From 8b46ff6938d2c78a16520a37086944ecbaa8ab8b Mon Sep 17 00:00:00 2001 From: Petr Mladek Date: Mon, 7 Sep 2015 14:38:37 +0200 Subject: ring_buffer: Do no not complete benchmark reader too early It seems that complete(&read_done) might be called too early in some situations. 1st scenario: ------------- CPU0 CPU1 ring_buffer_producer_thread() wake_up_process(consumer); wait_for_completion(&read_start); ring_buffer_consumer_thread() complete(&read_start); ring_buffer_producer() # producing data in # the do-while cycle ring_buffer_consumer(); # reading data # got error # set kill_test = 1; set_current_state( TASK_INTERRUPTIBLE); if (reader_finish) # false schedule(); # producer still in the middle of # do-while cycle if (consumer && !(cnt % wakeup_interval)) wake_up_process(consumer); # spurious wakeup while (!reader_finish && !kill_test) # leaving because # kill_test == 1 reader_finish = 0; complete(&read_done); 1st BANG: We might access uninitialized "read_done" if this is the the first round. # producer finally leaving # the do-while cycle because kill_test == 1; if (consumer) { reader_finish = 1; wake_up_process(consumer); wait_for_completion(&read_done); 2nd BANG: This will never complete because consumer already did the completion. 2nd scenario: ------------- CPU0 CPU1 ring_buffer_producer_thread() wake_up_process(consumer); wait_for_completion(&read_start); ring_buffer_consumer_thread() complete(&read_start); ring_buffer_producer() # CPU3 removes the module <--- difference from # and stops producer <--- the 1st scenario if (kthread_should_stop()) kill_test = 1; ring_buffer_consumer(); while (!reader_finish && !kill_test) # kill_test == 1 => we never go # into the top level while() reader_finish = 0; complete(&read_done); # producer still in the middle of # do-while cycle if (consumer && !(cnt % wakeup_interval)) wake_up_process(consumer); # spurious wakeup while (!reader_finish && !kill_test) # leaving because kill_test == 1 reader_finish = 0; complete(&read_done); BANG: We are in the same "bang" situations as in the 1st scenario. Root of the problem: -------------------- ring_buffer_consumer() must complete "read_done" only when "reader_finish" variable is set. It must not be skipped due to other conditions. Note that we still must keep the check for "reader_finish" in a loop because there might be spurious wakeups as described in the above scenarios. Solution: ---------- The top level cycle in ring_buffer_consumer() will finish only when "reader_finish" is set. The data will be read in "while-do" cycle so that they are not read after an error (kill_test == 1) or a spurious wake up. In addition, "reader_finish" is manipulated by the producer thread. Therefore we add READ_ONCE() to make sure that the fresh value is read in each cycle. Also we add the corresponding barrier to synchronize the sleep check. Next we set the state back to TASK_RUNNING for the situation where we did not sleep. Just from paranoid reasons, we initialize both completions statically. This is safer, in case there are other races that we are unaware of. As a side effect we could remove the memory barrier from ring_buffer_producer_thread(). IMHO, this was the reason for the barrier. ring_buffer_reset() uses spin locks that should provide the needed memory barrier for using the buffer. Link: http://lkml.kernel.org/r/1441629518-32712-2-git-send-email-pmladek@suse.com Signed-off-by: Petr Mladek Signed-off-by: Steven Rostedt --- kernel/trace/ring_buffer_benchmark.c | 25 ++++++++++++++++--------- 1 file changed, 16 insertions(+), 9 deletions(-) (limited to 'kernel/trace') diff --git a/kernel/trace/ring_buffer_benchmark.c b/kernel/trace/ring_buffer_benchmark.c index a1503a0..9ea7949 100644 --- a/kernel/trace/ring_buffer_benchmark.c +++ b/kernel/trace/ring_buffer_benchmark.c @@ -24,8 +24,8 @@ struct rb_page { static int wakeup_interval = 100; static int reader_finish; -static struct completion read_start; -static struct completion read_done; +static DECLARE_COMPLETION(read_start); +static DECLARE_COMPLETION(read_done); static struct ring_buffer *buffer; static struct task_struct *producer; @@ -178,10 +178,14 @@ static void ring_buffer_consumer(void) read_events ^= 1; read = 0; - while (!reader_finish && !kill_test) { - int found; + /* + * Continue running until the producer specifically asks to stop + * and is ready for the completion. + */ + while (!READ_ONCE(reader_finish)) { + int found = 1; - do { + while (found && !kill_test) { int cpu; found = 0; @@ -195,17 +199,23 @@ static void ring_buffer_consumer(void) if (kill_test) break; + if (stat == EVENT_FOUND) found = 1; + } - } while (found && !kill_test); + } + /* Wait till the producer wakes us up when there is more data + * available or when the producer wants us to finish reading. + */ set_current_state(TASK_INTERRUPTIBLE); if (reader_finish) break; schedule(); } + __set_current_state(TASK_RUNNING); reader_finish = 0; complete(&read_done); } @@ -389,13 +399,10 @@ static int ring_buffer_consumer_thread(void *arg) static int ring_buffer_producer_thread(void *arg) { - init_completion(&read_start); - while (!kthread_should_stop() && !kill_test) { ring_buffer_reset(buffer); if (consumer) { - smp_wmb(); wake_up_process(consumer); wait_for_completion(&read_start); } -- cgit v1.1