linux-kernel - Re: [PATCH - take 2] knfsd: nfsd: Handle ERESTARTSYS from syscalls.

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Date:	Thu, 19 Jun 2008 06:38:24 -0400
From:	Jeff Layton <jlayton@...hat.com>
To:	Neil Brown <neilb@...e.de>
Cc:	"J. Bruce Fields" <bfields@...ldses.org>,
	linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH - take 2] knfsd: nfsd: Handle ERESTARTSYS from syscalls.

On Thu, 19 Jun 2008 12:29:16 +1000
Neil Brown <neilb@...e.de> wrote:

> On Wednesday June 18, jlayton@...hat.com wrote:
> > 
> > No objection to the patch, but what signal was being sent to nfsd when
> > you saw this? If it's anything but a SIGKILL, then I wonder if we have
> > a race that we need to deal with. My understanding is that we have nfsd
> > flip between 2 sigmasks to prevent anything but a SIGKILL from being
> > delivered while we're handling the local filesystem operation.
> 
> SuSE /etc/init.d/nfsserver does
> 
>         killproc -n -KILL nfsd 
> 
> so it looks like a SIGKILL.
> 
> 
> > 
> > From nfsd():
> > 
> > ----------[snip]-----------
> >                 sigprocmask(SIG_SETMASK, &shutdown_mask, NULL);
> > 
> >                 /*
> >                  * Find a socket with data available and call its
> >                  * recvfrom routine.
> >                  */
> >                 while ((err = svc_recv(rqstp, 60*60*HZ)) == -EAGAIN)
> >                         ;
> >                 if (err < 0)
> >                         break;
> >                 update_thread_usage(atomic_read(&nfsd_busy));
> >                 atomic_inc(&nfsd_busy);
> > 
> >                 /* Lock the export hash tables for reading. */
> >                 exp_readlock();
> > 
> >                 /* Process request with signals blocked.  */
> >                 sigprocmask(SIG_SETMASK, &allowed_mask, NULL);
> > 
> >                 svc_process(rqstp);
> > 
> > ----------[snip]-----------
> > 
> > What happens if this catches a SIGINT after the err<0 check, but before
> > the mask is set to allowed_mask? Does svc_process() then get called with
> > a signal pending?
> 
> Yes, I suspect it does.
> 
> I wonder why we have all this mucking about this signal masks anyway.
> Anyone have any ideas about what it actually achieves?
> 

HCH asked me the same question when I did the conversion to kthreads.
My interpretation (based on guesswork here) was that we wanted to
distinguish between SIGKILL and other allowed signals. A SIGKILL is
allowed to interrupt the underlying I/O, but other signals should not.

The question to answer here, I suppose, is whether masking a pending
signal is sufficient to make signal_pending() return false. If I'm
looking correctly then the answer should be "yes". So I don't think we
have a race here after all. I suspect that if SuSE used a different
signal here, that would prevent this from happening. For the record,
both RHEL and Fedora's init scripts use SIGINT for this.

-- 
Jeff Layton <jlayton@...hat.com>
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