linux-kernel - Re: [PATCH] rfc: threaded epoll

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Date:	Mon, 7 May 2007 15:35:38 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Ulrich Drepper <drepper@...il.com>
cc:	Davi Arnaut <davi@...ent.com.br>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rfc: threaded epoll_wait thundering herd

On Mon, 7 May 2007, Ulrich Drepper wrote:

> On 5/7/07, Davi Arnaut <davi@...ent.com.br> wrote:
> > See Linus's message on this same thread.
> 
> No.  I'm talking about the userlevel side, not kernel side.
> 
> If a thread is canceled *after* it returns from the syscall but before
> it reports the event to the call (i.e., while still in the syscall
> wrapper, thread cancellation rules require a check there) the event is
> lost.

read(2) is a cancellation point too. So if the fine userspace code issues 
a random pthread_cancel() to a thread handling that, data is lost together 
with the session that thread was handling. Hmm, I wonder how the world 
could have functioned so far.
Bottom line is, if you really want to throw random cancels to your worker 
threads, you better wrap them into pthread_cleanup_push(). Because 
otherwise, no matter where your cancel hits, you end up with a broken 
system.

- Davide

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