| Message ID | 20240822-b4-regmap-maple-nolock-v1-1-d5e6dbae3396@kernel.org (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | regmap: Improve lock handling with maple tree | expand |
On Thu, Aug 22, 2024 at 08:13:35PM +0100, Mark Brown wrote: > Currently the maple tree code allows external locks to be configured by > passing the lock itself. This is generally helpful and convenient but is No, it's a really bad idea. Stop doing it. Use the internal lock. It's a temporary hack we put in and I'm really regretting allowing it. > not ideal for situations like the regmap maple tree cache where we support > configurable locking at the regmap level and don't have the lock type when > we are configuring the maple tree. Add a helper that allows us to pass the > dep map directly to help with these situations. Since such code is already > peering at the lockdep internals enough to be looking at the map no stub > is provided. > > Signed-off-by: Mark Brown <broonie@kernel.org> > --- > include/linux/maple_tree.h | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/include/linux/maple_tree.h b/include/linux/maple_tree.h > index a53ad4dabd7e..bdc6b133abdc 100644 > --- a/include/linux/maple_tree.h > +++ b/include/linux/maple_tree.h > @@ -193,6 +193,9 @@ typedef struct lockdep_map *lockdep_map_p; > #define mt_set_external_lock(mt, lock) \ > (mt)->ma_external_lock = &(lock)->dep_map > > +#define mt_set_external_lock_dep_map(mt, dep_map) \ > + (mt)->ma_external_lock = dep_map > + > #define mt_on_stack(mt) (mt).ma_external_lock = NULL > #else > typedef struct { /* nothing */ } lockdep_map_p; > > -- > 2.39.2 > > > -- > maple-tree mailing list > maple-tree@lists.infradead.org > https://lists.infradead.org/mailman/listinfo/maple-tree
On Thu, Aug 22, 2024 at 08:21:40PM +0100, Matthew Wilcox wrote: > On Thu, Aug 22, 2024 at 08:13:35PM +0100, Mark Brown wrote: > > Currently the maple tree code allows external locks to be configured by > > passing the lock itself. This is generally helpful and convenient but is > No, it's a really bad idea. Stop doing it. Use the internal lock. > It's a temporary hack we put in and I'm really regretting allowing it. I mean, we do use the internal lock here since otherwise lockdep moans but it's pure overhead which just complicates the code. It's only ever taken within another lock, meaning it winds up protecting nothing for these maple trees. We can't go the other way round and use the maple tree lock as the regmap lock since apart from anything else it's a spin lock and we need to use a mutex most of the time to support busses that sleep during I/O.
On Thu, Aug 22, 2024 at 08:48:56PM +0100, Mark Brown wrote: > On Thu, Aug 22, 2024 at 08:21:40PM +0100, Matthew Wilcox wrote: > > On Thu, Aug 22, 2024 at 08:13:35PM +0100, Mark Brown wrote: > > > > Currently the maple tree code allows external locks to be configured by > > > passing the lock itself. This is generally helpful and convenient but is > > > No, it's a really bad idea. Stop doing it. Use the internal lock. > > It's a temporary hack we put in and I'm really regretting allowing it. > > I mean, we do use the internal lock here since otherwise lockdep moans > but it's pure overhead which just complicates the code. It's only ever > taken within another lock, meaning it winds up protecting nothing for > these maple trees. We can't go the other way round and use the maple > tree lock as the regmap lock since apart from anything else it's a spin > lock and we need to use a mutex most of the time to support busses that > sleep during I/O. When it's an uncontended spinlock, there's really no overhead. I wish I'd been firmer on that point earlier and prohibited the external lock hack. The point is that the lock protects the tree. If we are ever going to be able to defragment slabs (and I believe this is an ability that Linux must gain), we must be able to go from the object (the maple node) to a lock that will let us reallocate the node. If there's some external lock that protects the tree, we can't possibly do that.
On Thu, Aug 22, 2024 at 08:55:20PM +0100, Matthew Wilcox wrote: > On Thu, Aug 22, 2024 at 08:48:56PM +0100, Mark Brown wrote: > > I mean, we do use the internal lock here since otherwise lockdep moans > > but it's pure overhead which just complicates the code. It's only ever > When it's an uncontended spinlock, there's really no overhead. I wish I'd > been firmer on that point earlier and prohibited the external lock hack. > The point is that the lock protects the tree. If we are ever going to > be able to defragment slabs (and I believe this is an ability that Linux > must gain), we must be able to go from the object (the maple node) to > a lock that will let us reallocate the node. If there's some external > lock that protects the tree, we can't possibly do that. If the external lock guarantees that nothing can possibly be contending access to the tree (including the read side) I don't see any issue there?
diff --git a/include/linux/maple_tree.h b/include/linux/maple_tree.h index a53ad4dabd7e..bdc6b133abdc 100644 --- a/include/linux/maple_tree.h +++ b/include/linux/maple_tree.h @@ -193,6 +193,9 @@ typedef struct lockdep_map *lockdep_map_p; #define mt_set_external_lock(mt, lock) \ (mt)->ma_external_lock = &(lock)->dep_map +#define mt_set_external_lock_dep_map(mt, dep_map) \ + (mt)->ma_external_lock = dep_map + #define mt_on_stack(mt) (mt).ma_external_lock = NULL #else typedef struct { /* nothing */ } lockdep_map_p;
Currently the maple tree code allows external locks to be configured by passing the lock itself. This is generally helpful and convenient but is not ideal for situations like the regmap maple tree cache where we support configurable locking at the regmap level and don't have the lock type when we are configuring the maple tree. Add a helper that allows us to pass the dep map directly to help with these situations. Since such code is already peering at the lockdep internals enough to be looking at the map no stub is provided. Signed-off-by: Mark Brown <broonie@kernel.org> --- include/linux/maple_tree.h | 3 +++ 1 file changed, 3 insertions(+)