From: "Michael S. Tsirkin" <mst@redhat.com> Date: 2011-01-18 11:09:39
When built with rcu checks enabled, vhost triggers
bogus warnings as vhost features are read without
dev->mutex sometimes.
Fixing it properly is not trivial as vhost.h does not
know which lockdep classes it will be used under.
Disable the warning by stubbing out the check for now.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
drivers/vhost/vhost.h | 4 +---
1 files changed, 1 insertions(+), 3 deletions(-)
From: Paul E. McKenney <hidden> Date: 2011-01-18 17:48:41
On Tue, Jan 18, 2011 at 01:08:45PM +0200, Michael S. Tsirkin wrote:
quoted hunk
When built with rcu checks enabled, vhost triggers
bogus warnings as vhost features are read without
dev->mutex sometimes.
Fixing it properly is not trivial as vhost.h does not
know which lockdep classes it will be used under.
Disable the warning by stubbing out the check for now.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
drivers/vhost/vhost.h | 4 +---
1 files changed, 1 insertions(+), 3 deletions(-)
@@ -173,9 +173,7 @@ static inline int vhost_has_feature(struct vhost_dev *dev, int bit){unsignedacked_features;-acked_features=-rcu_dereference_index_check(dev->acked_features,-lockdep_is_held(&dev->mutex));+acked_features=rcu_dereference_index_check(dev->acked_features,1);
Ouch!!!
Could you please at least add a comment?
Alternatively, pass in the lock that is held and check for that? Given
that this is a static inline, the compiler should be able to optimize
the argument away when !PROVE_RCU, correct?
Thanx, Paul
return acked_features & (1 << bit);
}
--
1.7.3.2.91.g446ac
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From: "Michael S. Tsirkin" <mst@redhat.com> Date: 2011-01-18 17:55:53
On Tue, Jan 18, 2011 at 09:48:34AM -0800, Paul E. McKenney wrote:
On Tue, Jan 18, 2011 at 01:08:45PM +0200, Michael S. Tsirkin wrote:
quoted
When built with rcu checks enabled, vhost triggers
bogus warnings as vhost features are read without
dev->mutex sometimes.
Fixing it properly is not trivial as vhost.h does not
know which lockdep classes it will be used under.
Disable the warning by stubbing out the check for now.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
drivers/vhost/vhost.h | 4 +---
1 files changed, 1 insertions(+), 3 deletions(-)
@@ -173,9 +173,7 @@ static inline int vhost_has_feature(struct vhost_dev *dev, int bit){unsignedacked_features;-acked_features=-rcu_dereference_index_check(dev->acked_features,-lockdep_is_held(&dev->mutex));+acked_features=rcu_dereference_index_check(dev->acked_features,1);
Ouch!!!
Could you please at least add a comment?
Yes, OK.
Alternatively, pass in the lock that is held and check for that? Given
that this is a static inline, the compiler should be able to optimize
the argument away when !PROVE_RCU, correct?
Thanx, Paul
Hopefully, yes. We don't always have a lock: the idea was
to create a lockdep for these cases. But we can't pass
the pointer to that ...
quoted
return acked_features & (1 << bit);
}
--
1.7.3.2.91.g446ac
--
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From: Paul E. McKenney <hidden> Date: 2011-01-18 19:02:38
On Tue, Jan 18, 2011 at 07:55:00PM +0200, Michael S. Tsirkin wrote:
On Tue, Jan 18, 2011 at 09:48:34AM -0800, Paul E. McKenney wrote:
quoted
On Tue, Jan 18, 2011 at 01:08:45PM +0200, Michael S. Tsirkin wrote:
quoted
When built with rcu checks enabled, vhost triggers
bogus warnings as vhost features are read without
dev->mutex sometimes.
Fixing it properly is not trivial as vhost.h does not
know which lockdep classes it will be used under.
Disable the warning by stubbing out the check for now.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
drivers/vhost/vhost.h | 4 +---
1 files changed, 1 insertions(+), 3 deletions(-)
@@ -173,9 +173,7 @@ static inline int vhost_has_feature(struct vhost_dev *dev, int bit){unsignedacked_features;-acked_features=-rcu_dereference_index_check(dev->acked_features,-lockdep_is_held(&dev->mutex));+acked_features=rcu_dereference_index_check(dev->acked_features,1);
Ouch!!!
Could you please at least add a comment?
Yes, OK.
quoted
Alternatively, pass in the lock that is held and check for that? Given
that this is a static inline, the compiler should be able to optimize
the argument away when !PROVE_RCU, correct?
Thanx, Paul
Hopefully, yes. We don't always have a lock: the idea was
to create a lockdep for these cases. But we can't pass
the pointer to that ...
I suppose you could pass a pointer to the lockdep map structure.
Not sure if this makes sense, but it would handle the situation.
Alternatively, create a helper function that checks the possibilities
and screams if none of them are in effect.
Thanx, Paul
quoted
quoted
return acked_features & (1 << bit);
}
--
1.7.3.2.91.g446ac
--
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From: "Michael S. Tsirkin" <mst@redhat.com> Date: 2011-01-18 20:11:21
On Tue, Jan 18, 2011 at 11:02:33AM -0800, Paul E. McKenney wrote:
On Tue, Jan 18, 2011 at 07:55:00PM +0200, Michael S. Tsirkin wrote:
quoted
On Tue, Jan 18, 2011 at 09:48:34AM -0800, Paul E. McKenney wrote:
quoted
On Tue, Jan 18, 2011 at 01:08:45PM +0200, Michael S. Tsirkin wrote:
quoted
When built with rcu checks enabled, vhost triggers
bogus warnings as vhost features are read without
dev->mutex sometimes.
Fixing it properly is not trivial as vhost.h does not
know which lockdep classes it will be used under.
Disable the warning by stubbing out the check for now.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
drivers/vhost/vhost.h | 4 +---
1 files changed, 1 insertions(+), 3 deletions(-)
@@ -173,9 +173,7 @@ static inline int vhost_has_feature(struct vhost_dev *dev, int bit){unsignedacked_features;-acked_features=-rcu_dereference_index_check(dev->acked_features,-lockdep_is_held(&dev->mutex));+acked_features=rcu_dereference_index_check(dev->acked_features,1);
Ouch!!!
Could you please at least add a comment?
Yes, OK.
quoted
Alternatively, pass in the lock that is held and check for that? Given
that this is a static inline, the compiler should be able to optimize
the argument away when !PROVE_RCU, correct?
Thanx, Paul
Hopefully, yes. We don't always have a lock: the idea was
to create a lockdep for these cases. But we can't pass
the pointer to that ...
I suppose you could pass a pointer to the lockdep map structure.
Not sure if this makes sense, but it would handle the situation.
Will it compile with lockdep disabled too? What will the pointer be?
Alternatively, create a helper function that checks the possibilities
and screams if none of them are in effect.
Thanx, Paul
The problem here is the callee needs to know about all callers.
quoted
quoted
quoted
return acked_features & (1 << bit);
}
--
1.7.3.2.91.g446ac
--
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From: Paul E. McKenney <hidden> Date: 2011-01-18 20:31:18
On Tue, Jan 18, 2011 at 10:10:31PM +0200, Michael S. Tsirkin wrote:
On Tue, Jan 18, 2011 at 11:02:33AM -0800, Paul E. McKenney wrote:
quoted
On Tue, Jan 18, 2011 at 07:55:00PM +0200, Michael S. Tsirkin wrote:
quoted
On Tue, Jan 18, 2011 at 09:48:34AM -0800, Paul E. McKenney wrote:
quoted
On Tue, Jan 18, 2011 at 01:08:45PM +0200, Michael S. Tsirkin wrote:
quoted
When built with rcu checks enabled, vhost triggers
bogus warnings as vhost features are read without
dev->mutex sometimes.
Fixing it properly is not trivial as vhost.h does not
know which lockdep classes it will be used under.
Disable the warning by stubbing out the check for now.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
drivers/vhost/vhost.h | 4 +---
1 files changed, 1 insertions(+), 3 deletions(-)
@@ -173,9 +173,7 @@ static inline int vhost_has_feature(struct vhost_dev *dev, int bit){unsignedacked_features;-acked_features=-rcu_dereference_index_check(dev->acked_features,-lockdep_is_held(&dev->mutex));+acked_features=rcu_dereference_index_check(dev->acked_features,1);
Ouch!!!
Could you please at least add a comment?
Yes, OK.
quoted
Alternatively, pass in the lock that is held and check for that? Given
that this is a static inline, the compiler should be able to optimize
the argument away when !PROVE_RCU, correct?
Thanx, Paul
Hopefully, yes. We don't always have a lock: the idea was
to create a lockdep for these cases. But we can't pass
the pointer to that ...
I suppose you could pass a pointer to the lockdep map structure.
Not sure if this makes sense, but it would handle the situation.
Will it compile with lockdep disabled too? What will the pointer be?
One (crude) approach would be to make the pointer void* if lockdep
is disabled.
quoted
Alternatively, create a helper function that checks the possibilities
and screams if none of them are in effect.
Thanx, Paul
The problem here is the callee needs to know about all callers.
As does the guy reading the code. ;-)
Thanx, Paul
quoted
quoted
quoted
quoted
return acked_features & (1 << bit);
}
--
1.7.3.2.91.g446ac
--
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Please read the FAQ at http://www.tux.org/lkml/
On Tue, Jan 18, 2011 at 01:08:45PM +0200, Michael S. Tsirkin wrote:
When built with rcu checks enabled, vhost triggers
bogus warnings as vhost features are read without
dev->mutex sometimes.
Fixing it properly is not trivial as vhost.h does not
know which lockdep classes it will be used under.
Disable the warning by stubbing out the check for now.
What is the harm in leaving the bogus warnings until the difficult fix
happens? RCU checks enabled does not seem like something that is enabled
in production. If this patch is applied, there is always the risk that
it'll be simply forgotten about.
--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
From: "Michael S. Tsirkin" <mst@redhat.com> Date: 2011-01-19 05:19:53
On Wed, Jan 19, 2011 at 12:40:40AM +0000, Mel Gorman wrote:
On Tue, Jan 18, 2011 at 01:08:45PM +0200, Michael S. Tsirkin wrote:
quoted
When built with rcu checks enabled, vhost triggers
bogus warnings as vhost features are read without
dev->mutex sometimes.
Fixing it properly is not trivial as vhost.h does not
know which lockdep classes it will be used under.
Disable the warning by stubbing out the check for now.
What is the harm in leaving the bogus warnings until the difficult fix
happens? RCU checks enabled does not seem like something that is enabled
in production.
I would like to run with rcu checks enabled sometimes to debug kvm,
which has an elaborate rcu strategy. Bogus warnings in the log
make it easy to overlook the real ones. Further, the rcu macros
used are a form of documentation. If we have
- rcu_dereference_index_check(dev->acked_features,
- lockdep_is_held(&dev->mutex));
this means 'taken in rcu read side critical section or under mutex',
+ acked_features = rcu_dereference_index_check(dev->acked_features, 1);
means 'not checked'.
If this patch is applied, there is always the risk that
it'll be simply forgotten about.
Well, that's why I put in a TODO.
If there's a demand for that, I can add a Kconfig option to
trigger a warning at each unchecked rcu call in vhost-net
but I doubt it'll get a lof of use :)
--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab