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    SubjectRe: [patch/rfc] eventfd semaphore-like behavior
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    On Thu, Feb 5, 2009 at 11:58 AM, Davide Libenzi <davidel@xmailserver.org> 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@xmailserver.org>

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

    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
    man-pages online: http://www.kernel.org/doc/man-pages/online_pages.html
    Found a bug? http://www.kernel.org/doc/man-pages/reporting_bugs.html


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