Re: [PATCH] rfc: threaded epoll_wait thundering herd

[Davide Libenzi]

On Mon, 7 May 2007, Ulrich Drepper wrote:

> On 5/5/07, Davi Arnaut <davi@xxxxxxxxxxxxx> wrote:
> > A google search turns up a few users. It also addresses some complaints
> > from Drepper.
> 
> There is a huge problem with this approach and we're back at the
> inadequate interface.
> 
> select/poll/epoll are thread cancellation points.  I.e., the thread
> can be canceled before returning to the user.  If this cancellation
> happens between the kernel deciding to give this thread the event (and
> no other thread) and the thread testing for cancellation in the libc
> wrapper around the syscall, then the event is lost and the process(es)
> might hang.
> 
> With kevent we in the end fixed the problem by requiring that part of
> the cancellation handling the thread tries to wake up another thread
> waiting for the event queue.  This is easily possible since the event
> data is in the shared memory segment and it's just purely the thread
> wakeup that is needed.

So, by the same logic, every API that 1) returns something to userspace 
by canceling its internal kernel state 2) is not based on shared 
kernel/userspace memory, will break under your assumptions.
Scary, because there's a pretty long list.



- Davide


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