From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Mikulas Patocka <mpatocka@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@elte.hu>,
Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
Anton Arapov <anton@redhat.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH -mm] percpu_rw_semaphore-reimplement-to-not-block-the-readers-unnecessari ly.fix
Date: Mon, 12 Nov 2012 10:31:28 -0800 [thread overview]
Message-ID: <20121112183128.GE2518@linux.vnet.ibm.com> (raw)
In-Reply-To: <20121111182744.GA25282@redhat.com>
On Sun, Nov 11, 2012 at 07:27:44PM +0100, Oleg Nesterov wrote:
> More include's and more comments, no changes in code.
>
> To remind, once/if I am sure you agree with this patch I'll send 2 additional
> and simple patches:
>
> 1. lockdep annotations
>
> 2. CONFIG_PERCPU_RWSEM
>
> It seems that we can do much more improvements to a) speedup the writers and
> b) make percpu_rw_semaphore more useful, but not right now.
>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Looks good to me!
Thanx, Paul
> ---
> lib/percpu-rwsem.c | 35 +++++++++++++++++++++++++++++++++--
> 1 files changed, 33 insertions(+), 2 deletions(-)
>
> diff --git a/lib/percpu-rwsem.c b/lib/percpu-rwsem.c
> index 0e3bc0f..02bd157 100644
> --- a/lib/percpu-rwsem.c
> +++ b/lib/percpu-rwsem.c
> @@ -1,6 +1,11 @@
> +#include <linux/mutex.h>
> +#include <linux/rwsem.h>
> +#include <linux/percpu.h>
> +#include <linux/wait.h>
> #include <linux/percpu-rwsem.h>
> #include <linux/rcupdate.h>
> #include <linux/sched.h>
> +#include <linux/errno.h>
>
> int percpu_init_rwsem(struct percpu_rw_semaphore *brw)
> {
> @@ -21,6 +26,29 @@ void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
> brw->fast_read_ctr = NULL; /* catch use after free bugs */
> }
>
> +/*
> + * This is the fast-path for down_read/up_read, it only needs to ensure
> + * there is no pending writer (!mutex_is_locked() check) and inc/dec the
> + * fast per-cpu counter. The writer uses synchronize_sched() to serialize
> + * with the preempt-disabled section below.
> + *
> + * The nontrivial part is that we should guarantee acquire/release semantics
> + * in case when
> + *
> + * R_W: down_write() comes after up_read(), the writer should see all
> + * changes done by the reader
> + * or
> + * W_R: down_read() comes after up_write(), the reader should see all
> + * changes done by the writer
> + *
> + * If this helper fails the callers rely on the normal rw_semaphore and
> + * atomic_dec_and_test(), so in this case we have the necessary barriers.
> + *
> + * But if it succeeds we do not have any barriers, mutex_is_locked() or
> + * __this_cpu_add() below can be reordered with any LOAD/STORE done by the
> + * reader inside the critical section. See the comments in down_write and
> + * up_write below.
> + */
> static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val)
> {
> bool success = false;
> @@ -98,6 +126,7 @@ void percpu_down_write(struct percpu_rw_semaphore *brw)
> *
> * 3. Ensures that if any reader has exited its critical section via
> * fast-path, it executes a full memory barrier before we return.
> + * See R_W case in the comment above update_fast_ctr().
> */
> synchronize_sched();
>
> @@ -116,8 +145,10 @@ void percpu_up_write(struct percpu_rw_semaphore *brw)
> /* allow the new readers, but only the slow-path */
> up_write(&brw->rw_sem);
>
> - /* insert the barrier before the next fast-path in down_read */
> + /*
> + * Insert the barrier before the next fast-path in down_read,
> + * see W_R case in the comment above update_fast_ctr().
> + */
> synchronize_sched();
> -
> mutex_unlock(&brw->writer_mutex);
> }
> --
> 1.5.5.1
>
>
next prev parent reply other threads:[~2012-11-12 18:38 UTC|newest]
Thread overview: 103+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-10-15 19:09 [RFC PATCH 0/2] uprobes: register/unregister can race with fork Oleg Nesterov
2012-10-15 19:10 ` [PATCH 1/2] brw_mutex: big read-write mutex Oleg Nesterov
2012-10-15 23:28 ` Paul E. McKenney
2012-10-16 15:56 ` Oleg Nesterov
2012-10-16 18:58 ` Paul E. McKenney
2012-10-17 16:37 ` Oleg Nesterov
2012-10-17 22:28 ` Paul E. McKenney
2012-10-16 19:56 ` Linus Torvalds
2012-10-17 16:59 ` Oleg Nesterov
2012-10-17 22:44 ` Paul E. McKenney
2012-10-18 16:24 ` Oleg Nesterov
2012-10-18 16:38 ` Paul E. McKenney
2012-10-18 17:57 ` Oleg Nesterov
2012-10-18 19:28 ` Mikulas Patocka
2012-10-19 12:38 ` Peter Zijlstra
2012-10-19 15:32 ` Mikulas Patocka
2012-10-19 17:40 ` Peter Zijlstra
2012-10-19 17:57 ` Oleg Nesterov
2012-10-19 22:54 ` Mikulas Patocka
2012-10-24 3:08 ` Dave Chinner
2012-10-25 14:09 ` Mikulas Patocka
2012-10-25 23:40 ` Dave Chinner
2012-10-26 12:06 ` Oleg Nesterov
2012-10-26 13:22 ` Mikulas Patocka
2012-10-26 14:12 ` Oleg Nesterov
2012-10-26 15:23 ` mark_files_ro && sb_end_write Oleg Nesterov
2012-10-26 16:09 ` [PATCH 1/2] brw_mutex: big read-write mutex Mikulas Patocka
2012-10-19 17:49 ` Oleg Nesterov
2012-10-22 23:09 ` Mikulas Patocka
2012-10-23 15:12 ` Oleg Nesterov
2012-10-19 19:28 ` Paul E. McKenney
2012-10-22 23:36 ` [PATCH 0/2] fix and improvements for percpu-rw-semaphores (was: brw_mutex: big read-write mutex) Mikulas Patocka
2012-10-22 23:37 ` [PATCH 1/2] percpu-rw-semaphores: use light/heavy barriers Mikulas Patocka
2012-10-22 23:39 ` [PATCH 2/2] percpu-rw-semaphores: use rcu_read_lock_sched Mikulas Patocka
2012-10-24 16:16 ` Paul E. McKenney
2012-10-24 17:18 ` Oleg Nesterov
2012-10-24 18:20 ` Paul E. McKenney
2012-10-24 18:43 ` Oleg Nesterov
2012-10-24 19:43 ` Paul E. McKenney
2012-10-25 14:54 ` Mikulas Patocka
2012-10-25 15:07 ` Paul E. McKenney
2012-10-25 16:15 ` Mikulas Patocka
2012-10-23 16:59 ` [PATCH 1/2] percpu-rw-semaphores: use light/heavy barriers Oleg Nesterov
2012-10-23 18:05 ` Paul E. McKenney
2012-10-23 18:27 ` Oleg Nesterov
2012-10-23 18:41 ` Oleg Nesterov
2012-10-23 20:29 ` Paul E. McKenney
2012-10-23 20:32 ` Paul E. McKenney
2012-10-23 21:39 ` Mikulas Patocka
2012-10-24 16:23 ` Paul E. McKenney
2012-10-24 20:22 ` Mikulas Patocka
2012-10-24 20:36 ` Paul E. McKenney
2012-10-24 20:44 ` Mikulas Patocka
2012-10-24 23:57 ` Paul E. McKenney
2012-10-25 12:39 ` Paul E. McKenney
2012-10-25 13:48 ` Mikulas Patocka
2012-10-23 19:23 ` Oleg Nesterov
2012-10-23 20:45 ` Peter Zijlstra
2012-10-23 20:57 ` Peter Zijlstra
2012-10-24 15:11 ` Oleg Nesterov
2012-10-23 21:26 ` Mikulas Patocka
2012-10-23 20:32 ` Peter Zijlstra
2012-10-30 18:48 ` [PATCH 0/2] fix and improvements for percpu-rw-semaphores (was: brw_mutex: big read-write mutex) Oleg Nesterov
2012-10-31 19:41 ` [PATCH 0/1] percpu_rw_semaphore: reimplement to not block the readers unnecessarily Oleg Nesterov
2012-10-31 19:41 ` [PATCH 1/1] " Oleg Nesterov
2012-11-01 15:10 ` Linus Torvalds
2012-11-01 15:34 ` Oleg Nesterov
2012-11-02 18:06 ` [PATCH v2 0/1] " Oleg Nesterov
2012-11-02 18:06 ` [PATCH v2 1/1] " Oleg Nesterov
2012-11-07 17:04 ` [PATCH v3 " Mikulas Patocka
2012-11-07 17:47 ` Oleg Nesterov
2012-11-07 19:17 ` Mikulas Patocka
2012-11-08 13:42 ` Oleg Nesterov
2012-11-08 1:23 ` Paul E. McKenney
2012-11-08 1:16 ` [PATCH v2 " Paul E. McKenney
2012-11-08 13:33 ` Oleg Nesterov
2012-11-08 16:27 ` Paul E. McKenney
2012-11-08 13:48 ` [PATCH RESEND v2 0/1] " Oleg Nesterov
2012-11-08 13:48 ` [PATCH RESEND v2 1/1] " Oleg Nesterov
2012-11-08 20:07 ` Andrew Morton
2012-11-08 21:08 ` Paul E. McKenney
2012-11-08 23:41 ` Mikulas Patocka
2012-11-09 0:41 ` Paul E. McKenney
2012-11-09 3:23 ` Paul E. McKenney
2012-11-09 16:35 ` Oleg Nesterov
2012-11-09 16:59 ` Paul E. McKenney
2012-11-09 12:47 ` Mikulas Patocka
2012-11-09 15:46 ` Oleg Nesterov
2012-11-09 17:01 ` Paul E. McKenney
2012-11-09 18:10 ` Oleg Nesterov
2012-11-09 18:19 ` Oleg Nesterov
2012-11-10 0:55 ` Paul E. McKenney
2012-11-11 15:45 ` Oleg Nesterov
2012-11-12 18:38 ` Paul E. McKenney
2012-11-11 18:27 ` [PATCH -mm] percpu_rw_semaphore-reimplement-to-not-block-the-readers-unnecessari ly.fix Oleg Nesterov
2012-11-12 18:31 ` Paul E. McKenney [this message]
2012-11-16 23:22 ` Andrew Morton
2012-11-18 19:32 ` Oleg Nesterov
2012-11-01 15:43 ` [PATCH 1/1] percpu_rw_semaphore: reimplement to not block the readers unnecessarily Paul E. McKenney
2012-11-01 18:33 ` Oleg Nesterov
2012-11-02 16:18 ` Oleg Nesterov
2012-10-15 19:10 ` [PATCH 2/2] uprobes: Use brw_mutex to fix register/unregister vs dup_mmap() race Oleg Nesterov
2012-10-18 7:03 ` Srikar Dronamraju
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20121112183128.GE2518@linux.vnet.ibm.com \
--to=paulmck@linux.vnet.ibm.com \
--cc=akpm@linux-foundation.org \
--cc=ananth@in.ibm.com \
--cc=anton@redhat.com \
--cc=linux-kernel@vger.kernel.org \
--cc=mingo@elte.hu \
--cc=mpatocka@redhat.com \
--cc=oleg@redhat.com \
--cc=peterz@infradead.org \
--cc=srikar@linux.vnet.ibm.com \
--cc=torvalds@linux-foundation.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).