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eventfd - allow atomic read and waitqueue remove

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KVM needs a wait to atomically remove themselves from the eventfd ->poll()
wait queue head, in order to handle correctly their IRQfd deassign
operation.

This patch introduces such API, plus a way to read an eventfd from its
context.

Signed-off-by: Davide Libenzi <davidel@xmailserver.org>
Signed-off-by: Avi Kivity <avi@redhat.com>
This commit is contained in:
Davide Libenzi 2010-01-13 09:34:36 -08:00 committed by Marcelo Tosatti
parent a6085fbaf6
commit cb289d6244
2 changed files with 90 additions and 15 deletions
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89
fs/eventfd.c
View File

@ -135,26 +135,71 @@ static unsigned int eventfd_poll(struct file *file, poll_table *wait)
return events;
}
static ssize_t eventfd_read(struct file *file, char __user *buf, size_t count,
loff_t *ppos)
static void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt)
{
*cnt = (ctx->flags & EFD_SEMAPHORE) ? 1 : ctx->count;
ctx->count -= *cnt;
}
/**
* eventfd_ctx_remove_wait_queue - Read the current counter and removes wait queue.
* @ctx: [in] Pointer to eventfd context.
* @wait: [in] Wait queue to be removed.
* @cnt: [out] Pointer to the 64bit conter value.
*
* Returns zero if successful, or the following error codes:
*
* -EAGAIN : The operation would have blocked.
*
* This is used to atomically remove a wait queue entry from the eventfd wait
* queue head, and read/reset the counter value.
*/
int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_t *wait,
__u64 *cnt)
{
unsigned long flags;
spin_lock_irqsave(&ctx->wqh.lock, flags);
eventfd_ctx_do_read(ctx, cnt);
__remove_wait_queue(&ctx->wqh, wait);
if (*cnt != 0 && waitqueue_active(&ctx->wqh))
wake_up_locked_poll(&ctx->wqh, POLLOUT);
spin_unlock_irqrestore(&ctx->wqh.lock, flags);
return *cnt != 0 ? 0 : -EAGAIN;
}
EXPORT_SYMBOL_GPL(eventfd_ctx_remove_wait_queue);
/**
* eventfd_ctx_read - Reads the eventfd counter or wait if it is zero.
* @ctx: [in] Pointer to eventfd context.
* @no_wait: [in] Different from zero if the operation should not block.
* @cnt: [out] Pointer to the 64bit conter value.
*
* Returns zero if successful, or the following error codes:
*
* -EAGAIN : The operation would have blocked but @no_wait was nonzero.
* -ERESTARTSYS : A signal interrupted the wait operation.
*
* If @no_wait is zero, the function might sleep until the eventfd internal
* counter becomes greater than zero.
*/
ssize_t eventfd_ctx_read(struct eventfd_ctx *ctx, int no_wait, __u64 *cnt)
{
struct eventfd_ctx *ctx = file->private_data;
ssize_t res;
__u64 ucnt = 0;
DECLARE_WAITQUEUE(wait, current);
if (count < sizeof(ucnt))
return -EINVAL;
spin_lock_irq(&ctx->wqh.lock);
*cnt = 0;
res = -EAGAIN;
if (ctx->count > 0)
res = sizeof(ucnt);
else if (!(file->f_flags & O_NONBLOCK)) {
res = 0;
else if (!no_wait) {
__add_wait_queue(&ctx->wqh, &wait);
for (res = 0;;) {
for (;;) {
set_current_state(TASK_INTERRUPTIBLE);
if (ctx->count > 0) {
res = sizeof(ucnt);
res = 0;
break;
}
if (signal_pending(current)) {
@ -168,18 +213,32 @@ static ssize_t eventfd_read(struct file *file, char __user *buf, size_t count,
__remove_wait_queue(&ctx->wqh, &wait);
__set_current_state(TASK_RUNNING);
}
if (likely(res > 0)) {
ucnt = (ctx->flags & EFD_SEMAPHORE) ? 1 : ctx->count;
ctx->count -= ucnt;
if (likely(res == 0)) {
eventfd_ctx_do_read(ctx, cnt);
if (waitqueue_active(&ctx->wqh))
wake_up_locked_poll(&ctx->wqh, POLLOUT);
}
spin_unlock_irq(&ctx->wqh.lock);
if (res > 0 && put_user(ucnt, (__u64 __user *) buf))
return -EFAULT;
return res;
}
EXPORT_SYMBOL_GPL(eventfd_ctx_read);
static ssize_t eventfd_read(struct file *file, char __user *buf, size_t count,
loff_t *ppos)
{
struct eventfd_ctx *ctx = file->private_data;
ssize_t res;
__u64 cnt;
if (count < sizeof(cnt))
return -EINVAL;
res = eventfd_ctx_read(ctx, file->f_flags & O_NONBLOCK, &cnt);
if (res < 0)
return res;
return put_user(cnt, (__u64 __user *) buf) ? -EFAULT : sizeof(cnt);
}
static ssize_t eventfd_write(struct file *file, const char __user *buf, size_t count,
loff_t *ppos)

16
include/linux/eventfd.h
View File

@ -10,6 +10,7 @@
#include <linux/fcntl.h>
#include <linux/file.h>
#include <linux/wait.h>
/*
* CAREFUL: Check include/asm-generic/fcntl.h when defining
@ -34,6 +35,9 @@ struct file *eventfd_fget(int fd);
struct eventfd_ctx *eventfd_ctx_fdget(int fd);
struct eventfd_ctx *eventfd_ctx_fileget(struct file *file);
int eventfd_signal(struct eventfd_ctx *ctx, int n);
ssize_t eventfd_ctx_read(struct eventfd_ctx *ctx, int no_wait, __u64 *cnt);
int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_t *wait,
__u64 *cnt);
#else /* CONFIG_EVENTFD */
@ -61,6 +65,18 @@ static inline void eventfd_ctx_put(struct eventfd_ctx *ctx)
}
static inline ssize_t eventfd_ctx_read(struct eventfd_ctx *ctx, int no_wait,
__u64 *cnt)
{
return -ENOSYS;
}
static inline int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx,
wait_queue_t *wait, __u64 *cnt)
{
return -ENOSYS;
}
#endif
#endif /* _LINUX_EVENTFD_H */