From: "Patel, Vedang" <vedang.patel@intel.com>
To: "rostedt@goodmis.org" <rostedt@goodmis.org>,
"tom.zanussi@linux.intel.com" <tom.zanussi@linux.intel.com>
Cc: "bigeasy@linutronix.de" <bigeasy@linutronix.de>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"tglx@linutronix.de" <tglx@linutronix.de>,
"namhyung@kernel.org" <namhyung@kernel.org>,
"joelaf@google.com" <joelaf@google.com>,
"mhiramat@kernel.org" <mhiramat@kernel.org>,
"linux-rt-users@vger.kernel.org" <linux-rt-users@vger.kernel.org>,
"mathieu.desnoyers@efficios.com" <mathieu.desnoyers@efficios.com>,
"joel.opensrc@gmail.com" <joel.opensrc@gmail.com>,
"Liu, Baohong" <baohong.liu@intel.com>
Subject: Re: [PATCH v2 02/40] tracing: Add support to detect and avoid duplicates
Date: Wed, 6 Sep 2017 20:58:03 +0000 [thread overview]
Message-ID: <1504731482.21216.8.camel@intel.com> (raw)
In-Reply-To: <20170906144722.6eb71312@gandalf.local.home>
On Wed, 2017-09-06 at 14:47 -0400, Steven Rostedt wrote:
> On Tue, 5 Sep 2017 16:57:14 -0500
> Tom Zanussi <tom.zanussi@linux.intel.com> wrote:
>
>
> >
> > diff --git a/kernel/trace/tracing_map.c
> > b/kernel/trace/tracing_map.c
> > index 305039b..437b490 100644
> > --- a/kernel/trace/tracing_map.c
> > +++ b/kernel/trace/tracing_map.c
> > @@ -414,6 +414,7 @@ static inline bool keys_match(void *key, void
> > *test_key, unsigned key_size)
> > __tracing_map_insert(struct tracing_map *map, void *key, bool
> > lookup_only)
> > {
> > u32 idx, key_hash, test_key;
> > + int dup_try = 0;
> > struct tracing_map_entry *entry;
> >
> > key_hash = jhash(key, map->key_size, 0);
> > @@ -426,10 +427,31 @@ static inline bool keys_match(void *key, void
> > *test_key, unsigned key_size)
> > entry = TRACING_MAP_ENTRY(map->map, idx);
> > test_key = entry->key;
> >
> > - if (test_key && test_key == key_hash && entry->val
> > &&
> > - keys_match(key, entry->val->key, map-
> > >key_size)) {
> > - atomic64_inc(&map->hits);
> > - return entry->val;
> > + if (test_key && test_key == key_hash) {
> > + if (entry->val &&
> > + keys_match(key, entry->val->key, map-
> > >key_size)) {
> > + atomic64_inc(&map->hits);
> > + return entry->val;
> > + } else if (unlikely(!entry->val)) {
> I'm thinking we need a READ_ONCE() here.
>
> val = READ_ONCE(entry->val);
>
> then use "val" instead of entry->val. Otherwise, wont it be possible
> if two tasks are inserting at the same time, to have this:
>
> (Using reg as when the value is read into a register from memory)
>
> CPU0 CPU1
> ---- ----
> reg = entry->val
> (reg == zero)
>
> entry->val = elt;
>
> keys_match(key, reg)
> (false)
>
> reg = entry->val
> (reg = elt)
>
> if (unlikely(!reg))
>
> Causes the if to fail.
>
> A READ_ONCE(), would make sure the entry->val used to test against
> key
> would also be the same value used to test if it is zero.
>
Hi Steve,
Thanks for the input.
I agree with your change. Adding READ_ONCE will avoid a race condition
which might result in adding duplicates. Will add it in the next
version.
-Vedang
> -- Steve
>
>
>
> >
> > + /*
> > + * The key is present. But, val
> > (pointer to elt
> > + * struct) is still NULL. which
> > means some other
> > + * thread is in the process of
> > inserting an
> > + * element.
> > + *
> > + * On top of that, it's key_hash
> > is same as the
> > + * one being inserted right now.
> > So, it's
> > + * possible that the element has
> > the same
> > + * key as well.
> > + */
> > +
> > + dup_try++;
> > + if (dup_try > map->map_size) {
> > + atomic64_inc(&map->drops);
> > + break;
> > + }
> > + continue;
> > + }
> > }
> >
> > if (!test_key) {
> > @@ -451,6 +473,13 @@ static inline bool keys_match(void *key, void
> > *test_key, unsigned key_size)
> > atomic64_inc(&map->hits);
> >
> > return entry->val;
> > + } else {
> > + /*
> > + * cmpxchg() failed. Loop around
> > once
> > + * more to check what key was
> > inserted.
> > + */
> > + dup_try++;
> > + continue;
> > }
> > }
> >
next prev parent reply other threads:[~2017-09-06 20:58 UTC|newest]
Thread overview: 90+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-09-05 21:57 [PATCH v2 00/40] tracing: Inter-event (e.g. latency) support Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 01/40] tracing: Exclude 'generic fields' from histograms Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 02/40] tracing: Add support to detect and avoid duplicates Tom Zanussi
2017-09-06 18:32 ` Steven Rostedt
2017-09-06 18:47 ` Steven Rostedt
2017-09-06 20:58 ` Patel, Vedang [this message]
2017-09-05 21:57 ` [PATCH v2 03/40] tracing: Remove code which merges duplicates Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 04/40] tracing: Add hist_field_name() accessor Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 05/40] tracing: Reimplement log2 Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 06/40] ring-buffer: Add interface for setting absolute time stamps Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 07/40] tracing: Apply absolute timestamps to instance max buffer Tom Zanussi
2017-09-06 19:57 ` Steven Rostedt
2017-09-07 0:49 ` Steven Rostedt
2017-09-07 1:15 ` Liu, Baohong
2017-09-05 21:57 ` [PATCH v2 08/40] ring-buffer: Redefine the unimplemented RINGBUF_TIME_TIME_STAMP Tom Zanussi
2017-09-07 14:35 ` Steven Rostedt
2017-09-07 15:05 ` Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 09/40] tracing: Give event triggers access to ring_buffer_event Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 10/40] tracing: Add ring buffer event param to hist field functions Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 11/40] tracing: Increase tracing map KEYS_MAX size Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 12/40] tracing: Break out hist trigger assignment parsing Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 13/40] tracing: Make traceprobe parsing code reusable Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 14/40] tracing: Add hist trigger timestamp support Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 15/40] tracing: Add per-element variable support to tracing_map Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 16/40] tracing: Add hist_data member to hist_field Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 17/40] tracing: Add usecs modifier for hist trigger timestamps Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 18/40] tracing: Add variable support to hist triggers Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 19/40] tracing: Account for variables in named trigger compatibility Tom Zanussi
2017-09-07 16:40 ` Steven Rostedt
2017-09-07 17:00 ` Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 20/40] tracing: Add simple expression support to hist triggers Tom Zanussi
2017-09-07 16:46 ` Steven Rostedt
2017-09-07 17:01 ` Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 21/40] tracing: Generalize per-element hist trigger data Tom Zanussi
2017-09-07 17:56 ` Steven Rostedt
2017-09-07 18:14 ` Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 22/40] tracing: Pass tracing_map_elt to hist_field accessor functions Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 23/40] tracing: Add hist_field 'type' field Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 24/40] tracing: Add variable reference handling to hist triggers Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 25/40] tracing: Add support for dynamic tracepoints Tom Zanussi
2017-09-05 23:29 ` Mathieu Desnoyers
2017-09-06 2:35 ` Tom Zanussi
2017-09-07 22:02 ` Steven Rostedt
2017-09-08 14:18 ` Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 26/40] tracing: Add hist trigger action hook Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 27/40] tracing: Add support for 'synthetic' events Tom Zanussi
2017-09-07 23:40 ` Steven Rostedt
2017-09-08 14:30 ` Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 28/40] tracing: Add support for 'field variables' Tom Zanussi
2017-09-07 23:43 ` Steven Rostedt
2017-09-08 15:37 ` Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 29/40] tracing: Add 'onmatch' hist trigger action support Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 30/40] tracing: Add 'onmax' " Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 31/40] tracing: Allow whitespace to surround hist trigger filter Tom Zanussi
2017-09-08 18:50 ` Steven Rostedt
2017-09-08 19:08 ` Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 32/40] tracing: Add cpu field for hist triggers Tom Zanussi
2017-09-08 19:08 ` Steven Rostedt
2017-09-08 19:35 ` Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 33/40] tracing: Add hist trigger support for variable reference aliases Tom Zanussi
2017-09-08 19:09 ` Steven Rostedt
2017-09-08 19:41 ` Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 34/40] tracing: Add 'last error' error facility for hist triggers Tom Zanussi
2017-09-08 19:25 ` Steven Rostedt
2017-09-08 19:44 ` Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 35/40] tracing: Reverse the order event_mutex/trace_types_lock are taken Tom Zanussi
2017-09-08 19:31 ` Steven Rostedt
2017-09-08 19:41 ` Steven Rostedt
2017-09-08 20:00 ` Steven Rostedt
2017-09-05 21:57 ` [PATCH v2 36/40] tracing: Remove lookups from tracing_map hitcount Tom Zanussi
2017-09-12 2:16 ` Masami Hiramatsu
2017-09-12 14:16 ` Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 37/40] tracing: Add inter-event hist trigger Documentation Tom Zanussi
2017-09-20 14:44 ` Julia Cartwright
2017-09-20 17:15 ` Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 38/40] tracing: Make tracing_set_clock() non-static Tom Zanussi
2017-09-12 2:18 ` Masami Hiramatsu
2017-09-12 14:18 ` Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 39/40] tracing: Add a clock attribute for hist triggers Tom Zanussi
2017-09-05 21:57 ` [PATCH v2 40/40] tracing: Add trace_event_buffer_reserve() variant that allows recursion Tom Zanussi
2017-09-07 22:29 ` kbuild test robot
2017-09-07 22:35 ` kbuild test robot
2017-09-08 20:27 ` Steven Rostedt
2017-09-08 20:41 ` Tom Zanussi
2017-09-12 1:50 ` [PATCH v2 00/40] tracing: Inter-event (e.g. latency) support Masami Hiramatsu
2017-09-12 14:14 ` Tom Zanussi
2017-09-19 16:31 ` Steven Rostedt
2017-09-19 18:44 ` Tom Zanussi
2017-09-21 20:20 ` Steven Rostedt
2017-09-21 21:11 ` Tom Zanussi
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