex Williamson wrote:
> eventfd_fget() tests to see whether the file is an eventfd file, which
> we then immediately pass to eventfd_ctx_fileget(), which again tests
> whether the file is an eventfd file.  Simplify slightly by using
> fdget() so that we only test that we're looking at an eventfd once.
> fget() could also be used, but fdget() makes use of fget_light() for
> another slight optimization.

Umm...

> @@ -210,8 +210,8 @@ fail:
>   if (ctx && !IS_ERR(ctx))
>   eventfd_ctx_put(ctx);
>  
> - if (file && !IS_ERR(file))
> - fput(file);
> + if (irqfd.file)
> + fdput(irqfd);
>  
>   kfree(virqfd);

IMO it's a bad style; you have three failure exits leading here, and those
ifs are nothing but "how far did we get before we'd failed".

fail3:
eventfd_ctx_put(ctx);
fail2:
fdput(irqfd);
fail1:
kfree(virqfd);

is much easier to analyse.  It's a very common pattern and IME it's more
robust than this kind of "flexibility"...
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[PATCH] vfio-pci: Use fdget() rather than eventfd_fget()

[Alex Williamson]

eventfd_fget() tests to see whether the file is an eventfd file, which
we then immediately pass to eventfd_ctx_fileget(), which again tests
whether the file is an eventfd file.  Simplify slightly by using
fdget() so that we only test that we're looking at an eventfd once.
fget() could also be used, but fdget() makes use of fget_light() for
another slight optimization.

Signed-off-by: Alex Williamson 
---
 drivers/vfio/pci/vfio_pci_intrs.c |   18 +-
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/drivers/vfio/pci/vfio_pci_intrs.c 
b/drivers/vfio/pci/vfio_pci_intrs.c
index 4bc704e..7507975 100644
--- a/drivers/vfio/pci/vfio_pci_intrs.c
+++ b/drivers/vfio/pci/vfio_pci_intrs.c
@@ -130,7 +130,7 @@ static int virqfd_enable(struct vfio_pci_device *vdev,
 void (*thread)(struct vfio_pci_device *, void *),
 void *data, struct virqfd **pvirqfd, int fd)
 {
-   struct file *file = NULL;
+   struct fd irqfd;
struct eventfd_ctx *ctx = NULL;
struct virqfd *virqfd;
int ret = 0;
@@ -149,13 +149,13 @@ static int virqfd_enable(struct vfio_pci_device *vdev,
INIT_WORK(&virqfd->shutdown, virqfd_shutdown);
INIT_WORK(&virqfd->inject, virqfd_inject);
 
-   file = eventfd_fget(fd);
-   if (IS_ERR(file)) {
-   ret = PTR_ERR(file);
+   irqfd = fdget(fd);
+   if (!irqfd.file) {
+   ret = -EBADF;
goto fail;
}
 
-   ctx = eventfd_ctx_fileget(file);
+   ctx = eventfd_ctx_fileget(irqfd.file);
if (IS_ERR(ctx)) {
ret = PTR_ERR(ctx);
goto fail;
@@ -187,7 +187,7 @@ static int virqfd_enable(struct vfio_pci_device *vdev,
init_waitqueue_func_entry(&virqfd->wait, virqfd_wakeup);
init_poll_funcptr(&virqfd->pt, virqfd_ptable_queue_proc);
 
-   events = file->f_op->poll(file, &virqfd->pt);
+   events = irqfd.file->f_op->poll(irqfd.file, &virqfd->pt);
 
/*
 * Check if there was an event already pending on the eventfd
@@ -202,7 +202,7 @@ static int virqfd_enable(struct vfio_pci_device *vdev,
 * Do not drop the file until the irqfd is fully initialized,
 * otherwise we might race against the POLLHUP.
 */
-   fput(file);
+   fdput(irqfd);
 
return 0;
 
@@ -210,8 +210,8 @@ fail:
if (ctx && !IS_ERR(ctx))
eventfd_ctx_put(ctx);
 
-   if (file && !IS_ERR(file))
-   fput(file);
+   if (irqfd.file)
+   fdput(irqfd);
 
kfree(virqfd);
 

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