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Subject: Re: [GIT] Networking
From: Eric Dumazet <eric.dumazet@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: David Miller <davem@davemloft.net>, akpm@linux-foundation.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
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	 <AANLkTin32jUnBtHF-EAkSMnFQM6cTbn-G3NZQJ3xjTWU@mail.gmail.com>
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Date: Thu, 20 Jan 2011 22:40:46 +0100
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Le jeudi 20 janvier 2011 à 13:12 -0800, Linus Torvalds a écrit :



>   ...
>   read(3, 0x9806500, 4096)                = -1 EAGAIN (Resource
> temporarily unavailable)
>   poll([{fd=4, events=POLLIN}, {fd=3, events=POLLIN}, {fd=12,
> events=POLLIN|POLLPRI}, {fd=14, events=POLLIN|POLLPRI}, {fd=9,
> events=POLLIN|POLLPRI}, {fd=10, events=POLLIN|POLLPRI}, {fd=15,
> events=POLLIN}, {fd=16, events=POLLIN}, {fd=17, events=0}, {fd=19,
> events=POLLIN}], 10, -1) = ? ERESTART_RESTARTBLOCK (To be restarted)
>   restart_syscall(
> 

Hmm, poll() here on fd=3 only asks events=POLLIN

> and that's it - it's now hung. So why did it work when I straced it
> while hung? And why is it doing that ERESTART_RESTARTBLOCK in the
> first place, I'm not seeing any signals there?
> 
> So I tried sending it a useless signal, which will re-animate the
> strace, and now I get:
> 
>   restart_syscall(<... resuming interrupted call ...>) = 1
>   --- SIGWINCH (Window changed) @ 0 (0) ---
>   poll([{fd=3, events=POLLIN|POLLOUT}], 1, -1) = 1 ([{fd=3, revents=POLLOUT}])
> 

Then here ask for fd=3 both POLLIN and POLLOUT :
kernel answers : OK for POLLOUT (not POLLIN), so previous poll() call
was OK to be blocked after all...

So I'm wondering if it could be a userland bug, that triggers with
recent kernel changes.