Re: [PATCH] sunrpc.ko: RPC cache fix races

Re: [PATCH] sunrpc.ko: RPC cache fix races

[NeilBrown]

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On 20 Feb 2013 14:57:07 +0100 bstroesser@ts.fujitsu.com wrote:

> On 20 Feb 2013 04:09:00 +0100 neilb@suse.de wrote:
> 
> > On 19 Feb 2013 18:08:40 +0100 bstroesser@ts.fujitsu.com wrote:
> > 
> > > Second attempt using the correct FROM. Sorry for the noise.
> > > 
> > > 
> > > Hi,
> > > 
> > > I found a problem in sunrpc.ko on a SLES11 SP1 (2.6.32.59-0,7.1) and 
> > > fixed it (see first patch ifor 2.6.32.60 below).
> > > For us the patch works fine (tested on 2.6.32.59-0.7.1).
> > > 
> > > AFAICS from the code, the problem seems to persist in current kernel 
> > > versions also. Thus, I added the second patch for 3.7.9.
> > > As the setup to reproduce the problem is quite complex, I couldn't 
> > > test the second patch yet. So consider this one as a RFC.
> > > 
> > > Best regards,
> > > Bodo
> > > 
> > > Please CC me, I'm not on the list.
> > > 
> > > =========================================
> > > From: Bodo Stroesser <bstroesser@ts.fujitsu.com>
> > > Date: Fri, 08 Feb 2013
> > > Subject: [PATCH] net: sunrpc: fix races in RPC cache
> > > 
> > > We found the problem and tested the patch on a SLES11 SP1 
> > > 2.6.32.59-0.7.1
> > > 
> > > This patch applies to linux-2.6.32.60 (changed monotonic_seconds -->
> > > get_seconds())
> > > 
> > > Sporadically NFS3 RPC requests to the nfs server are dropped due to
> > > cache_check() (net/sunrpc/cache.c) returning -ETIMEDOUT for an entry 
> > > of the "auth.unix.gid" cache.
> > > In this case, no NFS reply is sent to the client.
> > > 
> > > The reason for the dropped requests are races in cache_check() when
> > > cache_make_upcall() returns -EINVAL (because it is called for a cache 
> > > without readers) and cache_check() therefore refreshes the cache entry 
> > > (rv == -EAGAIN).
> > > 
> > > There are three details that need to be changed:
> > >  1) cache_revisit_request() must not be called before cache_fresh_locked()
> > >     has updated the cache entry, as cache_revisit_request() wakes up
> > >     threads waiting for the cache entry to be updated.
> > 
> > This certainly seems correct.  It is wrong to call cache_revisit_request() so early.
> > 
> > >     The explicit call to cache_revisit_request() is not needed, as
> > >     cache_fresh_unlocked() calls it anyway.
> > 
> > But cache_fresh_unlocked is only called if "rv == -EAGAIN", however we also need to call it in the case where "age > refresh_age/2" - it must always be called after clearing CACHE_PENDING.
> > 
> > Also, cache_fresh_unlocked() only calls cache_revisit_request() if CACHE_PENDING is set, but we have just cleared it!  Some definitely something wrong here.
> > (Note that I'm looking at the SLES 2.6.32 code at the moment, mainline is a bit different).
> > 
> > 
> > >     (But in case of not updating the cache entry, cache_revisit_request()
> > >     must be called. Thus, we use a fall through in the "case").
> > 
> > Hmm... I don't like case fallthroughs unless they have nice big comments:
> >     /* FALLTHROUGH */
> > or similar. :-)
> > 
> > >  2) CACHE_PENDING must not be cleared before cache_fresh_locked() has
> > >     updated the cache entry, as cache_defer_req() can return without really
> > >     sleeping if it detects CACHE_PENDING unset.
> > 
> > Agreed.  So we should leave the clearing of CACHE_PENDING to cache_fresh_unlocked().
> > 
> > 
> > >     (In case of not updating the cache entry again we use the fall 
> > > through)
> > >  3) Imagine a thread that calls cache_check() and gets rv = -ENOENT from
> > >     cache_is_valid(). Then it sets CACHE_PENDINGs and calls
> > >     cache_make_upcall().
> > >     We assume that meanwhile get_seconds() advances to the next
> > >     sec. and a second thread also calls cache_check(). It gets -EAGAIN from
> > >     cache_is_valid() for the same cache entry. As CACHE_PENDING still is
> > >     set, it calls cache_defer_req() immediately and waits for a wakeup from
> > >     the first thread.
> > >     After cache_make_upcall() returned -EINVAL, the first thread does not
> > >     update the cache entry as it had got rv = -ENOENT, but wakes up the
> > >     second thread by calling cache_revisit_request().
> > >     Thread two wakes up, calls cache_is_valid() and again gets -EAGAIN.
> > >     Thus, the result of the second cache_check() is -ETIMEDOUT and the
> > >     NFS request is dropped.
> > 
> > Yep, that's not so good....
> > 
> > 
> > >     To solve this, the cache entry now is updated not only if rv == -EAGAIN
> > >     but if rv == -ENOENT also. This is a sufficient workaround, as the
> > >     first thread would have to stay in cache_check() between its call to
> > >     cache_is_valid() and clearing CACHE_PENDING for more than 60 seconds
> > >     to break the workaround.
> > 
> > Still, it isn't nice to just have a work-around.  It would be best to have a fix.
> > The key problem here is that cache_is_valid() is time-sensitive.  This have been address in mainline - cache_is_valid doesn't depend on the current time there.
> 
> So, the solution would be a backport of the current mainline code ...

That would always be my preference, when it is practical.

> 
> Anyway, I think for SLES11 SP1 and 2.6.32.60 the work-around would be sufficient.
> BTW: it has the positive side effect, that - while a cache entry is in its second
> half of life - no longer each cache_check() tries to do a cache_make_upcall().

I agree that is an improvement.

> 
> 
> > 
> > 
> > >     
> > > Signed-off-by: Bodo Stroesser <bstroesser@ts.fujitsu.com>
> > > ---
> > > 
> > > --- a/net/sunrpc/cache.c	2012-08-08 21:35:09.000000000 +0200
> > > +++ b/net/sunrpc/cache.c	2013-02-08 14:29:41.000000000 +0100
> > > @@ -233,15 +233,14 @@ int cache_check(struct cache_detail *det
> > >  		if (!test_and_set_bit(CACHE_PENDING, &h->flags)) {
> > >  			switch (cache_make_upcall(detail, h)) {
> > >  			case -EINVAL:
> > > -				clear_bit(CACHE_PENDING, &h->flags);
> > > -				cache_revisit_request(h);
> > > -				if (rv == -EAGAIN) {
> > > +				if (rv == -EAGAIN || rv == -ENOENT) {
> > >  					set_bit(CACHE_NEGATIVE, &h->flags);
> > >  					cache_fresh_locked(h, get_seconds()+CACHE_NEW_EXPIRY);
> > > +					clear_bit(CACHE_PENDING, &h->flags);
> > >  					cache_fresh_unlocked(h, detail);
> > >  					rv = -ENOENT;
> > > +					break;
> > >  				}
> > > -				break;
> > >  
> > >  			case -EAGAIN:
> > >  				clear_bit(CACHE_PENDING, &h->flags);
> > 
> > I agree with some of this....
> > Maybe:
> > 
> >   switch(cache_make_upcall(detail, h)) {
> >   case -EINVAL:
> >         if (rv) {
> > 		set_bit(CACHE_NEGATIVE, &h->flags);
> > 		cache_fresh_locked(h, get_seconds() + CACHE_NEW_EXPIRY);
> > 		rv = -ENOENT;
> > 	}
> > 	/* FALLTHROUGH */
> >   case -EAGAIN:
> > 	cache_fresh_unlocked(h, detail);
> >   }
> 
> I agree, your patch is obviously better than the mine.
> But let me suggest one little change: I would like to substitute
> cache_fresh_unlocked() by clear_bit() and cache_revisit_request(),
> as the call to cache_dequeue() in cache_fresh_unlocked() seems to
> be obsolete here:

It is exactly this sort of thinking (on my part) that got us into this mess
in the first place.  I reasoned that the full locking/testing/whatever wasn't
necessary and took a short cut.  It wasn't a good idea.

Given that this is obviously difficult code to get right, we should make it
as easy to review as possible.  Have "cache_fresh_unlocked" makes it more
obviously correct, and that is a good thing.
Maybe cache_dequeue isn't needed here, but it won't hurt so I'd much rather
have the clearer code.
In fact, I'd also like to change

			if (test_and_clear_bit(CACHE_PENDING, &ch->flags))
				cache_dequeue(current_detail, ch);
			cache_revisit_request(ch);

near the end of cache_clean to call  cache_fresh_unlocked().


NeilBrown


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Re: [PATCH] sunrpc.ko: RPC cache fix races

[NeilBrown]

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On 25 Feb 2013 20:55:50 +0100 Bodo Stroesser <bstroesser@ts.fujitsu.com>
wrote:

> On 24 Feb 2013 23:53:00 +0100 neilb@suse.de wrote:
> 
> > On 21 Feb 2013 16:44:41 +0100 Bodo Stroesser <bstroesser@ts.fujitsu.com>
> > wrote:
> > 
> > > On 21 Feb 2013 00:55:00 +0100 neilb@suse.de wrote:
> > > 
> > > > On 20 Feb 2013 14:57:07 +0100 bstroesser@ts.fujitsu.com wrote:
> > > > 
> > > > > On 20 Feb 2013 04:09:00 +0100 neilb@suse.de wrote:
> > > 
> > > snip
> > > 
> > > > > > Maybe:
> > > > > > 
> > > > > >   switch(cache_make_upcall(detail, h)) {
> > > > > >   case -EINVAL:
> > > > > >         if (rv) {
> > > > > > 		set_bit(CACHE_NEGATIVE, &h->flags);
> > > > > > 		cache_fresh_locked(h, get_seconds() + CACHE_NEW_EXPIRY);
> > > > > > 		rv = -ENOENT;
> > > > > > 	}
> > > > > > 	/* FALLTHROUGH */
> > > > > >   case -EAGAIN:
> > > > > > 	cache_fresh_unlocked(h, detail);
> > > > > >   }
> > > > > 
> > > > > I agree, your patch is obviously better than the mine.
> > > > > But let me suggest one little change: I would like to substitute
> > > > > cache_fresh_unlocked() by clear_bit() and cache_revisit_request(),
> > > > > as the call to cache_dequeue() in cache_fresh_unlocked() seems to
> > > > > be obsolete here:
> > > > 
> > > > It is exactly this sort of thinking (on my part) that got us into this mess
> > > > in the first place.  I reasoned that the full locking/testing/whatever wasn't
> > > > necessary and took a short cut.  It wasn't a good idea.
> > > 
> > > Maybe I'm totally wrong, but AFAICS, calling cache_dequeue() here in extreme
> > > situations (two threads in parallel calling check_cache() while a first reader
> > > is going to open cache access) could again cause a race (?)
> > 
> > Can you explain the race you see?
> 
> O.k., let me try ...
> 
> Suppposed there are 2 threads calling cache_check concurrently and a
> user process that is going to become a reader for the involved cache.
> 
> The first thread calls cache_is_valid() and gets -EAGAIN.
> Next, it sets CACHE_PENDING and calls cache_make_upcall(), which
> returns -EINVAL, as there is no reader yet.
> 
> Meanwhile the second thread also calls cache_is_valid(), also gets
> -EAGAIN, but now is interrupted and delayed for a while.
> 
> The first thread continues its work, negates the entry and calls
> cache_fresh_locked() followed by cache_fresh_unlocked().
> In cache_fresh_unlocked it resets CACHE_PENDING and calls
> cache_revisit_request(). While this, it is interrupted and delayed.
> 
> The user process is scheduled and opens the channel to become a reader.
> 
> The second thread wakes up again and sets CACHE_PENDING. Next it calls
> cache_make_upcall(), which results in a request being queued, as there
> is a reader now.
> 
> The first thread wakes up and continues its work calling cache_dequeue().
> Maybe the request is dropped before the reader could process it.
> 
> Do you agree or did I miss something?

Yes, I see what you mean, thanks.

I would close that race with:
@@ -1022,6 +1016,9 @@ static void cache_dequeue(struct cache_detail *detail, struct cache_head *ch)
 			struct cache_request *cr = container_of(cq, struct cache_request, q);
 			if (cr->item != ch)
 				continue;
+			if (test_bit(CACHE_PENDING, &ch->flags))
+				/* Lost a race and it is pending again */
+				break;
 			if (cr->readers != 0)
 				continue;
 			list_del(&cr->q.list);

> 
> 
> > 
> > > 
> > > BTW: if there is a reader for a cache, is there any protection against many
> > > upcalls being queued for the same cache entry?
> > 
> > The CACHE_PENDING flag should provide that protection.  We only queue an
> > upcall after "test_and_set", and always dequeue after "clear_bit".
> 
> Sorry, my question wasn't very clear. CACHE_PENDING *does* provide the
> protection against parallel upcalls.
> 
> OTOH, as cache_is_valid() is called before test_and_set_bit(CACHE_PENDING)
> and then the upcall is made without again calling cache_is_valid(),
> I see a small chance of a second request being queued unnecessarily just
> after the first request was answered. 

A small chance, yes.  However  I don't think it is harmful, just unnecessary.
So I'm not sure it is worth fixing.

Thanks,
NeilBrown


> 
> Bodo
> 
> > 
> > NeilBrown
> > 
> > 
> > > 
> > > Bodo
> > > 
> > > > 
> > > > Given that this is obviously difficult code to get right, we should make it
> > > > as easy to review as possible.  Have "cache_fresh_unlocked" makes it more
> > > > obviously correct, and that is a good thing.
> > > > Maybe cache_dequeue isn't needed here, but it won't hurt so I'd much rather
> > > > have the clearer code.
> > > > In fact, I'd also like to change
> > > > 
> > > > 			if (test_and_clear_bit(CACHE_PENDING, &ch->flags))
> > > > 				cache_dequeue(current_detail, ch);
> > > > 			cache_revisit_request(ch);
> > > > 
> > > > near the end of cache_clean to call  cache_fresh_unlocked().
> > > > 


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Re: [PATCH] sunrpc.ko: RPC cache fix races

[Bodo Stroesser]

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Cg==

Re: [PATCH] sunrpc.ko: RPC cache fix races

[NeilBrown]

[-- Attachment #1: Type: text/plain, Size: 2389 bytes --]

On 21 Feb 2013 16:44:41 +0100 Bodo Stroesser <bstroesser@ts.fujitsu.com>
wrote:

> On 21 Feb 2013 00:55:00 +0100 neilb@suse.de wrote:
> 
> > On 20 Feb 2013 14:57:07 +0100 bstroesser@ts.fujitsu.com wrote:
> > 
> > > On 20 Feb 2013 04:09:00 +0100 neilb@suse.de wrote:
> 
> snip
> 
> > > > Maybe:
> > > > 
> > > >   switch(cache_make_upcall(detail, h)) {
> > > >   case -EINVAL:
> > > >         if (rv) {
> > > > 		set_bit(CACHE_NEGATIVE, &h->flags);
> > > > 		cache_fresh_locked(h, get_seconds() + CACHE_NEW_EXPIRY);
> > > > 		rv = -ENOENT;
> > > > 	}
> > > > 	/* FALLTHROUGH */
> > > >   case -EAGAIN:
> > > > 	cache_fresh_unlocked(h, detail);
> > > >   }
> > > 
> > > I agree, your patch is obviously better than the mine.
> > > But let me suggest one little change: I would like to substitute
> > > cache_fresh_unlocked() by clear_bit() and cache_revisit_request(),
> > > as the call to cache_dequeue() in cache_fresh_unlocked() seems to
> > > be obsolete here:
> > 
> > It is exactly this sort of thinking (on my part) that got us into this mess
> > in the first place.  I reasoned that the full locking/testing/whatever wasn't
> > necessary and took a short cut.  It wasn't a good idea.
> 
> Maybe I'm totally wrong, but AFAICS, calling cache_dequeue() here in extreme
> situations (two threads in parallel calling check_cache() while a first reader
> is going to open cache access) could again cause a race (?)

Can you explain the race you see?

> 
> BTW: if there is a reader for a cache, is there any protection against many
> upcalls being queued for the same cache entry?

The CACHE_PENDING flag should provide that protection.  We only queue an
upcall after "test_and_set", and always dequeue after "clear_bit".

NeilBrown


> 
> Bodo
> 
> > 
> > Given that this is obviously difficult code to get right, we should make it
> > as easy to review as possible.  Have "cache_fresh_unlocked" makes it more
> > obviously correct, and that is a good thing.
> > Maybe cache_dequeue isn't needed here, but it won't hurt so I'd much rather
> > have the clearer code.
> > In fact, I'd also like to change
> > 
> > 			if (test_and_clear_bit(CACHE_PENDING, &ch->flags))
> > 				cache_dequeue(current_detail, ch);
> > 			cache_revisit_request(ch);
> > 
> > near the end of cache_clean to call  cache_fresh_unlocked().
> > 


[-- Attachment #2: signature.asc --]
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Re: [PATCH] sunrpc.ko: RPC cache fix races

[Bodo Stroesser]

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Re: [PATCH] sunrpc.ko: RPC cache fix races

[bstroesser]

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Re: [PATCH] sunrpc.ko: RPC cache fix races

[NeilBrown]

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On 19 Feb 2013 18:08:40 +0100 bstroesser@ts.fujitsu.com wrote:

> Second attempt using the correct FROM. Sorry for the noise.
> 
> 
> Hi,
> 
> I found a problem in sunrpc.ko on a SLES11 SP1 (2.6.32.59-0,7.1)
> and fixed it (see first patch ifor 2.6.32.60 below).
> For us the patch works fine (tested on 2.6.32.59-0.7.1).
> 
> AFAICS from the code, the problem seems to persist in current
> kernel versions also. Thus, I added the second patch for 3.7.9.
> As the setup to reproduce the problem is quite complex, I couldn't
> test the second patch yet. So consider this one as a RFC.
> 
> Best regards,
> Bodo
> 
> Please CC me, I'm not on the list.
> 
> =========================================
> From: Bodo Stroesser <bstroesser@ts.fujitsu.com>
> Date: Fri, 08 Feb 2013
> Subject: [PATCH] net: sunrpc: fix races in RPC cache
> 
> We found the problem and tested the patch on a SLES11 SP1 2.6.32.59-0.7.1
> 
> This patch applies to linux-2.6.32.60 (changed monotonic_seconds -->
> get_seconds())
> 
> Sporadically NFS3 RPC requests to the nfs server are dropped due to
> cache_check() (net/sunrpc/cache.c) returning -ETIMEDOUT for an entry
> of the "auth.unix.gid" cache.
> In this case, no NFS reply is sent to the client.
> 
> The reason for the dropped requests are races in cache_check() when
> cache_make_upcall() returns -EINVAL (because it is called for a cache
> without readers) and cache_check() therefore refreshes the cache entry 
> (rv == -EAGAIN).
> 
> There are three details that need to be changed:
>  1) cache_revisit_request() must not be called before cache_fresh_locked()
>     has updated the cache entry, as cache_revisit_request() wakes up
>     threads waiting for the cache entry to be updated.

This certainly seems correct.  It is wrong to call cache_revisit_request() so
early.

>     The explicit call to cache_revisit_request() is not needed, as
>     cache_fresh_unlocked() calls it anyway.

But cache_fresh_unlocked is only called if "rv == -EAGAIN", however we also
need to call it in the case where "age > refresh_age/2" - it must always be
called after clearing CACHE_PENDING.

Also, cache_fresh_unlocked() only calls cache_revisit_request() if
CACHE_PENDING is set, but we have just cleared it!  Some definitely something
wrong here.
(Note that I'm looking at the SLES 2.6.32 code at the moment, mainline is a
bit different).


>     (But in case of not updating the cache entry, cache_revisit_request()
>     must be called. Thus, we use a fall through in the "case").

Hmm... I don't like case fallthroughs unless they have nice big comments:
    /* FALLTHROUGH */
or similar. :-)

>  2) CACHE_PENDING must not be cleared before cache_fresh_locked() has
>     updated the cache entry, as cache_defer_req() can return without really
>     sleeping if it detects CACHE_PENDING unset.

Agreed.  So we should leave the clearing of CACHE_PENDING to
cache_fresh_unlocked().


>     (In case of not updating the cache entry again we use the fall through)
>  3) Imagine a thread that calls cache_check() and gets rv = -ENOENT from
>     cache_is_valid(). Then it sets CACHE_PENDINGs and calls
>     cache_make_upcall().
>     We assume that meanwhile get_seconds() advances to the next
>     sec. and a second thread also calls cache_check(). It gets -EAGAIN from
>     cache_is_valid() for the same cache entry. As CACHE_PENDING still is
>     set, it calls cache_defer_req() immediately and waits for a wakeup from
>     the first thread.
>     After cache_make_upcall() returned -EINVAL, the first thread does not
>     update the cache entry as it had got rv = -ENOENT, but wakes up the
>     second thread by calling cache_revisit_request().
>     Thread two wakes up, calls cache_is_valid() and again gets -EAGAIN.
>     Thus, the result of the second cache_check() is -ETIMEDOUT and the
>     NFS request is dropped.

Yep, that's not so good....


>     To solve this, the cache entry now is updated not only if rv == -EAGAIN
>     but if rv == -ENOENT also. This is a sufficient workaround, as the
>     first thread would have to stay in cache_check() between its call to
>     cache_is_valid() and clearing CACHE_PENDING for more than 60 seconds
>     to break the workaround.

Still, it isn't nice to just have a work-around.  It would be best to have a
fix.
The key problem here is that cache_is_valid() is time-sensitive.  This have
been address in mainline - cache_is_valid doesn't depend on the current time
there.


>     
> Signed-off-by: Bodo Stroesser <bstroesser@ts.fujitsu.com>
> ---
> 
> --- a/net/sunrpc/cache.c	2012-08-08 21:35:09.000000000 +0200
> +++ b/net/sunrpc/cache.c	2013-02-08 14:29:41.000000000 +0100
> @@ -233,15 +233,14 @@ int cache_check(struct cache_detail *det
>  		if (!test_and_set_bit(CACHE_PENDING, &h->flags)) {
>  			switch (cache_make_upcall(detail, h)) {
>  			case -EINVAL:
> -				clear_bit(CACHE_PENDING, &h->flags);
> -				cache_revisit_request(h);
> -				if (rv == -EAGAIN) {
> +				if (rv == -EAGAIN || rv == -ENOENT) {
>  					set_bit(CACHE_NEGATIVE, &h->flags);
>  					cache_fresh_locked(h, get_seconds()+CACHE_NEW_EXPIRY);
> +					clear_bit(CACHE_PENDING, &h->flags);
>  					cache_fresh_unlocked(h, detail);
>  					rv = -ENOENT;
> +					break;
>  				}
> -				break;
>  
>  			case -EAGAIN:
>  				clear_bit(CACHE_PENDING, &h->flags);

I agree with some of this....
Maybe:

  switch(cache_make_upcall(detail, h)) {
  case -EINVAL:
        if (rv) {
		set_bit(CACHE_NEGATIVE, &h->flags);
		cache_fresh_locked(h, get_seconds() + CACHE_NEW_EXPIRY);
		rv = -ENOENT;
	}
	/* FALLTHROUGH */
  case -EAGAIN:
	cache_fresh_unlocked(h, detail);
  }

Though it isn't good that cache_fresh_locked() is being called without
holding a lock!  Maybe we should import try_to_negate_entry() from mainline.

> =========================================
> From: Bodo Stroesser <bstroesser@ts.fujitsu.com>
> Date: Fri, 08 Feb 2013
> Subject: [PATCH] net: sunrpc: fix races in RPC cache
> 
> This patch applies to SLES 11 SP2 linux-3.0.51-0.7.9 and also to
> vanilla linux-3.7.9
> 
> It is untested and is only based on a code review after we
> analyzed the reason for NFS requests being dropped on a
> SLES11 SP1 (linux-2.6.32.59-0.7.1)
> 
> Sporadically NFS3 RPC requests to the nfs server are dropped due to
> cache_check() (net/sunrpc/cache.c) returning -ETIMEDOUT for an entry
> of the "auth.unix.gid" cache.
> In this case, no NFS reply is sent to the client.
> 
> The reason for the dropped requests are races in cache_check() when
> cache_make_upcall() returns -EINVAL (because it is called for a cache
> without readers) and cache_check() therefore refreshes the cache entry 
> (rv == -EAGAIN).
> 
> There are two details that need to be changed:
>  1) cache_revisit_request() must not be called before cache_fresh_locked()
>     has updated the cache entry, as cache_revisit_request() wakes up
>     threads waiting for the cache entry to be updated.
>     The explicit call to cache_revisit_request() is not needed, as
>     cache_fresh_unlocked() calls it anyway.
>     (But in case of not updating the cache entry, cache_revisit_request()
>     must be called).
>  2) CACHE_PENDING must not be cleared before cache_fresh_locked() has
>     updated the cache entry, as cache_wait_req() called by
>     cache_defer_req() can return without really sleeping if it detects
>     CACHE_PENDING unset.
> 
> Signed-off-by: Bodo Stroesser <bstroesser@ts.fujitsu.com>
> ---
> 
> --- a/net/sunrpc/cache.c	2013-02-08 15:56:07.000000000 +0100
> +++ b/net/sunrpc/cache.c	2013-02-08 16:04:32.000000000 +0100
> @@ -230,11 +230,14 @@ static int try_to_negate_entry(struct ca
>  	rv = cache_is_valid(detail, h);
>  	if (rv != -EAGAIN) {
>  		write_unlock(&detail->hash_lock);
> +		clear_bit(CACHE_PENDING, &h->flags);
> +		cache_revisit_request(h);

This should just be cache_fresh_unlocked(), as below.
>  		return rv;
>  	}
>  	set_bit(CACHE_NEGATIVE, &h->flags);
>  	cache_fresh_locked(h, seconds_since_boot()+CACHE_NEW_EXPIRY);
>  	write_unlock(&detail->hash_lock);
> +	clear_bit(CACHE_PENDING, &h->flags);
Clearing this bit is wrong  - cache_frsh_unlocked will do that.

>  	cache_fresh_unlocked(h, detail);
>  	return -ENOENT;
>  }
So maybe:

static int try_to_negate_entry(....)
{
	int rv;

	write_lock(&detail->hash_lock);
	rv = cache_is_valid(detail, h);
	if (rv == -EAGAIN) {
		set_bit(CACHE_NEGATIVE, &h->flags);
		cache_fresh_locked(h, ....);
		rv = -ENOENT;
	}
	write_unlock(&detail->hash_lock);
	cache_fresh_unlocked(h, detail);
	return rv;
}
????


> @@ -275,8 +278,6 @@ int cache_check(struct cache_detail *det
>  		if (!test_and_set_bit(CACHE_PENDING, &h->flags)) {
>  			switch (cache_make_upcall(detail, h)) {
>  			case -EINVAL:
> -				clear_bit(CACHE_PENDING, &h->flags);
> -				cache_revisit_request(h);
>  				rv = try_to_negate_entry(detail, h);
>  				break;
>  			case -EAGAIN:

Yes, those lines should definitely be removed.

So maybe this against mainline:

diff --git a/net/sunrpc/cache.c b/net/sunrpc/cache.c
index 9afa439..7296644 100644
--- a/net/sunrpc/cache.c
+++ b/net/sunrpc/cache.c
@@ -228,12 +228,11 @@ static int try_to_negate_entry(struct cache_detail *detail, struct cache_head *h
 
 	write_lock(&detail->hash_lock);
 	rv = cache_is_valid(detail, h);
-	if (rv != -EAGAIN) {
-		write_unlock(&detail->hash_lock);
-		return rv;
+	if (rv == -EAGAIN) {
+		set_bit(CACHE_NEGATIVE, &h->flags);
+		cache_fresh_locked(h, seconds_since_boot()+CACHE_NEW_EXPIRY);
+		rv = -ENOENT;
 	}
-	set_bit(CACHE_NEGATIVE, &h->flags);
-	cache_fresh_locked(h, seconds_since_boot()+CACHE_NEW_EXPIRY);
 	write_unlock(&detail->hash_lock);
 	cache_fresh_unlocked(h, detail);
 	return -ENOENT;
@@ -275,13 +274,10 @@ int cache_check(struct cache_detail *detail,
 		if (!test_and_set_bit(CACHE_PENDING, &h->flags)) {
 			switch (cache_make_upcall(detail, h)) {
 			case -EINVAL:
-				clear_bit(CACHE_PENDING, &h->flags);
-				cache_revisit_request(h);
 				rv = try_to_negate_entry(detail, h);
 				break;
 			case -EAGAIN:
-				clear_bit(CACHE_PENDING, &h->flags);
-				cache_revisit_request(h);
+				cache_fresh_unlocked(h, detail);
 				break;
 			}
 		}


Is that convincing?

Thanks a lot for your very thorough analysis!

NeilBrown

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Re: [PATCH] sunrpc.ko: RPC cache fix races

[J. Bruce Fields]

On Tue, Feb 19, 2013 at 06:08:40PM +0100, bstroesser@ts.fujitsu.com wrote:
> Second attempt using the correct FROM. Sorry for the noise.
> 
> 
> Hi,
> 
> I found a problem in sunrpc.ko on a SLES11 SP1 (2.6.32.59-0,7.1)
> and fixed it (see first patch ifor 2.6.32.60 below).
> For us the patch works fine (tested on 2.6.32.59-0.7.1).
> 
> AFAICS from the code, the problem seems to persist in current
> kernel versions also. Thus, I added the second patch for 3.7.9.
> As the setup to reproduce the problem is quite complex, I couldn't
> test the second patch yet. So consider this one as a RFC.
> 
> Best regards,
> Bodo
> 
> Please CC me, I'm not on the list.
> 
> =========================================
> From: Bodo Stroesser <bstroesser@ts.fujitsu.com>
> Date: Fri, 08 Feb 2013
> Subject: [PATCH] net: sunrpc: fix races in RPC cache
> 
> We found the problem and tested the patch on a SLES11 SP1 2.6.32.59-0.7.1
> 
> This patch applies to linux-2.6.32.60 (changed monotonic_seconds -->
> get_seconds())

This patch may well be correct, but for stable I can't normally take
patches that aren't backports of specific upstream fixes.  That means:
first, we need to fix whatever remains to be fixed on the latest
upstream kernel.  Then, we need to identify the other upstream patches
are needed and aren't yet in 2.6.32.60.

...
> =========================================
> From: Bodo Stroesser <bstroesser@ts.fujitsu.com>
> Date: Fri, 08 Feb 2013
> Subject: [PATCH] net: sunrpc: fix races in RPC cache
> 
> This patch applies to SLES 11 SP2 linux-3.0.51-0.7.9 and also to
> vanilla linux-3.7.9
> 
> It is untested and is only based on a code review after we
> analyzed the reason for NFS requests being dropped on a
> SLES11 SP1 (linux-2.6.32.59-0.7.1)
> 
> Sporadically NFS3 RPC requests to the nfs server are dropped due to
> cache_check() (net/sunrpc/cache.c) returning -ETIMEDOUT for an entry
> of the "auth.unix.gid" cache.
> In this case, no NFS reply is sent to the client.
> 
> The reason for the dropped requests are races in cache_check() when
> cache_make_upcall() returns -EINVAL (because it is called for a cache
> without readers) and cache_check() therefore refreshes the cache entry 
> (rv == -EAGAIN).
> 
> There are two details that need to be changed:
>  1) cache_revisit_request() must not be called before cache_fresh_locked()
>     has updated the cache entry, as cache_revisit_request() wakes up
>     threads waiting for the cache entry to be updated.
>     The explicit call to cache_revisit_request() is not needed, as
>     cache_fresh_unlocked() calls it anyway.
>     (But in case of not updating the cache entry, cache_revisit_request()
>     must be called).
>  2) CACHE_PENDING must not be cleared before cache_fresh_locked() has
>     updated the cache entry, as cache_wait_req() called by
>     cache_defer_req() can return without really sleeping if it detects
>     CACHE_PENDING unset.

I think that makes sense.  Thanks for the explanation.  This is all a
bit subtle, so any additional review or testing would be appreciated....

--b.

> 
> Signed-off-by: Bodo Stroesser <bstroesser@ts.fujitsu.com>
> ---
> 
> --- a/net/sunrpc/cache.c	2013-02-08 15:56:07.000000000 +0100
> +++ b/net/sunrpc/cache.c	2013-02-08 16:04:32.000000000 +0100
> @@ -230,11 +230,14 @@ static int try_to_negate_entry(struct ca
>  	rv = cache_is_valid(detail, h);
>  	if (rv != -EAGAIN) {
>  		write_unlock(&detail->hash_lock);
> +		clear_bit(CACHE_PENDING, &h->flags);
> +		cache_revisit_request(h);
>  		return rv;
>  	}
>  	set_bit(CACHE_NEGATIVE, &h->flags);
>  	cache_fresh_locked(h, seconds_since_boot()+CACHE_NEW_EXPIRY);
>  	write_unlock(&detail->hash_lock);
> +	clear_bit(CACHE_PENDING, &h->flags);
>  	cache_fresh_unlocked(h, detail);
>  	return -ENOENT;
>  }
> @@ -275,8 +278,6 @@ int cache_check(struct cache_detail *det
>  		if (!test_and_set_bit(CACHE_PENDING, &h->flags)) {
>  			switch (cache_make_upcall(detail, h)) {
>  			case -EINVAL:
> -				clear_bit(CACHE_PENDING, &h->flags);
> -				cache_revisit_request(h);
>  				rv = try_to_negate_entry(detail, h);
>  				break;
>  			case -EAGAIN:

[PATCH] sunrpc.ko: RPC cache fix races

[bstroesser]

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