From: Oleg Nesterov <oleg@redhat.com> To: Peter Zijlstra <peterz@infradead.org> Cc: Victor Kaplansky <VICTORK@il.ibm.com>, Frederic Weisbecker <fweisbec@gmail.com>, Anton Blanchard <anton@samba.org>, Benjamin Herrenschmidt <benh@kernel.crashing.org>, LKML <linux-kernel@vger.kernel.org>, Linux PPC dev <linuxppc-dev@ozlabs.org>, Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>, Michael Ellerman <michael@ellerman.id.au>, Michael Neuling <mikey@neuling.org>, Paul McKenney <paulmck@linux.vnet.ibm.com> Subject: Re: perf events ring buffer memory barrier on powerpc Date: Mon, 28 Oct 2013 20:09:28 +0100 [thread overview] Message-ID: <20131028190928.GA11449@redhat.com> (raw) In-Reply-To: <20131028132634.GO19466@laptop.lan> On 10/28, Peter Zijlstra wrote: > > Lets add Paul and Oleg to the thread; this is getting far more 'fun' > that it should be ;-) Heh. All I can say is that I would like to see the authoritative answer, you know who can shed a light ;) But to avoid the confusion, wmb() added by this patch looks "obviously correct" to me. > + * Since the mmap() consumer (userspace) can run on a different CPU: > + * > + * kernel user > + * > + * READ ->data_tail READ ->data_head > + * smp_rmb() (A) smp_rmb() (C) > + * WRITE $data READ $data > + * smp_wmb() (B) smp_mb() (D) > + * STORE ->data_head WRITE ->data_tail > + * > + * Where A pairs with D, and B pairs with C. > + * > + * I don't think A needs to be a full barrier because we won't in fact > + * write data until we see the store from userspace. this matches the intuition, but ... > So we simply don't > + * issue the data WRITE until we observe it. why do we need any barrier (rmb) then? how it can help to serialize with "WRITE $data" ? (of course there could be other reasons for this rmb(), just I can't really understand "A pairs with D"). And this reminds me about the memory barrier in kfifo.c which I was not able to understand. Hmm, it has already gone away, and now I do not understand kfifo.c at all ;) But I have found the commit, attached below. Note that that commit added the full barrier into __kfifo_put(). And to me it looks the same as "A" above. Following the logic above we could say that we do not need a barrier (at least the full one), we won't in fact write into the "unread" area until we see the store to ->out from __kfifo_get() ? In short. I am confused, I _feel_ that "A" has to be a full barrier, but I can't prove this. And let me suggest the artificial/simplified example, bool BUSY; data_t DATA; bool try_to_get(data_t *data) { if (!BUSY) return false; rmb(); *data = DATA; mb(); BUSY = false; return true; } bool try_to_put(data_t *data) { if (BUSY) return false; mb(); // XXXXXXXX: do we really need it? I think yes. DATA = *data; wmb(); BUSY = true; return true; } Again, following the description above we could turn the mb() in _put() into barrier(), but I do not think we can rely on the contorl dependency. Oleg. --- commit a45bce49545739a940f8bd4ca85c3b7435564893 Author: Paul E. McKenney <paulmck@us.ibm.com> Date: Fri Sep 29 02:00:11 2006 -0700 [PATCH] memory ordering in __kfifo primitives Both __kfifo_put() and __kfifo_get() have header comments stating that if there is but one concurrent reader and one concurrent writer, locking is not necessary. This is almost the case, but a couple of memory barriers are needed. Another option would be to change the header comments to remove the bit about locking not being needed, and to change the those callers who currently don't use locking to add the required locking. The attachment analyzes this approach, but the patch below seems simpler. Signed-off-by: Paul E. McKenney <paulmck@us.ibm.com> Cc: Stelian Pop <stelian@popies.net> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org> diff --git a/kernel/kfifo.c b/kernel/kfifo.c index 64ab045..5d1d907 100644 --- a/kernel/kfifo.c +++ b/kernel/kfifo.c @@ -122,6 +122,13 @@ unsigned int __kfifo_put(struct kfifo *fifo, len = min(len, fifo->size - fifo->in + fifo->out); + /* + * Ensure that we sample the fifo->out index -before- we + * start putting bytes into the kfifo. + */ + + smp_mb(); + /* first put the data starting from fifo->in to buffer end */ l = min(len, fifo->size - (fifo->in & (fifo->size - 1))); memcpy(fifo->buffer + (fifo->in & (fifo->size - 1)), buffer, l); @@ -129,6 +136,13 @@ unsigned int __kfifo_put(struct kfifo *fifo, /* then put the rest (if any) at the beginning of the buffer */ memcpy(fifo->buffer, buffer + l, len - l); + /* + * Ensure that we add the bytes to the kfifo -before- + * we update the fifo->in index. + */ + + smp_wmb(); + fifo->in += len; return len; @@ -154,6 +168,13 @@ unsigned int __kfifo_get(struct kfifo *fifo, len = min(len, fifo->in - fifo->out); + /* + * Ensure that we sample the fifo->in index -before- we + * start removing bytes from the kfifo. + */ + + smp_rmb(); + /* first get the data from fifo->out until the end of the buffer */ l = min(len, fifo->size - (fifo->out & (fifo->size - 1))); memcpy(buffer, fifo->buffer + (fifo->out & (fifo->size - 1)), l); @@ -161,6 +182,13 @@ unsigned int __kfifo_get(struct kfifo *fifo, /* then get the rest (if any) from the beginning of the buffer */ memcpy(buffer + l, fifo->buffer, len - l); + /* + * Ensure that we remove the bytes from the kfifo -before- + * we update the fifo->out index. + */ + + smp_mb(); + fifo->out += len; return len;
WARNING: multiple messages have this Message-ID (diff)
From: Oleg Nesterov <oleg@redhat.com> To: Peter Zijlstra <peterz@infradead.org> Cc: Michael Neuling <mikey@neuling.org>, Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>, Frederic Weisbecker <fweisbec@gmail.com>, LKML <linux-kernel@vger.kernel.org>, Linux PPC dev <linuxppc-dev@ozlabs.org>, Anton Blanchard <anton@samba.org>, Victor Kaplansky <VICTORK@il.ibm.com>, Paul McKenney <paulmck@linux.vnet.ibm.com> Subject: Re: perf events ring buffer memory barrier on powerpc Date: Mon, 28 Oct 2013 20:09:28 +0100 [thread overview] Message-ID: <20131028190928.GA11449@redhat.com> (raw) In-Reply-To: <20131028132634.GO19466@laptop.lan> On 10/28, Peter Zijlstra wrote: > > Lets add Paul and Oleg to the thread; this is getting far more 'fun' > that it should be ;-) Heh. All I can say is that I would like to see the authoritative answer, you know who can shed a light ;) But to avoid the confusion, wmb() added by this patch looks "obviously correct" to me. > + * Since the mmap() consumer (userspace) can run on a different CPU: > + * > + * kernel user > + * > + * READ ->data_tail READ ->data_head > + * smp_rmb() (A) smp_rmb() (C) > + * WRITE $data READ $data > + * smp_wmb() (B) smp_mb() (D) > + * STORE ->data_head WRITE ->data_tail > + * > + * Where A pairs with D, and B pairs with C. > + * > + * I don't think A needs to be a full barrier because we won't in fact > + * write data until we see the store from userspace. this matches the intuition, but ... > So we simply don't > + * issue the data WRITE until we observe it. why do we need any barrier (rmb) then? how it can help to serialize with "WRITE $data" ? (of course there could be other reasons for this rmb(), just I can't really understand "A pairs with D"). And this reminds me about the memory barrier in kfifo.c which I was not able to understand. Hmm, it has already gone away, and now I do not understand kfifo.c at all ;) But I have found the commit, attached below. Note that that commit added the full barrier into __kfifo_put(). And to me it looks the same as "A" above. Following the logic above we could say that we do not need a barrier (at least the full one), we won't in fact write into the "unread" area until we see the store to ->out from __kfifo_get() ? In short. I am confused, I _feel_ that "A" has to be a full barrier, but I can't prove this. And let me suggest the artificial/simplified example, bool BUSY; data_t DATA; bool try_to_get(data_t *data) { if (!BUSY) return false; rmb(); *data = DATA; mb(); BUSY = false; return true; } bool try_to_put(data_t *data) { if (BUSY) return false; mb(); // XXXXXXXX: do we really need it? I think yes. DATA = *data; wmb(); BUSY = true; return true; } Again, following the description above we could turn the mb() in _put() into barrier(), but I do not think we can rely on the contorl dependency. Oleg. --- commit a45bce49545739a940f8bd4ca85c3b7435564893 Author: Paul E. McKenney <paulmck@us.ibm.com> Date: Fri Sep 29 02:00:11 2006 -0700 [PATCH] memory ordering in __kfifo primitives Both __kfifo_put() and __kfifo_get() have header comments stating that if there is but one concurrent reader and one concurrent writer, locking is not necessary. This is almost the case, but a couple of memory barriers are needed. Another option would be to change the header comments to remove the bit about locking not being needed, and to change the those callers who currently don't use locking to add the required locking. The attachment analyzes this approach, but the patch below seems simpler. Signed-off-by: Paul E. McKenney <paulmck@us.ibm.com> Cc: Stelian Pop <stelian@popies.net> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org> diff --git a/kernel/kfifo.c b/kernel/kfifo.c index 64ab045..5d1d907 100644 --- a/kernel/kfifo.c +++ b/kernel/kfifo.c @@ -122,6 +122,13 @@ unsigned int __kfifo_put(struct kfifo *fifo, len = min(len, fifo->size - fifo->in + fifo->out); + /* + * Ensure that we sample the fifo->out index -before- we + * start putting bytes into the kfifo. + */ + + smp_mb(); + /* first put the data starting from fifo->in to buffer end */ l = min(len, fifo->size - (fifo->in & (fifo->size - 1))); memcpy(fifo->buffer + (fifo->in & (fifo->size - 1)), buffer, l); @@ -129,6 +136,13 @@ unsigned int __kfifo_put(struct kfifo *fifo, /* then put the rest (if any) at the beginning of the buffer */ memcpy(fifo->buffer, buffer + l, len - l); + /* + * Ensure that we add the bytes to the kfifo -before- + * we update the fifo->in index. + */ + + smp_wmb(); + fifo->in += len; return len; @@ -154,6 +168,13 @@ unsigned int __kfifo_get(struct kfifo *fifo, len = min(len, fifo->in - fifo->out); + /* + * Ensure that we sample the fifo->in index -before- we + * start removing bytes from the kfifo. + */ + + smp_rmb(); + /* first get the data from fifo->out until the end of the buffer */ l = min(len, fifo->size - (fifo->out & (fifo->size - 1))); memcpy(buffer, fifo->buffer + (fifo->out & (fifo->size - 1)), l); @@ -161,6 +182,13 @@ unsigned int __kfifo_get(struct kfifo *fifo, /* then get the rest (if any) from the beginning of the buffer */ memcpy(buffer + l, fifo->buffer, len - l); + /* + * Ensure that we remove the bytes from the kfifo -before- + * we update the fifo->out index. + */ + + smp_mb(); + fifo->out += len; return len;
next prev parent reply other threads:[~2013-10-28 18:17 UTC|newest] Thread overview: 215+ messages / expand[flat|nested] mbox.gz Atom feed top 2013-10-22 23:54 perf events ring buffer memory barrier on powerpc Michael Neuling 2013-10-23 7:39 ` Victor Kaplansky 2013-10-23 7:39 ` Victor Kaplansky 2013-10-23 14:19 ` Frederic Weisbecker 2013-10-23 14:19 ` Frederic Weisbecker 2013-10-23 14:25 ` Frederic Weisbecker 2013-10-23 14:25 ` Frederic Weisbecker 2013-10-25 17:37 ` Peter Zijlstra 2013-10-25 17:37 ` Peter Zijlstra 2013-10-25 20:31 ` Michael Neuling 2013-10-25 20:31 ` Michael Neuling 2013-10-27 9:00 ` Victor Kaplansky 2013-10-27 9:00 ` Victor Kaplansky 2013-10-28 9:22 ` Peter Zijlstra 2013-10-28 9:22 ` Peter Zijlstra 2013-10-28 10:02 ` Frederic Weisbecker 2013-10-28 10:02 ` Frederic Weisbecker 2013-10-28 12:38 ` Victor Kaplansky 2013-10-28 12:38 ` Victor Kaplansky 2013-10-28 13:26 ` Peter Zijlstra 2013-10-28 13:26 ` Peter Zijlstra 2013-10-28 16:34 ` Paul E. McKenney 2013-10-28 16:34 ` Paul E. McKenney 2013-10-28 20:17 ` Oleg Nesterov 2013-10-28 20:17 ` Oleg Nesterov 2013-10-28 20:58 ` Victor Kaplansky 2013-10-28 20:58 ` Victor Kaplansky 2013-10-29 10:21 ` Peter Zijlstra 2013-10-29 10:21 ` Peter Zijlstra 2013-10-29 10:30 ` Peter Zijlstra 2013-10-29 10:30 ` Peter Zijlstra 2013-10-29 10:35 ` Peter Zijlstra 2013-10-29 10:35 ` Peter Zijlstra 2013-10-29 20:15 ` Oleg Nesterov 2013-10-29 20:15 ` Oleg Nesterov 2013-10-29 19:27 ` Vince Weaver 2013-10-29 19:27 ` Vince Weaver 2013-10-30 10:42 ` Peter Zijlstra 2013-10-30 10:42 ` Peter Zijlstra 2013-10-30 11:48 ` James Hogan 2013-10-30 11:48 ` James Hogan 2013-10-30 12:48 ` Peter Zijlstra 2013-10-30 12:48 ` Peter Zijlstra 2013-11-06 13:19 ` [tip:perf/core] tools/perf: Add required memory barriers tip-bot for Peter Zijlstra 2013-11-06 13:50 ` Vince Weaver 2013-11-06 14:00 ` Peter Zijlstra 2013-11-06 14:28 ` Peter Zijlstra 2013-11-06 14:55 ` Vince Weaver 2013-11-06 15:10 ` Peter Zijlstra 2013-11-06 15:23 ` Peter Zijlstra 2013-11-06 14:44 ` Peter Zijlstra 2013-11-06 16:07 ` Peter Zijlstra 2013-11-06 17:31 ` Vince Weaver 2013-11-06 18:24 ` Peter Zijlstra 2013-11-07 8:21 ` Ingo Molnar 2013-11-07 14:27 ` Vince Weaver 2013-11-07 15:55 ` Ingo Molnar 2013-11-11 16:24 ` Peter Zijlstra 2013-11-11 21:10 ` Ingo Molnar 2013-10-29 21:23 ` perf events ring buffer memory barrier on powerpc Michael Neuling 2013-10-29 21:23 ` Michael Neuling 2013-10-30 9:27 ` Paul E. McKenney 2013-10-30 9:27 ` Paul E. McKenney 2013-10-30 11:25 ` Peter Zijlstra 2013-10-30 11:25 ` Peter Zijlstra 2013-10-30 14:52 ` Victor Kaplansky 2013-10-30 14:52 ` Victor Kaplansky 2013-10-30 15:39 ` Peter Zijlstra 2013-10-30 15:39 ` Peter Zijlstra 2013-10-30 17:14 ` Victor Kaplansky 2013-10-30 17:14 ` Victor Kaplansky 2013-10-30 17:44 ` Peter Zijlstra 2013-10-30 17:44 ` Peter Zijlstra 2013-10-31 6:16 ` Paul E. McKenney 2013-10-31 6:16 ` Paul E. McKenney 2013-11-01 13:12 ` Victor Kaplansky 2013-11-01 13:12 ` Victor Kaplansky 2013-11-02 16:36 ` Paul E. McKenney 2013-11-02 16:36 ` Paul E. McKenney 2013-11-02 17:26 ` Paul E. McKenney 2013-11-02 17:26 ` Paul E. McKenney 2013-10-31 6:40 ` Paul E. McKenney 2013-10-31 6:40 ` Paul E. McKenney 2013-11-01 14:25 ` Victor Kaplansky 2013-11-01 14:25 ` Victor Kaplansky 2013-11-02 17:28 ` Paul E. McKenney 2013-11-02 17:28 ` Paul E. McKenney 2013-11-01 14:56 ` Peter Zijlstra 2013-11-01 14:56 ` Peter Zijlstra 2013-11-02 17:32 ` Paul E. McKenney 2013-11-02 17:32 ` Paul E. McKenney 2013-11-03 14:40 ` Paul E. McKenney 2013-11-03 14:40 ` Paul E. McKenney 2013-11-03 15:17 ` [RFC] arch: Introduce new TSO memory barrier smp_tmb() Peter Zijlstra 2013-11-03 15:17 ` Peter Zijlstra 2013-11-03 18:08 ` Linus Torvalds 2013-11-03 18:08 ` Linus Torvalds 2013-11-03 20:01 ` Peter Zijlstra 2013-11-03 20:01 ` Peter Zijlstra 2013-11-03 22:42 ` Paul E. McKenney 2013-11-03 22:42 ` Paul E. McKenney 2013-11-03 23:34 ` Linus Torvalds 2013-11-03 23:34 ` Linus Torvalds 2013-11-04 10:51 ` Paul E. McKenney 2013-11-04 10:51 ` Paul E. McKenney 2013-11-04 11:22 ` Peter Zijlstra 2013-11-04 11:22 ` Peter Zijlstra 2013-11-04 16:27 ` Paul E. McKenney 2013-11-04 16:27 ` Paul E. McKenney 2013-11-04 16:48 ` Peter Zijlstra 2013-11-04 16:48 ` Peter Zijlstra 2013-11-04 19:11 ` Peter Zijlstra 2013-11-04 19:11 ` Peter Zijlstra 2013-11-04 19:18 ` Peter Zijlstra 2013-11-04 19:18 ` Peter Zijlstra 2013-11-04 20:54 ` Paul E. McKenney 2013-11-04 20:54 ` Paul E. McKenney 2013-11-04 20:53 ` Paul E. McKenney 2013-11-04 20:53 ` Paul E. McKenney 2013-11-05 14:05 ` Will Deacon 2013-11-05 14:05 ` Will Deacon 2013-11-05 14:49 ` Paul E. McKenney 2013-11-05 14:49 ` Paul E. McKenney 2013-11-05 18:49 ` Peter Zijlstra 2013-11-05 18:49 ` Peter Zijlstra 2013-11-06 11:00 ` Will Deacon 2013-11-06 11:00 ` Will Deacon 2013-11-06 12:39 ` Peter Zijlstra 2013-11-06 12:39 ` Peter Zijlstra 2013-11-06 12:51 ` Geert Uytterhoeven 2013-11-06 12:51 ` Geert Uytterhoeven 2013-11-06 13:57 ` Peter Zijlstra 2013-11-06 13:57 ` Peter Zijlstra 2013-11-06 18:48 ` Paul E. McKenney 2013-11-06 18:48 ` Paul E. McKenney 2013-11-06 19:42 ` Peter Zijlstra 2013-11-06 19:42 ` Peter Zijlstra 2013-11-07 11:17 ` Will Deacon 2013-11-07 11:17 ` Will Deacon 2013-11-07 13:36 ` Peter Zijlstra 2013-11-07 13:36 ` Peter Zijlstra 2013-11-07 23:50 ` Mathieu Desnoyers 2013-11-07 23:50 ` Mathieu Desnoyers 2013-11-04 11:05 ` Will Deacon 2013-11-04 11:05 ` Will Deacon 2013-11-04 16:34 ` Paul E. McKenney 2013-11-04 16:34 ` Paul E. McKenney 2013-11-03 20:59 ` Benjamin Herrenschmidt 2013-11-03 20:59 ` Benjamin Herrenschmidt 2013-11-03 22:43 ` Paul E. McKenney 2013-11-03 22:43 ` Paul E. McKenney 2013-11-03 17:07 ` perf events ring buffer memory barrier on powerpc Will Deacon 2013-11-03 22:47 ` Paul E. McKenney 2013-11-04 9:57 ` Will Deacon 2013-11-04 10:52 ` Paul E. McKenney 2013-11-01 16:11 ` Peter Zijlstra 2013-11-01 16:11 ` Peter Zijlstra 2013-11-02 17:46 ` Paul E. McKenney 2013-11-02 17:46 ` Paul E. McKenney 2013-11-01 16:18 ` Peter Zijlstra 2013-11-01 16:18 ` Peter Zijlstra 2013-11-02 17:49 ` Paul E. McKenney 2013-11-02 17:49 ` Paul E. McKenney 2013-10-30 13:28 ` Victor Kaplansky 2013-10-30 13:28 ` Victor Kaplansky 2013-10-30 15:51 ` Peter Zijlstra 2013-10-30 15:51 ` Peter Zijlstra 2013-10-30 18:29 ` Peter Zijlstra 2013-10-30 18:29 ` Peter Zijlstra 2013-10-30 19:11 ` Peter Zijlstra 2013-10-30 19:11 ` Peter Zijlstra 2013-10-31 4:33 ` Paul E. McKenney 2013-10-31 4:33 ` Paul E. McKenney 2013-10-31 4:32 ` Paul E. McKenney 2013-10-31 4:32 ` Paul E. McKenney 2013-10-31 9:04 ` Peter Zijlstra 2013-10-31 9:04 ` Peter Zijlstra 2013-10-31 15:07 ` Paul E. McKenney 2013-10-31 15:07 ` Paul E. McKenney 2013-10-31 15:19 ` Peter Zijlstra 2013-10-31 15:19 ` Peter Zijlstra 2013-11-01 9:28 ` Paul E. McKenney 2013-11-01 9:28 ` Paul E. McKenney 2013-11-01 10:30 ` Peter Zijlstra 2013-11-01 10:30 ` Peter Zijlstra 2013-11-02 15:20 ` Paul E. McKenney 2013-11-02 15:20 ` Paul E. McKenney 2013-11-04 9:07 ` Peter Zijlstra 2013-11-04 9:07 ` Peter Zijlstra 2013-11-04 10:00 ` Paul E. McKenney 2013-11-04 10:00 ` Paul E. McKenney 2013-10-31 9:59 ` Victor Kaplansky 2013-10-31 9:59 ` Victor Kaplansky 2013-10-31 12:28 ` David Laight 2013-10-31 12:28 ` David Laight 2013-10-31 12:55 ` Victor Kaplansky 2013-10-31 12:55 ` Victor Kaplansky 2013-10-31 15:25 ` Paul E. McKenney 2013-10-31 15:25 ` Paul E. McKenney 2013-11-01 16:06 ` Victor Kaplansky 2013-11-01 16:06 ` Victor Kaplansky 2013-11-01 16:25 ` David Laight 2013-11-01 16:25 ` David Laight 2013-11-01 16:30 ` Victor Kaplansky 2013-11-01 16:30 ` Victor Kaplansky 2013-11-03 20:57 ` Benjamin Herrenschmidt 2013-11-03 20:57 ` Benjamin Herrenschmidt 2013-11-02 15:46 ` Paul E. McKenney 2013-11-02 15:46 ` Paul E. McKenney 2013-10-28 19:09 ` Oleg Nesterov [this message] 2013-10-28 19:09 ` Oleg Nesterov 2013-10-29 14:06 ` [tip:perf/urgent] perf: Fix perf ring buffer memory ordering tip-bot for Peter Zijlstra 2014-05-08 20:46 perf events ring buffer memory barrier on powerpc Mikulas Patocka [not found] ` <OF667059AA.7F151BCC-ONC2257CD3.0036CFEB-C2257CD3.003BBF01@il.ibm.com> 2014-05-09 12:20 ` Mikulas Patocka 2014-05-09 13:47 ` Paul E. McKenney
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