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eventfd: Export eventfd_ctx_do_read()

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[ Upstream commit 28f1326710555bbe666f64452d08f2d7dd657cae ]

Where events are consumed in the kernel, for example by KVM's
irqfd_wakeup() and VFIO's virqfd_wakeup(), they currently lack a
mechanism to drain the eventfd's counter.

Since the wait queue is already locked while the wakeup functions are
invoked, all they really need to do is call eventfd_ctx_do_read().

Add a check for the lock, and export it for them.

Signed-off-by: David Woodhouse <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
Stable-dep-of: 758b49204781 ("eventfd: prevent underflow for eventfd semaphores")
Signed-off-by: Sasha Levin <[email protected]>
This commit is contained in:
David Woodhouse 2020-10-27 13:55:21 +00:00 • committed by Greg Kroah-Hartman
commit 3e9617d63e
2 changed files with 10 additions and 1 deletions
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5
fs/eventfd.c
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@ -181,11 +181,14 @@ static __poll_t eventfd_poll(struct file *file, poll_table *wait)
return events;
}
static void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt)
void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt)
{
lockdep_assert_held(&ctx->wqh.lock);
*cnt = (ctx->flags & EFD_SEMAPHORE) ? 1 : ctx->count;
ctx->count -= *cnt;
}
EXPORT_SYMBOL_GPL(eventfd_ctx_do_read);
/**
* eventfd_ctx_remove_wait_queue - Read the current counter and removes wait queue.