From: Peter Zijlstra <peterz@infradead.org>
To: David Howells <dhowells@redhat.com>
Cc: Greg KH <gregkh@linuxfoundation.org>,
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 10:47:14 +0200 [thread overview]
Message-ID: <20190531084714.GL2677@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <31936.1559146000@warthog.procyon.org.uk>
On Wed, May 29, 2019 at 05:06:40PM +0100, David Howells wrote:
> Looking at the perf ring buffer, there appears to be a missing barrier in
> perf_aux_output_end():
>
> rb->user_page->aux_head = rb->aux_head;
>
> should be:
>
> smp_store_release(&rb->user_page->aux_head, rb->aux_head);
I've answered that in another email; the aux bit is 'magic'.
> It should also be using smp_load_acquire(). See
> Documentation/core-api/circular-buffers.rst
We use the control dependency instead, as described in the comment of
perf_output_put_handle():
* kernel user
*
* if (LOAD ->data_tail) { LOAD ->data_head
* (A) smp_rmb() (C)
* STORE $data LOAD $data
* smp_wmb() (B) smp_mb() (D)
* STORE ->data_head STORE ->data_tail
* }
*
* Where A pairs with D, and B pairs with C.
*
* In our case (A) is a control dependency that separates the load of
* the ->data_tail and the stores of $data. In case ->data_tail
* indicates there is no room in the buffer to store $data we do not.
*
* D needs to be a full barrier since it separates the data READ
* from the tail WRITE.
*
* For B a WMB is sufficient since it separates two WRITEs, and for C
* an RMB is sufficient since it separates two READs.
Userspace can choose to use smp_load_acquire() over the first smp_rmb()
if that is efficient for the architecture (for w ahole bunch of archs
load-acquire would end up using mb() while rmb() is adequate and
cheaper).
WARNING: multiple messages have this Message-ID (diff)
From: Peter Zijlstra <peterz@infradead.org>
To: David Howells <dhowells@redhat.com>
Cc: Greg KH <gregkh@linuxfoundation.org>,
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 08:47:14 +0000 [thread overview]
Message-ID: <20190531084714.GL2677@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <31936.1559146000@warthog.procyon.org.uk>
On Wed, May 29, 2019 at 05:06:40PM +0100, David Howells wrote:
> Looking at the perf ring buffer, there appears to be a missing barrier in
> perf_aux_output_end():
>
> rb->user_page->aux_head = rb->aux_head;
>
> should be:
>
> smp_store_release(&rb->user_page->aux_head, rb->aux_head);
I've answered that in another email; the aux bit is 'magic'.
> It should also be using smp_load_acquire(). See
> Documentation/core-api/circular-buffers.rst
We use the control dependency instead, as described in the comment of
perf_output_put_handle():
* kernel user
*
* if (LOAD ->data_tail) { LOAD ->data_head
* (A) smp_rmb() (C)
* STORE $data LOAD $data
* smp_wmb() (B) smp_mb() (D)
* STORE ->data_head STORE ->data_tail
* }
*
* Where A pairs with D, and B pairs with C.
*
* In our case (A) is a control dependency that separates the load of
* the ->data_tail and the stores of $data. In case ->data_tail
* indicates there is no room in the buffer to store $data we do not.
*
* D needs to be a full barrier since it separates the data READ
* from the tail WRITE.
*
* For B a WMB is sufficient since it separates two WRITEs, and for C
* an RMB is sufficient since it separates two READs.
Userspace can choose to use smp_load_acquire() over the first smp_rmb()
if that is efficient for the architecture (for w ahole bunch of archs
load-acquire would end up using mb() while rmb() is adequate and
cheaper).
next prev parent reply other threads:[~2019-05-31 8:47 UTC|newest]
Thread overview: 131+ 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 ` David Howells
2019-05-28 16:01 ` [PATCH 1/7] General notification queue with user mmap()'able ring buffer David Howells
2019-05-28 16:01 ` David Howells
2019-05-28 16:26 ` Greg KH
2019-05-28 16:26 ` Greg KH
2019-05-28 17:30 ` David Howells
2019-05-28 17:30 ` David Howells
2019-05-28 23:12 ` Greg KH
2019-05-28 23:12 ` Greg KH
2019-05-29 16:06 ` David Howells
2019-05-29 16:06 ` David Howells
2019-05-29 17:46 ` Jann Horn
2019-05-29 17:46 ` Jann Horn
2019-05-29 21:02 ` David Howells
2019-05-29 21:02 ` David Howells
2019-05-31 11:14 ` Peter Zijlstra
2019-05-31 11:14 ` Peter Zijlstra
2019-05-31 12:02 ` David Howells
2019-05-31 12:02 ` David Howells
2019-05-31 13:26 ` Peter Zijlstra
2019-05-31 13:26 ` Peter Zijlstra
2019-05-31 14:20 ` David Howells
2019-05-31 14:20 ` David Howells
2019-05-31 16:44 ` Peter Zijlstra
2019-05-31 16:44 ` Peter Zijlstra
2019-05-31 17:12 ` David Howells
2019-05-31 17:12 ` David Howells
2019-06-17 16:24 ` Peter Zijlstra
2019-06-17 16:24 ` Peter Zijlstra
2019-05-29 23:09 ` Greg KH
2019-05-29 23:09 ` Greg KH
2019-05-29 23:11 ` Greg KH
2019-05-29 23:11 ` Greg KH
2019-05-30 9:50 ` Andrea Parri
2019-05-30 9:50 ` Andrea Parri
2019-05-31 8:35 ` Peter Zijlstra
2019-05-31 8:35 ` Peter Zijlstra
2019-05-31 8:47 ` Peter Zijlstra [this message]
2019-05-31 8:47 ` Peter Zijlstra
2019-05-31 12:42 ` David Howells
2019-05-31 12:42 ` David Howells
2019-05-31 14:55 ` David Howells
2019-05-31 14:55 ` David Howells
2019-05-28 19:14 ` Jann Horn
2019-05-28 19:14 ` Jann Horn
2019-05-28 22:28 ` David Howells
2019-05-28 22:28 ` David Howells
2019-05-28 23:16 ` Jann Horn
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 ` David Howells
2019-05-28 16:02 ` [PATCH 3/7] vfs: Add a mount-notification facility David Howells
2019-05-28 16:02 ` David Howells
2019-05-28 20:06 ` Jann Horn
2019-05-28 20:06 ` Jann Horn
2019-05-28 23:04 ` David Howells
2019-05-28 23:04 ` David Howells
2019-05-28 23:23 ` Jann Horn
2019-05-28 23:23 ` Jann Horn
2019-05-29 11:16 ` David Howells
2019-05-29 11:16 ` David Howells
2019-05-28 23:08 ` David Howells
2019-05-28 23:08 ` David Howells
2019-05-29 10:55 ` David Howells
2019-05-29 10:55 ` David Howells
2019-05-29 11:00 ` David Howells
2019-05-29 11:00 ` David Howells
2019-05-29 15:53 ` Casey Schaufler
2019-05-29 15:53 ` Casey Schaufler
2019-05-29 16:12 ` Jann Horn
2019-05-29 16:12 ` Jann Horn
2019-05-29 17:04 ` Casey Schaufler
2019-05-29 17:04 ` Casey Schaufler
2019-06-03 16:30 ` David Howells
2019-06-03 16:30 ` David Howells
2019-05-29 17:13 ` Andy Lutomirski
2019-05-29 17:13 ` Andy Lutomirski
2019-05-29 17:46 ` Casey Schaufler
2019-05-29 17:46 ` Casey Schaufler
2019-05-29 18:11 ` Jann Horn
2019-05-29 18:11 ` Jann Horn
2019-05-29 19:28 ` Casey Schaufler
2019-05-29 19:28 ` Casey Schaufler
2019-05-29 19:47 ` Jann Horn
2019-05-29 19:47 ` Jann Horn
2019-05-29 20:50 ` Casey Schaufler
2019-05-29 20:50 ` Casey Schaufler
2019-05-29 23:12 ` Andy Lutomirski
2019-05-29 23:12 ` Andy Lutomirski
2019-05-29 23:56 ` Casey Schaufler
2019-05-29 23:56 ` Casey Schaufler
2019-05-28 16:02 ` [PATCH 4/7] vfs: Add superblock notifications David Howells
2019-05-28 16:02 ` David Howells
2019-05-28 20:27 ` Jann Horn
2019-05-28 20:27 ` Jann Horn
2019-05-29 12:58 ` David Howells
2019-05-29 12:58 ` David Howells
2019-05-29 14:16 ` Jann Horn
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 ` David Howells
2019-05-28 16:02 ` [PATCH 6/7] block: Add block layer notifications David Howells
2019-05-28 16:02 ` David Howells
2019-05-28 20:37 ` Jann Horn
2019-05-28 20:37 ` Jann Horn
2019-05-28 16:02 ` [PATCH 7/7] Add sample notification program David Howells
2019-05-28 16:02 ` David Howells
2019-05-28 23:58 ` [RFC][PATCH 0/7] Mount, FS, Block and Keyrings notifications Greg KH
2019-05-28 23:58 ` Greg KH
2019-05-29 6:33 ` Amir Goldstein
2019-05-29 6:33 ` Amir Goldstein
2019-05-29 6:33 ` Amir Goldstein
2019-05-29 14:25 ` Jan Kara
2019-05-29 14:25 ` Jan Kara
2019-05-29 15:10 ` Greg KH
2019-05-29 15:10 ` Greg KH
2019-05-29 15:53 ` Amir Goldstein
2019-05-29 15:53 ` Amir Goldstein
2019-05-30 11:00 ` Jan Kara
2019-05-30 11:00 ` Jan Kara
2019-06-04 12:33 ` David Howells
2019-06-04 12:33 ` David Howells
2019-05-29 6:45 ` David Howells
2019-05-29 6:45 ` David Howells
2019-05-29 7:40 ` Amir Goldstein
2019-05-29 7:40 ` Amir Goldstein
2019-05-29 9:09 ` David Howells
2019-05-29 9:09 ` David Howells
2019-05-29 15:41 ` Casey Schaufler
2019-05-29 15:41 ` Casey Schaufler
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