Message-ID: <20080630171019.GA4877@fieldses.org>
Date:	Mon, 30 Jun 2008 13:10:19 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Jeff Layton <jlayton@...hat.com>
Cc:	Neil Brown <neilb@...e.de>, linux-nfs@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH - take 2] knfsd: nfsd: Handle ERESTARTSYS from
	syscalls. (and possible kthread_stop changes)

On Mon, Jun 30, 2008 at 08:35:36AM -0400, Jeff Layton wrote:
> The idea looks reasonable, but the patch above doesn't apply to Bruce's
> for-2.6.27 tree. The nfsd() function has changed a bit. Actually, I
> think we can make this even simpler. kthreads start with all signals
> ignored and the new patch enables all of the ones in SHUTDOWN_SIGS. So
> I don't think we need any signal masking at all.
> 
> How about this patch? It should apply on top of the patchset already
> in Bruce's tree.

Looks reasonable to me; applied for 2.6.27 barring any test failures or
objections.

--b.

> 
> ------------[snip]--------------
> 
> Subject: [PATCH] knfsd: treat all shutdown signals as equivalent
> 
> knfsd currently uses 2 signal masks when processing requests. A "loose"
> mask (SHUTDOWN_SIGS) that it uses when receiving network requests, and
> then a more "strict" mask (ALLOWED_SIGS, which is just SIGKILL) that it
> allows when doing the actual operation on the local storage.
> 
> This is apparently unnecessarily complicated. The underlying filesystem
> should be able to sanely handle a signal in the middle of an operation.
> This patch removes the signal mask handling from knfsd altogether. When
> knfsd is started as a kthread, all signals are ignored. It then allows
> all of the signals in SHUTDOWN_SIGS. There's no need to set the mask
> as well.
> 
> Signed-off-by: Jeff Layton <jlayton@...hat.com>
> ---
>  fs/nfsd/nfssvc.c |   20 ++------------------
>  1 files changed, 2 insertions(+), 18 deletions(-)
> 
> diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
> index 96fdbca..d20a087 100644
> --- a/fs/nfsd/nfssvc.c
> +++ b/fs/nfsd/nfssvc.c
> @@ -37,13 +37,7 @@
>  
>  #define NFSDDBG_FACILITY	NFSDDBG_SVC
>  
> -/* these signals will be delivered to an nfsd thread 
> - * when handling a request
> - */
> -#define ALLOWED_SIGS	(sigmask(SIGKILL))
> -/* these signals will be delivered to an nfsd thread
> - * when not handling a request. i.e. when waiting
> - */
> +/* These signals can be used to shutdown an nfsd thread. */
>  #define SHUTDOWN_SIGS	(sigmask(SIGKILL) | sigmask(SIGHUP) | sigmask(SIGINT) | sigmask(SIGQUIT))
>  
>  extern struct svc_program	nfsd_program;
> @@ -414,7 +408,6 @@ nfsd(void *vrqstp)
>  {
>  	struct svc_rqst *rqstp = (struct svc_rqst *) vrqstp;
>  	struct fs_struct *fsp;
> -	sigset_t shutdown_mask, allowed_mask;
>  	int err, preverr = 0;
>  	unsigned int signo;
>  
> @@ -433,15 +426,12 @@ nfsd(void *vrqstp)
>  	current->fs = fsp;
>  	current->fs->umask = 0;
>  
> -	siginitsetinv(&shutdown_mask, SHUTDOWN_SIGS);
> -	siginitsetinv(&allowed_mask, ALLOWED_SIGS);
> -
>  	/*
>  	 * thread is spawned with all signals set to SIG_IGN, re-enable
>  	 * the ones that matter
>  	 */
>  	for (signo = 1; signo <= _NSIG; signo++) {
> -		if (!sigismember(&shutdown_mask, signo))
> +		if (siginmask(signo, SHUTDOWN_SIGS))
>  			allow_signal(signo);
>  	}
>  
> @@ -460,9 +450,6 @@ nfsd(void *vrqstp)
>  	 * The main request loop
>  	 */
>  	for (;;) {
> -		/* Block all but the shutdown signals */
> -		sigprocmask(SIG_SETMASK, &shutdown_mask, NULL);
> -
>  		/*
>  		 * Find a socket with data available and call its
>  		 * recvfrom routine.
> @@ -487,9 +474,6 @@ nfsd(void *vrqstp)
>  		/* Lock the export hash tables for reading. */
>  		exp_readlock();
>  
> -		/* Process request with signals blocked. */
> -		sigprocmask(SIG_SETMASK, &allowed_mask, NULL);
> -
>  		svc_process(rqstp);
>  
>  		/* Unlock export hash tables */
> -- 
> 1.5.5.1
> 
> 
> 
> > > 
> > > The latest patch doesn't use kthread_stop with nfsd. I figured it was
> > > best to avoid having multiple takedown methods, and since we're using
> > > signals here anyway, I just kept signaling as the only way to take down
> > > nfsd. Plus, as you mention -- all kthread_stop's are serialized, which
> > > could have meant that nfsd's would take a long time to come down on a
> > > busy box with a lot of them.
> > 
> > Oh, good.  I must have been looking at an old patch.
> > 
> > > 
> > > As a side note, I'm not terribly thrilled with how kthread_stop is
> > > implemented. I worry that a stuck kthread would block unrelated
> > > kthreads from coming down. I did a patch 6 mos ago or so to make it so
> > > that kthread_stop's could be done in parallel. The downside was that it
> > > added a new field to the task_struct (which is already too bloated,
> > > IMO). It might be worth resurrecting that at some point (possibly
> > > making the new field a union with another field that's unused in
> > > kthreads), or considering a different approach to parallelize
> > > kthread_stop.
> > 
> > One the one hand, kthreads are expected to not block and to check for
> > kthread_should_stop very often, so this wouldn't be a problem.
> > On the other hand, expectations like this are quickly invalidated, and
> > the code probably should be fixed.
> > I suspect we could do without adding anything to task_struct.
> > Instead of just one kthread_stop_info, have a linked list, one entry
> > for each thread being stopped at the moment.  kthread_stop adds to
> > the list, sets PF_EXITING (I hope that is safe) and wakes the process.
> > kthread_should_stop tests PF_EXITING.
> > When kthread() finishes, it does a linear search (the list should be
> > short) and calls complete and the relevant .done.
> > 
> > Something like this?
> > 
> > NeilBrown
> > 
> > 
> > diff --git a/kernel/kthread.c b/kernel/kthread.c
> > index bd1b9ea..107290a 100644
> > --- a/kernel/kthread.c
> > +++ b/kernel/kthread.c
> > @@ -39,12 +39,13 @@ struct kthread_stop_info
> >  	struct task_struct *k;
> >  	int err;
> >  	struct completion done;
> > +	struct kthread_stp_info *next;
> >  };
> >  
> >  /* Thread stopping is done by setthing this var: lock serializes
> >   * multiple kthread_stop calls. */
> >  static DEFINE_MUTEX(kthread_stop_lock);
> > -static struct kthread_stop_info kthread_stop_info;
> > +static struct kthread_stop_info *kthread_stop_info;
> >  
> >  /**
> >   * kthread_should_stop - should this kthread return now?
> > @@ -55,7 +56,7 @@ static struct kthread_stop_info kthread_stop_info;
> >   */
> >  int kthread_should_stop(void)
> >  {
> > -	return (kthread_stop_info.k == current);
> > +	return test_bit(PF_EXITING, &current.flags);
> >  }
> >  EXPORT_SYMBOL(kthread_should_stop);
> >  
> > @@ -80,8 +81,17 @@ static int kthread(void *_create)
> >  
> >  	/* It might have exited on its own, w/o kthread_stop.  Check. */
> >  	if (kthread_should_stop()) {
> > -		kthread_stop_info.err = ret;
> > -		complete(&kthread_stop_info.done);
> > +		struct kthread_stop_info **ksip;
> > +		mutex_lock(&kthread_stop_lock);
> > +		for (ksip = &kthread_stop_info ; *ksip ; ksip = &(*ksip)->next)
> > +			if ((*ksip)->k == current)
> > +				break;
> > +		*ksip = (*ksip)->next;
> > +		(*ksip)->next = NULL;
> > +		mutex_unlock(&kthread_stop_lock);
> > +
> > +		ksi->err = ret;
> > +		complete(&ksi->done);
> >  	}
> >  	return 0;
> >  }
> > @@ -199,26 +209,20 @@ EXPORT_SYMBOL(kthread_bind);
> >  int kthread_stop(struct task_struct *k)
> >  {
> >  	int ret;
> > +	struct kthread_stop_info ksi;
> >  
> >  	mutex_lock(&kthread_stop_lock);
> >  
> > -	/* It could exit after stop_info.k set, but before wake_up_process. */
> > -	get_task_struct(k);
> > +	init_completion(&ksi->done);
> > +	ksi->k = k;
> >  
> > -	/* Must init completion *before* thread sees kthread_stop_info.k */
> > -	init_completion(&kthread_stop_info.done);
> > -	smp_wmb();
> > -
> > -	/* Now set kthread_should_stop() to true, and wake it up. */
> > -	kthread_stop_info.k = k;
> > +	set_bit(PF_EXITING, &k->flags);
> >  	wake_up_process(k);
> > -	put_task_struct(k);
> > +	mutex_unlock(&kthread_stop_lock);
> >  
> > -	/* Once it dies, reset stop ptr, gather result and we're done. */
> > -	wait_for_completion(&kthread_stop_info.done);
> > -	kthread_stop_info.k = NULL;
> > +	/* Once it dies gather result and we're done. */
> > +	wait_for_completion(&ksi->done);
> >  	ret = kthread_stop_info.err;
> > -	mutex_unlock(&kthread_stop_lock);
> >  
> >  	return ret;
> >  }
> 
> 
> That looks reasonable to me, though I tend to find standard list
> functions easier to follow...
> 
> The main reason I was going to add a field to the task_struct was
> because I wanted to make sure that kthread_should_stop() was a quick
> operation. If we can just use PF_EXITING then that's definitely a
> better scheme.
> 
> -- 
> Jeff Layton <jlayton@...hat.com>
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