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Subject[patch/rfc] eventfd semaphore-like behavior
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>


- 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);


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