The struct used to be refcounted with regular inc/dec ops, atomic usage
showed up in commit 03adc61edad4 ("audit,io_uring: io_uring openat
triggers audit reference count underflow").
If putname spots a count of 1 there is no legitimate way of anyone to
bump it and these modifications are low traffic (names are not heavily)
shared, thus one can do a load first and if the value of 1 is found the
atomic can be elided -- this is the last reference..
When performing a failed open this reduces putname on the profile from
~1.60% to ~0.2% and bumps the syscall rate by just shy of 1% (the
discrepancy is due to now bigger stalls elsewhere).
Signed-off-by: Mateusz Guzik <mjguzik@xxxxxxxxx>
---
This is a lazy hack.
The race is only possible with io_uring which has a dedicated entry
point, thus a getname variant which takes it into account could store
the need to use atomics as a flag in struct filename. To that end
getname could take a boolean indicating this, fronted with some inlines
and the current entry point renamed to __getname_flags to hide it.
Option B is to add a routine which "upgrades" to atomics after getname
returns, but that's a littly fishy vs audit_reusename.
At the end of the day all spots which modify the ref could branch on the
atomics flag.
I opted to not do it since the hack below undoes the problem for me.
I'm not going to fight for this hack though, it is merely a placeholder
until someone(tm) fixes things.
If the hack is considered a no-go and the appraoch described above is
considered fine, I can submit a patch some time this month to sort it
out, provided someone tells me how to name a routine which grabs a ref
-- the op is currently opencoded and "getname" allocates instead of
merely refing. would "refname" do it?
fs/namei.c | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)
diff --git a/fs/namei.c b/fs/namei.c
index 37fb0a8aa09a..f9440bdb21d0 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -260,11 +260,13 @@ void putname(struct filename *name)
if (IS_ERR(name))
return;
- if (WARN_ON_ONCE(!atomic_read(&name->refcnt)))
- return;
+ if (unlikely(atomic_read(&name->refcnt) != 1)) {
+ if (WARN_ON_ONCE(!atomic_read(&name->refcnt)))
+ return;
- if (!atomic_dec_and_test(&name->refcnt))
- return;
+ if (!atomic_dec_and_test(&name->refcnt))
+ return;
+ }
if (name->name != name->iname) {
__putname(name->name);
--
2.39.2
