Re: [patch/rfc] eventfd semaphore-like behavior

From: Michael Kerrisk
Date: Mon Feb 09 2009 - 02:45:37 EST

On Thu, Feb 5, 2009 at 11:58 AM, Davide Libenzi <davidel@xxxxxxxxxxxxxxx> wrote:
> People started using eventfd in scnarios where before where using pipes.
> Many of them use eventfds in a semaphore-like way, like they were before
> with pipes. The problem with eventfd is that a read() on the fd returns
> and wipes the whole counter, making the use of it as semaphore a little
> bit more cumbersome. You can do a read() followed by a write() of
> COUNTER-1, but IMO it's pretty easy and cheap to make this work w/out
> extra steps. This patch introduces a new eventfd flag that tells eventfd
> to only dequeue 1 from the counter, allowing simple read/write to make it
> behave like a semaphore.
> Simple test here:
>
> http://www.xmailserver.org/eventfd-sem.c
>
>
> Signed-off-by: Davide Libenzi <davidel@xxxxxxxxxxxxxxx>

Tested-by: Michael Kerrisk <mtk.manpages@xxxxxxxxx>

Applied this patch against 2.6.29-rc3, and it works as I would expect.


A question or two.... This change is rather specific to a single use
case, so I wonder

a) Are there use cases that require the ability to read an arbitrary
number of units off the eventfd -- i.e., read N units off the eventfd,
rather than just 1?

b) Is it desirable to be able to toggle the EFD_SEMAPHORE behavior on
and off for an eventfd?

It's difficult to see how these use cases could be accommodated in the
current API, but I just thought it worth raising the ideas.

Cheers,

Michael

> - Davide
>
>
> ---
> fs/eventfd.c | 20 +++++++++++---------
> include/linux/eventfd.h | 12 +++++++++++-
> 2 files changed, 22 insertions(+), 10 deletions(-)
>
> Index: linux-2.6.mod/fs/eventfd.c
> ===================================================================
> --- linux-2.6.mod.orig/fs/eventfd.c 2009-02-03 18:13:33.000000000 -0800
> +++ linux-2.6.mod/fs/eventfd.c 2009-02-04 12:16:39.000000000 -0800
> @@ -28,6 +28,7 @@ struct eventfd_ctx {
> * issue a wakeup.
> */
> __u64 count;
> + unsigned int flags;
> };
>
> /*
> @@ -87,22 +88,20 @@ static ssize_t eventfd_read(struct file
> {
> struct eventfd_ctx *ctx = file->private_data;
> ssize_t res;
> - __u64 ucnt;
> + __u64 ucnt = 0;
> DECLARE_WAITQUEUE(wait, current);
>
> if (count < sizeof(ucnt))
> return -EINVAL;
> spin_lock_irq(&ctx->wqh.lock);
> res = -EAGAIN;
> - ucnt = ctx->count;
> - if (ucnt > 0)
> + if (ctx->count > 0)
> res = sizeof(ucnt);
> else if (!(file->f_flags & O_NONBLOCK)) {
> __add_wait_queue(&ctx->wqh, &wait);
> for (res = 0;;) {
> set_current_state(TASK_INTERRUPTIBLE);
> if (ctx->count > 0) {
> - ucnt = ctx->count;
> res = sizeof(ucnt);
> break;
> }
> @@ -117,8 +116,9 @@ static ssize_t eventfd_read(struct file
> __remove_wait_queue(&ctx->wqh, &wait);
> __set_current_state(TASK_RUNNING);
> }
> - if (res > 0) {
> - ctx->count = 0;
> + if (likely(res > 0)) {
> + ucnt = (ctx->flags & EFD_SEMAPHORE) ? 1 : ctx->count;
> + ctx->count -= ucnt;
> if (waitqueue_active(&ctx->wqh))
> wake_up_locked_poll(&ctx->wqh, POLLOUT);
> }
> @@ -166,7 +166,7 @@ static ssize_t eventfd_write(struct file
> __remove_wait_queue(&ctx->wqh, &wait);
> __set_current_state(TASK_RUNNING);
> }
> - if (res > 0) {
> + if (likely(res > 0)) {
> ctx->count += ucnt;
> if (waitqueue_active(&ctx->wqh))
> wake_up_locked_poll(&ctx->wqh, POLLIN);
> @@ -207,7 +207,7 @@ SYSCALL_DEFINE2(eventfd2, unsigned int,
> BUILD_BUG_ON(EFD_CLOEXEC != O_CLOEXEC);
> BUILD_BUG_ON(EFD_NONBLOCK != O_NONBLOCK);
>
> - if (flags & ~(EFD_CLOEXEC | EFD_NONBLOCK))
> + if (flags & ~EFD_FLAGS_SET)
> return -EINVAL;
>
> ctx = kmalloc(sizeof(*ctx), GFP_KERNEL);
> @@ -216,13 +216,14 @@ SYSCALL_DEFINE2(eventfd2, unsigned int,
>
> init_waitqueue_head(&ctx->wqh);
> ctx->count = count;
> + ctx->flags = flags;
>
> /*
> * When we call this, the initialization must be complete, since
> * anon_inode_getfd() will install the fd.
> */
> fd = anon_inode_getfd("[eventfd]", &eventfd_fops, ctx,
> - flags & (O_CLOEXEC | O_NONBLOCK));
> + flags & EFD_SHARED_FCNTL_FLAGS);
> if (fd < 0)
> kfree(ctx);
> return fd;
> @@ -232,3 +233,4 @@ SYSCALL_DEFINE1(eventfd, unsigned int, c
> {
> return sys_eventfd2(count, 0);
> }
> +
> Index: linux-2.6.mod/include/linux/eventfd.h
> ===================================================================
> --- linux-2.6.mod.orig/include/linux/eventfd.h 2009-02-03 18:13:33.000000000 -0800
> +++ linux-2.6.mod/include/linux/eventfd.h 2009-02-04 12:16:32.000000000 -0800
> @@ -13,10 +13,20 @@
> /* For O_CLOEXEC and O_NONBLOCK */
> #include <linux/fcntl.h>
>
> -/* Flags for eventfd2. */
> +/*
> + * CAREFUL: Check include/asm-generic/fcntl.h when defining
> + * new flags, since they might collide with O_* ones. We want
> + * to re-use O_* flags that couldn't possibly have a meaning
> + * from eventfd, in order to leave a free define-space for
> + * shared O_* flags.
> + */
> +#define EFD_SEMAPHORE (1 << 0)
> #define EFD_CLOEXEC O_CLOEXEC
> #define EFD_NONBLOCK O_NONBLOCK
>
> +#define EFD_SHARED_FCNTL_FLAGS (O_CLOEXEC | O_NONBLOCK)
> +#define EFD_FLAGS_SET (EFD_SHARED_FCNTL_FLAGS | EFD_SEMAPHORE)
> +
> struct file *eventfd_fget(int fd);
> int eventfd_signal(struct file *file, int n);
>
>



--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
git://git.kernel.org/pub/scm/docs/man-pages/man-pages.git
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