summaryrefslogtreecommitdiffstats
path: root/fs/pnode.c
diff options
context:
space:
mode:
authorNick Piggin <npiggin@kernel.dk>2011-01-07 17:50:11 +1100
committerNick Piggin <npiggin@kernel.dk>2011-01-07 17:50:33 +1100
commitb3e19d924b6eaf2ca7d22cba99a517c5171007b6 (patch)
tree8c1fa4074114a883a4e2de2f7d12eb29ed91bdf1 /fs/pnode.c
parentc6653a838b1b2738561aff0b8c0f62a9b714bdd9 (diff)
downloadlinux-stable-b3e19d924b6eaf2ca7d22cba99a517c5171007b6.tar.gz
linux-stable-b3e19d924b6eaf2ca7d22cba99a517c5171007b6.tar.bz2
linux-stable-b3e19d924b6eaf2ca7d22cba99a517c5171007b6.zip
fs: scale mntget/mntput
The problem that this patch aims to fix is vfsmount refcounting scalability. We need to take a reference on the vfsmount for every successful path lookup, which often go to the same mount point. The fundamental difficulty is that a "simple" reference count can never be made scalable, because any time a reference is dropped, we must check whether that was the last reference. To do that requires communication with all other CPUs that may have taken a reference count. We can make refcounts more scalable in a couple of ways, involving keeping distributed counters, and checking for the global-zero condition less frequently. - check the global sum once every interval (this will delay zero detection for some interval, so it's probably a showstopper for vfsmounts). - keep a local count and only taking the global sum when local reaches 0 (this is difficult for vfsmounts, because we can't hold preempt off for the life of a reference, so a counter would need to be per-thread or tied strongly to a particular CPU which requires more locking). - keep a local difference of increments and decrements, which allows us to sum the total difference and hence find the refcount when summing all CPUs. Then, keep a single integer "long" refcount for slow and long lasting references, and only take the global sum of local counters when the long refcount is 0. This last scheme is what I implemented here. Attached mounts and process root and working directory references are "long" references, and everything else is a short reference. This allows scalable vfsmount references during path walking over mounted subtrees and unattached (lazy umounted) mounts with processes still running in them. This results in one fewer atomic op in the fastpath: mntget is now just a per-CPU inc, rather than an atomic inc; and mntput just requires a spinlock and non-atomic decrement in the common case. However code is otherwise bigger and heavier, so single threaded performance is basically a wash. Signed-off-by: Nick Piggin <npiggin@kernel.dk>
Diffstat (limited to 'fs/pnode.c')
-rw-r--r--fs/pnode.c4
1 files changed, 2 insertions, 2 deletions
diff --git a/fs/pnode.c b/fs/pnode.c
index 8066b8dd748f..d42514e32380 100644
--- a/fs/pnode.c
+++ b/fs/pnode.c
@@ -288,7 +288,7 @@ out:
*/
static inline int do_refcount_check(struct vfsmount *mnt, int count)
{
- int mycount = atomic_read(&mnt->mnt_count) - mnt->mnt_ghosts;
+ int mycount = mnt_get_count(mnt) - mnt->mnt_ghosts;
return (mycount > count);
}
@@ -300,7 +300,7 @@ static inline int do_refcount_check(struct vfsmount *mnt, int count)
* Check if any of these mounts that **do not have submounts**
* have more references than 'refcnt'. If so return busy.
*
- * vfsmount lock must be held for read or write
+ * vfsmount lock must be held for write
*/
int propagate_mount_busy(struct vfsmount *mnt, int refcnt)
{