From: David Howells <dhowells@redhat.com>
To: Greg KH <gregkh@linuxfoundation.org>
Cc: dhowells@redhat.com, viro@zeniv.linux.org.uk, raven@themaw.net,
linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org,
linux-block@vger.kernel.org, keyrings@vger.kernel.org,
linux-security-module@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/7] General notification queue with user mmap()'able ring buffer
Date: Fri, 31 May 2019 13:42:21 +0100 [thread overview]
Message-ID: <24720.1559306541@warthog.procyon.org.uk> (raw)
In-Reply-To: <20190529230954.GA3164@kroah.com>
Greg KH <gregkh@linuxfoundation.org> wrote:
> > kref_put() enforces a very specific destructor signature. I know of places
> > where that doesn't work because the destructor takes more than one argument
> > (granted that this is not the case here). So why does kref_put() exist at
> > all? Why not kref_dec_and_test()?
>
> The destructor only takes one object pointer as you are finally freeing
> that object. What more do you need/want to "know" at that point in
> time?
Imagine that I have an object that's on a list rooted in a namespace and that
I have a lot of these objects. Imagine further that any time I want to put a
ref on one of these objects, it's in a context that has the namespace pinned.
I therefore don't need to store a pointer to the namespace in every object
because I can pass that in to the put function
Indeed, I can still access the namespace even after the decrement didn't
reduce the usage count to 0 - say for doing statistics.
> What would kref_dec_and_test() be needed for?
Why do you need kref_put() to take a destructor function pointer? Why cannot
that be replaced with, say:
static inline bool __kref_put(struct kref *k)
{
return refcount_dec_and_test(&k->refcount);
}
and then one could do:
void put_foo(struct foo_net *ns, struct foo *f)
{
if (__kref_put(&f->refcount)) {
// destroy foo
}
}
that way the destruction code does not have to be offloaded into its own
function and you still have your pattern to look for.
For tracing purposes, I could live with something like:
static inline
bool __kref_put_return(struct kref *k, unsigned int *_usage)
{
return refcount_dec_and_test_return(&k->refcount, _usage);
}
and then I could do:
void put_foo(struct foo_net *ns, struct foo *f)
{
unsigned int u;
bool is_zero = __kref_put_return(&f->refcount, &u);
trace_foo_refcount(f, u);
if (is_zero) {
// destroy foo
}
}
then it could be made such that you can disable the ability of
refcount_dec_and_test_return() to pass back a useful refcount value if you
want a bit of extra speed.
Or even if refcount_dec_return() is guaranteed to return 0 if the count hits
the floor and non-zero otherwise and there's a config switch to impose a
stronger guarantee that it will return a value that's appropriately
transformed to look as if I was using atomic_dec_return().
Similarly for refcount_inc_return() - it could just return gibberish unless
the same config switch is enabled.
Question for AMD/Intel guys: I'm curious if LOCK DECL faster than LOCK XADD -1
on x86_64?
> > Why doesn't refcount_t get merged into kref, or vice versa? Having both
> > would seem redundant.
>
> kref uses refcount_t and provides a different functionality on top of
> it. Not all uses of a refcount in the kernel is for object lifecycle
> reference counting, as you know :)
I do? I can't think of one offhand. Not that I'm saying you're wrong on
that - there's an awful lot of kernel.
David
next prev parent reply other threads:[~2019-05-31 12:42 UTC|newest]
Thread overview: 65+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-28 16:01 [RFC][PATCH 0/7] Mount, FS, Block and Keyrings notifications David Howells
2019-05-28 16:01 ` [PATCH 1/7] General notification queue with user mmap()'able ring buffer David Howells
2019-05-28 16:26 ` Greg KH
2019-05-28 17:30 ` David Howells
2019-05-28 23:12 ` Greg KH
2019-05-29 16:06 ` David Howells
2019-05-29 17:46 ` Jann Horn
2019-05-29 21:02 ` David Howells
2019-05-31 11:14 ` Peter Zijlstra
2019-05-31 12:02 ` David Howells
2019-05-31 13:26 ` Peter Zijlstra
2019-05-31 14:20 ` David Howells
2019-05-31 16:44 ` Peter Zijlstra
2019-05-31 17:12 ` David Howells
2019-06-17 16:24 ` Peter Zijlstra
2019-05-29 23:09 ` Greg KH
2019-05-29 23:11 ` Greg KH
2019-05-30 9:50 ` Andrea Parri
2019-05-31 8:35 ` Peter Zijlstra
2019-05-31 8:47 ` Peter Zijlstra
2019-05-31 12:42 ` David Howells [this message]
2019-05-31 14:55 ` David Howells
2019-05-28 19:14 ` Jann Horn
2019-05-28 22:28 ` David Howells
2019-05-28 23:16 ` Jann Horn
2019-05-28 16:02 ` [PATCH 2/7] keys: Add a notification facility David Howells
2019-05-28 16:02 ` [PATCH 3/7] vfs: Add a mount-notification facility David Howells
2019-05-28 20:06 ` Jann Horn
2019-05-28 23:04 ` David Howells
2019-05-28 23:23 ` Jann Horn
2019-05-29 11:16 ` David Howells
2019-05-28 23:08 ` David Howells
2019-05-29 10:55 ` David Howells
2019-05-29 11:00 ` David Howells
2019-05-29 15:53 ` Casey Schaufler
2019-05-29 16:12 ` Jann Horn
2019-05-29 17:04 ` Casey Schaufler
2019-06-03 16:30 ` David Howells
2019-05-29 17:13 ` Andy Lutomirski
2019-05-29 17:46 ` Casey Schaufler
2019-05-29 18:11 ` Jann Horn
2019-05-29 19:28 ` Casey Schaufler
2019-05-29 19:47 ` Jann Horn
2019-05-29 20:50 ` Casey Schaufler
2019-05-29 23:12 ` Andy Lutomirski
2019-05-29 23:56 ` Casey Schaufler
2019-05-28 16:02 ` [PATCH 4/7] vfs: Add superblock notifications David Howells
2019-05-28 20:27 ` Jann Horn
2019-05-29 12:58 ` David Howells
2019-05-29 14:16 ` Jann Horn
2019-05-28 16:02 ` [PATCH 5/7] fsinfo: Export superblock notification counter David Howells
2019-05-28 16:02 ` [PATCH 6/7] block: Add block layer notifications David Howells
2019-05-28 20:37 ` Jann Horn
2019-05-28 16:02 ` [PATCH 7/7] Add sample notification program David Howells
2019-05-28 23:58 ` [RFC][PATCH 0/7] Mount, FS, Block and Keyrings notifications Greg KH
2019-05-29 6:33 ` Amir Goldstein
2019-05-29 14:25 ` Jan Kara
2019-05-29 15:10 ` Greg KH
2019-05-29 15:53 ` Amir Goldstein
2019-05-30 11:00 ` Jan Kara
2019-06-04 12:33 ` David Howells
2019-05-29 6:45 ` David Howells
2019-05-29 7:40 ` Amir Goldstein
2019-05-29 9:09 ` David Howells
2019-05-29 15:41 ` Casey Schaufler
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