On Sun, Jan 10, 2010 at 11:04:23AM -0800, Davide Libenzi wrote:
> On Sun, 10 Jan 2010, Michael S. Tsirkin wrote:
>
> > On Sun, Jan 10, 2010 at 09:27:34AM -0800, Davide Libenzi wrote:
> > > On Sun, 10 Jan 2010, Michael S. Tsirkin wrote:
> > >
> > > > Not sure what you mean by irqfd locking. IMO we must take waitqueue
> > > > lock, otherwise we can not prevent wait queue callback from being
> > > > invoked, right? Note that if we take our own lock under wait queue
> > > > callback, we won't be able to use this lock around remove_wait_queue.
> > > > Also note how fs/evetfd.c uses __remove_wait_queue after taking wait
> > > > queue lock - we'd like to do the same with our wait queue.
> > > >
> > > > Could you clarify pls?
> > >
> > > Wait a second. Looking at the current code in Linus tree, when you
> > > deassign an irqfd, you are actually destroying it, so the idea of saving
> > > the counter on deassign is not going to work.
> > >
> > >
> > >
> > > - Davide
> >
> > Yes, the only context passed between deassign and assign is the eventfd
> > itself. So I think the following code for deassign would work provided
> > eventfd_ctx_read_locked works with wait queue lock taken:
> >
> > spin_lock_irqsave(&irqfd->wqh.lock, flags);
> > eventfd_ctx_read_locked(ctx->eventfd, 1, &ucnt);
> > __remove_wait_queue(irqfd->wqh, &irqfd->wait);
> > spin_lock_irqrestore(&irqfd->wqh.lock, flags);
>
> As I said, you cannot do that from KVM, since write-witers will nedd to be
> woke up.
> Use the eventfd_ctx_remove_wait_queue() below (NOTE: compiled, not tested).
>
>
>
> - Davide
>
Yes, I think this will work. Will test and report. Thanks!
> ---
> fs/eventfd.c | 88 +++++++++++++++++++++++++++++++++++++++---------
> include/linux/eventfd.h | 15 ++++++++
> 2 files changed, 88 insertions(+), 15 deletions(-)
>
> Index: linux-2.6.mod/fs/eventfd.c
> ===================================================================
> --- linux-2.6.mod.orig/fs/eventfd.c 2010-01-06 18:33:27.000000000 -0800
> +++ linux-2.6.mod/fs/eventfd.c 2010-01-10 11:02:00.000000000 -0800
> @@ -135,26 +135,71 @@ static unsigned int eventfd_poll(struct
> 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 +213,31 @@ static ssize_t eventfd_read(struct file
> __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;
> + if ((res = eventfd_ctx_read(ctx, file->f_flags & O_NONBLOCK, &cnt)) < 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)
> Index: linux-2.6.mod/include/linux/eventfd.h
> ===================================================================
> --- linux-2.6.mod.orig/include/linux/eventfd.h 2010-01-06 18:33:27.000000000 -0800
> +++ linux-2.6.mod/include/linux/eventfd.h 2010-01-10 10:57:06.000000000 -0800
> @@ -34,6 +34,9 @@ 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 +64,18 @@ static inline void eventfd_ctx_put(struc
>
> }
>
> +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 */
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