From: Peter Zijlstra <peterz@infradead.org>
To: peterz@infradead.org, mingo@kernel.org
Cc: yabinc@google.com, acme@kernel.org,
alexander.shishkin@linux.intel.com, jolsa@redhat.com,
namhyung@kernel.org, mark.rutland@arm.com,
linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>
Subject: [PATCH 1/4] perf/ring_buffer: Fix exposing a temporarily decreased data_head.
Date: Fri, 17 May 2019 13:52:31 +0200 [thread overview]
Message-ID: <20190517115418.224478157@infradead.org> (raw)
In-Reply-To: 20190517115230.437269790@infradead.org
In perf_output_put_handle(), an IRQ/NMI can happen in below location and
write records to the same ring buffer:
...
local_dec_and_test(&rb->nest)
... <-- an IRQ/NMI can happen here
rb->user_page->data_head = head;
...
In this case, a value A is written to data_head in the IRQ, then a value
B is written to data_head after the IRQ. And A > B. As a result,
data_head is temporarily decreased from A to B. And a reader may see
data_head < data_tail if it read the buffer frequently enough, which
creates unexpected behaviors.
This can be fixed by moving dec(&rb->nest) to after updating data_head,
which prevents the IRQ/NMI above from updating data_head.
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Fixes: ef60777c9abd ("perf: Optimize the perf_output() path by removing IRQ-disables")
Signed-off-by: Yabin Cui <yabinc@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20190516184010.167903-1-yabinc@google.com
---
kernel/events/ring_buffer.c | 24 ++++++++++++++++++++----
1 file changed, 20 insertions(+), 4 deletions(-)
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -51,11 +51,18 @@ static void perf_output_put_handle(struc
head = local_read(&rb->head);
/*
- * IRQ/NMI can happen here, which means we can miss a head update.
+ * IRQ/NMI can happen here and advance @rb->head, causing our
+ * load above to be stale.
*/
- if (!local_dec_and_test(&rb->nest))
+ /*
+ * If this isn't the outermost nesting, we don't have to update
+ * @rb->user_page->data_head.
+ */
+ if (local_read(&rb->nest) > 1) {
+ local_dec(&rb->nest);
goto out;
+ }
/*
* Since the mmap() consumer (userspace) can run on a different CPU:
@@ -87,9 +94,18 @@ static void perf_output_put_handle(struc
rb->user_page->data_head = head;
/*
- * Now check if we missed an update -- rely on previous implied
- * compiler barriers to force a re-read.
+ * We must publish the head before decrementing the nest count,
+ * otherwise an IRQ/NMI can publish a more recent head value and our
+ * write will (temporarily) publish a stale value.
+ */
+ barrier();
+ local_set(&rb->nest, 0);
+
+ /*
+ * Ensure we decrement @rb->nest before we validate the @rb->head.
+ * Otherwise we cannot be sure we caught the 'last' nested update.
*/
+ barrier();
if (unlikely(head != local_read(&rb->head))) {
local_inc(&rb->nest);
goto again;
next prev parent reply other threads:[~2019-05-17 11:55 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-17 11:52 [PATCH 0/4] perf ring-buffer fixes Peter Zijlstra
2019-05-17 11:52 ` Peter Zijlstra [this message]
2019-05-17 13:05 ` [PATCH 1/4] perf/ring_buffer: Fix exposing a temporarily decreased data_head Ingo Molnar
2019-05-17 14:26 ` Peter Zijlstra
2019-05-24 7:01 ` Ingo Molnar
2019-05-24 8:08 ` [tip:perf/urgent] " tip-bot for Yabin Cui
2019-05-17 11:52 ` [PATCH 2/4] perf/ring_buffer: Add ordering to rb->nest increment Peter Zijlstra
2019-05-24 8:08 ` [tip:perf/urgent] " tip-bot for Peter Zijlstra
2019-05-17 11:52 ` [PATCH 3/4] perf/ring-buffer: Always use {READ,WRITE}_ONCE() for rb->user_page data Peter Zijlstra
2019-05-24 8:09 ` [tip:perf/urgent] " tip-bot for Peter Zijlstra
2019-05-17 11:52 ` [PATCH 4/4] perf/ring-buffer: Use regular variables for nesting Peter Zijlstra
2019-05-24 8:10 ` [tip:perf/urgent] " tip-bot for Peter Zijlstra
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=20190517115418.224478157@infradead.org \
--to=peterz@infradead.org \
--cc=acme@kernel.org \
--cc=alexander.shishkin@linux.intel.com \
--cc=jolsa@redhat.com \
--cc=linux-kernel@vger.kernel.org \
--cc=mark.rutland@arm.com \
--cc=mingo@kernel.org \
--cc=mingo@redhat.com \
--cc=namhyung@kernel.org \
--cc=yabinc@google.com \
/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).