On Thu, 2020-12-17 at 18:09 +0800, Ian Kent wrote: > On Thu, 2020-12-17 at 16:54 +0800, Fox Chen wrote: > > On Thu, Dec 17, 2020 at 12:46 PM Ian Kent <raven@xxxxxxxxxx> wrote: > > > On Tue, 2020-12-15 at 20:59 +0800, Ian Kent wrote: > > > > On Tue, 2020-12-15 at 16:33 +0800, Fox Chen wrote: > > > > > On Mon, Dec 14, 2020 at 9:30 PM Ian Kent <raven@xxxxxxxxxx> > > > > > wrote: > > > > > > On Mon, 2020-12-14 at 14:14 +0800, Fox Chen wrote: > > > > > > > On Sun, Dec 13, 2020 at 11:46 AM Ian Kent < > > > > > > > raven@xxxxxxxxxx > > > > > > > wrote: > > > > > > > > On Fri, 2020-12-11 at 10:17 +0800, Ian Kent wrote: > > > > > > > > > On Fri, 2020-12-11 at 10:01 +0800, Ian Kent wrote: > > > > > > > > > > > For the patches, there is a mutex_lock in kn- > > > > > > > > > > > > attr_mutex, > > > > > > > > > > > as > > > > > > > > > > > Tejun > > > > > > > > > > > mentioned here > > > > > > > > > > > ( > > > > > > > > > > > https://lore.kernel.org/lkml/X8fe0cmu+aq1gi7O@xxxxxxxxxxxxxxx/ > > > > > > > > > > > ), > > > > > > > > > > > maybe a global > > > > > > > > > > > rwsem for kn->iattr will be better?? > > > > > > > > > > > > > > > > > > > > I wasn't sure about that, IIRC a spin lock could be > > > > > > > > > > used > > > > > > > > > > around > > > > > > > > > > the > > > > > > > > > > initial check and checked again at the end which > > > > > > > > > > would > > > > > > > > > > probably > > > > > > > > > > have > > > > > > > > > > been much faster but much less conservative and a > > > > > > > > > > bit > > > > > > > > > > more > > > > > > > > > > ugly > > > > > > > > > > so > > > > > > > > > > I just went the conservative path since there was > > > > > > > > > > so > > > > > > > > > > much > > > > > > > > > > change > > > > > > > > > > already. > > > > > > > > > > > > > > > > > > Sorry, I hadn't looked at Tejun's reply yet and TBH > > > > > > > > > didn't > > > > > > > > > remember > > > > > > > > > it. > > > > > > > > > > > > > > > > > > Based on what Tejun said it sounds like that needs > > > > > > > > > work. > > > > > > > > > > > > > > > > Those attribute handling patches were meant to allow > > > > > > > > taking > > > > > > > > the > > > > > > > > rw > > > > > > > > sem read lock instead of the write lock for > > > > > > > > kernfs_refresh_inode() > > > > > > > > updates, with the added locking to protect the inode > > > > > > > > attributes > > > > > > > > update since it's called from the VFS both with and > > > > > > > > without > > > > > > > > the > > > > > > > > inode lock. > > > > > > > > > > > > > > Oh, understood. I was asking also because lock on kn- > > > > > > > > attr_mutex > > > > > > > drags > > > > > > > concurrent performance. > > > > > > > > > > > > > > > Looking around it looks like kernfs_iattrs() is called > > > > > > > > from > > > > > > > > multiple > > > > > > > > places without a node database lock at all. > > > > > > > > > > > > > > > > I'm thinking that, to keep my proposed change straight > > > > > > > > forward > > > > > > > > and on topic, I should just leave > > > > > > > > kernfs_refresh_inode() > > > > > > > > taking > > > > > > > > the node db write lock for now and consider the > > > > > > > > attributes > > > > > > > > handling > > > > > > > > as a separate change. Once that's done we could > > > > > > > > reconsider > > > > > > > > what's > > > > > > > > needed to use the node db read lock in > > > > > > > > kernfs_refresh_inode(). > > > > > > > > > > > > > > You meant taking write lock of kernfs_rwsem for > > > > > > > kernfs_refresh_inode()?? > > > > > > > It may be a lot slower in my benchmark, let me test it. > > > > > > > > > > > > Yes, but make sure the write lock of kernfs_rwsem is being > > > > > > taken > > > > > > not the read lock. > > > > > > > > > > > > That's a mistake I had initially? > > > > > > > > > > > > Still, that attributes handling is, I think, sufficient to > > > > > > warrant > > > > > > a separate change since it looks like it might need work, > > > > > > the > > > > > > kernfs > > > > > > node db probably should be kept stable for those attribute > > > > > > updates > > > > > > but equally the existence of an instantiated dentry might > > > > > > mitigate > > > > > > the it. > > > > > > > > > > > > Some people might just know whether it's ok or not but I > > > > > > would > > > > > > like > > > > > > to check the callers to work out what's going on. > > > > > > > > > > > > In any case it's academic if GCH isn't willing to consider > > > > > > the > > > > > > series > > > > > > for review and possible merge. > > > > > > > > > > > Hi Ian > > > > > > > > > > I removed kn->attr_mutex and changed read lock to write lock > > > > > for > > > > > kernfs_refresh_inode > > > > > > > > > > down_write(&kernfs_rwsem); > > > > > kernfs_refresh_inode(kn, inode); > > > > > up_write(&kernfs_rwsem); > > > > > > > > > > > > > > > Unfortunate, changes in this way make things worse, my > > > > > benchmark > > > > > runs > > > > > 100% slower than upstream sysfs. :( > > > > > open+read+close a sysfs file concurrently took 1000us. > > > > > (Currently, > > > > > sysfs with a big mutex kernfs_mutex only takes ~500us > > > > > for one open+read+close operation concurrently) > > > > > > > > Right, so it does need attention nowish. > > > > > > > > I'll have a look at it in a while, I really need to get a new > > > > autofs > > > > release out, and there are quite a few changes, and testing is > > > > seeing > > > > a number of errors, some old, some newly introduced. It's > > > > proving > > > > difficult. > > > > > > I've taken a breather for the autofs testing and had a look at > > > this. > > > > Thanks. :) > > > > > I think my original analysis of this was wrong. > > > > > > Could you try this patch please. > > > I'm not sure how much difference it will make but, in principle, > > > it's much the same as the previous approach except it doesn't > > > increase the kernfs node struct size or mess with the other > > > attribute handling code. > > > > > > Note, this is not even compile tested. > > > > I failed to apply this patch. So based on the original six patches, > > I > > manually removed kn->attr_mutex, and added > > inode_lock/inode_unlock to those two functions, they were like: > > > > int kernfs_iop_getattr(const struct path *path, struct kstat *stat, > > u32 request_mask, unsigned int query_flags) > > { > > struct inode *inode = d_inode(path->dentry); > > struct kernfs_node *kn = inode->i_private; > > > > inode_lock(inode); > > down_read(&kernfs_rwsem); > > kernfs_refresh_inode(kn, inode); > > up_read(&kernfs_rwsem); > > inode_unlock(inode); > > > > generic_fillattr(inode, stat); > > return 0; > > } > > > > int kernfs_iop_permission(struct inode *inode, int mask) > > { > > struct kernfs_node *kn; > > > > if (mask & MAY_NOT_BLOCK) > > return -ECHILD; > > > > kn = inode->i_private; > > > > inode_lock(inode); > > down_read(&kernfs_rwsem); > > kernfs_refresh_inode(kn, inode); > > up_read(&kernfs_rwsem); > > inode_unlock(inode); > > > > return generic_permission(inode, mask); > > } > > > > But I couldn't boot the kernel and there was no error on the > > screen. > > I guess it was deadlocked on /sys creation?? :D > > Right, I guess the locking documentation is out of date. I'm guessing > the inode lock is taken somewhere over the .permission() call. If > that > usage is consistent it's easy fixed, if the usage is inconsistent > it's > hard to deal with and amounts to a bug. Yes, it is called, both shared on open, and exclusive on open create, and without the inode lock at all at the start of path resolution. That can't really be called a VFS bug since .permission() is meant to check permissions not update the inode. This is probably what lead to the attr patches I had. If a suitable place to put a local per-object lock can't be found for this, other than in the kernfs_node, then it's a real problem from a contention POV. What could be done is to make the kernfs node attr_mutex a pointer and dynamically allocate it but even that is too costly a size addition to the kernfs node structure as Tejun has said. Those patches I referred to clearly aren't finished because the eighth one is empty, which followed a patch I have titled "kernfs: make attr_mutex a local kernfs node lock". I obviously gave up on it when the series was rejected. But I'll give it some more thought. Ian > > I'll have another look at it. > > Also, it sounds like I'm working from a more recent series. > > I had 8 patches, dropped the last three and added the one I posted. > If I can work out what's going on I'll post the series for you to > check. > > Ian > > > > kernfs: use kernfs read lock in .getattr() and .permission() > > > > > > From: Ian Kent <raven@xxxxxxxxxx> > > > > > > From Documenation/filesystems.rst and (slightly outdated) > > > comments > > > in fs/attr.c the inode i_rwsem is used for attribute handling. > > > > > > This lock satisfies the requirememnts needed to reduce lock > > > contention, > > > namely a per-object lock needs to be used rather than a file > > > system > > > global lock with the kernfs node db held stable for read > > > operations. > > > > > > In particular it should reduce lock contention seen when calling > > > the > > > kernfs .permission() method. > > > > > > The inode methods .getattr() and .permission() do not hold the > > > inode > > > i_rwsem lock when called as they are usually read operations. > > > Also > > > the .permission() method checks for rcu-walk mode and returns > > > -ECHILD > > > to the VFS if it is set. So the i_rwsem lock can be used in > > > kernfs_iop_getattr() and kernfs_iop_permission() to protect the > > > inode > > > update done by kernfs_refresh_inode(). Using this lock allows the > > > kernfs node db write lock in these functions to be changed to a > > > read > > > lock. > > > > > > Signed-off-by: Ian Kent <raven@xxxxxxxxxx> > > > --- > > > fs/kernfs/inode.c | 12 ++++++++---- > > > 1 file changed, 8 insertions(+), 4 deletions(-) > > > > > > diff --git a/fs/kernfs/inode.c b/fs/kernfs/inode.c > > > index ddaf18198935..568037e9efe9 100644 > > > --- a/fs/kernfs/inode.c > > > +++ b/fs/kernfs/inode.c > > > @@ -189,9 +189,11 @@ int kernfs_iop_getattr(const struct path > > > *path, struct kstat *stat, > > > struct inode *inode = d_inode(path->dentry); > > > struct kernfs_node *kn = inode->i_private; > > > > > > - down_write(&kernfs_rwsem); > > > + inode_lock(inode); > > > + down_read(&kernfs_rwsem); > > > kernfs_refresh_inode(kn, inode); > > > - up_write(&kernfs_rwsem); > > > + up_read(&kernfs_rwsem); > > > + inode_unlock(inode); > > > > > > generic_fillattr(inode, stat); > > > return 0; > > > @@ -281,9 +283,11 @@ int kernfs_iop_permission(struct inode > > > *inode, > > > int mask) > > > > > > kn = inode->i_private; > > > > > > - down_write(&kernfs_rwsem); > > > + inode_lock(inode); > > > + down_read(&kernfs_rwsem); > > > kernfs_refresh_inode(kn, inode); > > > - up_write(&kernfs_rwsem); > > > + up_read(&kernfs_rwsem); > > > + inode_unlock(inode); > > > > > > return generic_permission(inode, mask); > > > } > > > > > > > thanks, > > fox
