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perf_counter: fix race in perf_output_* 

When two (or more) contexts output to the same buffer, it is possible
to observe half written output.

Suppose we have CPU0 doing perf_counter_mmap(), CPU1 doing
perf_counter_overflow(). If CPU1 does a wakeup and exposes head to
user-space, then CPU2 can observe the data CPU0 is still writing.

[ Impact: fix occasionally corrupted profiling records ]

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
LKML-Reference: <20090501102533.007821627@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Peter Zijlstra 2009-05-01 12:23:16 +02:00 committed by Ingo Molnar
parent 3c56999eec
commit c33a0bc4e4
2 changed files with 105 additions and 30 deletions
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5
include/linux/perf_counter.h
View File

@ -358,10 +358,13 @@ struct perf_mmap_data {
struct rcu_head rcu_head;
int nr_pages; /* nr of data pages */
atomic_t wakeup; /* POLL_ for wakeups */
atomic_t poll; /* POLL_ for wakeups */
atomic_t head; /* write position */
atomic_t events; /* event limit */
atomic_t wakeup_head; /* completed head */
atomic_t lock; /* concurrent writes */
struct perf_counter_mmap_page *user_page;
void *data_pages[0];
};

130
kernel/perf_counter.c
View File

@ -1279,14 +1279,12 @@ static unsigned int perf_poll(struct file *file, poll_table *wait)
{
struct perf_counter *counter = file->private_data;
struct perf_mmap_data *data;
unsigned int events;
unsigned int events = POLL_HUP;
rcu_read_lock();
data = rcu_dereference(counter->data);
if (data)
events = atomic_xchg(&data->wakeup, 0);
else
events = POLL_HUP;
events = atomic_xchg(&data->poll, 0);
rcu_read_unlock();
poll_wait(file, &counter->waitq, wait);
@ -1568,22 +1566,6 @@ static const struct file_operations perf_fops = {
void perf_counter_wakeup(struct perf_counter *counter)
{
struct perf_mmap_data *data;
rcu_read_lock();
data = rcu_dereference(counter->data);
if (data) {
atomic_set(&data->wakeup, POLL_IN);
/*
* Ensure all data writes are issued before updating the
* user-space data head information. The matching rmb()
* will be in userspace after reading this value.
*/
smp_wmb();
data->user_page->data_head = atomic_read(&data->head);
}
rcu_read_unlock();
wake_up_all(&counter->waitq);
if (counter->pending_kill) {
@ -1721,10 +1703,14 @@ struct perf_output_handle {
int wakeup;
int nmi;
int overflow;
int locked;
unsigned long flags;
};
static inline void __perf_output_wakeup(struct perf_output_handle *handle)
static void perf_output_wakeup(struct perf_output_handle *handle)
{
atomic_set(&handle->data->poll, POLL_IN);
if (handle->nmi) {
handle->counter->pending_wakeup = 1;
perf_pending_queue(&handle->counter->pending,
@ -1733,6 +1719,86 @@ static inline void __perf_output_wakeup(struct perf_output_handle *handle)
perf_counter_wakeup(handle->counter);
}
/*
* Curious locking construct.
*
* We need to ensure a later event doesn't publish a head when a former
* event isn't done writing. However since we need to deal with NMIs we
* cannot fully serialize things.
*
* What we do is serialize between CPUs so we only have to deal with NMI
* nesting on a single CPU.
*
* We only publish the head (and generate a wakeup) when the outer-most
* event completes.
*/
static void perf_output_lock(struct perf_output_handle *handle)
{
struct perf_mmap_data *data = handle->data;
int cpu;
handle->locked = 0;
local_irq_save(handle->flags);
cpu = smp_processor_id();
if (in_nmi() && atomic_read(&data->lock) == cpu)
return;
while (atomic_cmpxchg(&data->lock, 0, cpu) != 0)
cpu_relax();
handle->locked = 1;
}
static void perf_output_unlock(struct perf_output_handle *handle)
{
struct perf_mmap_data *data = handle->data;
int head, cpu;
if (handle->wakeup)
data->wakeup_head = data->head;
if (!handle->locked)
goto out;
again:
/*
* The xchg implies a full barrier that ensures all writes are done
* before we publish the new head, matched by a rmb() in userspace when
* reading this position.
*/
while ((head = atomic_xchg(&data->wakeup_head, 0))) {
data->user_page->data_head = head;
handle->wakeup = 1;
}
/*
* NMI can happen here, which means we can miss a wakeup_head update.
*/
cpu = atomic_xchg(&data->lock, 0);
WARN_ON_ONCE(cpu != smp_processor_id());
/*
* Therefore we have to validate we did not indeed do so.
*/
if (unlikely(atomic_read(&data->wakeup_head))) {
/*
* Since we had it locked, we can lock it again.
*/
while (atomic_cmpxchg(&data->lock, 0, cpu) != 0)
cpu_relax();
goto again;
}
if (handle->wakeup)
perf_output_wakeup(handle);
out:
local_irq_restore(handle->flags);
}
static int perf_output_begin(struct perf_output_handle *handle,
struct perf_counter *counter, unsigned int size,
int nmi, int overflow)
@ -1745,6 +1811,7 @@ static int perf_output_begin(struct perf_output_handle *handle,
if (!data)
goto out;
handle->data = data;
handle->counter = counter;
handle->nmi = nmi;
handle->overflow = overflow;
@ -1752,12 +1819,13 @@ static int perf_output_begin(struct perf_output_handle *handle,
if (!data->nr_pages)
goto fail;
perf_output_lock(handle);
do {
offset = head = atomic_read(&data->head);
head += size;
} while (atomic_cmpxchg(&data->head, offset, head) != offset);
handle->data = data;
handle->offset = offset;
handle->head = head;
handle->wakeup = (offset >> PAGE_SHIFT) != (head >> PAGE_SHIFT);
@ -1765,7 +1833,7 @@ static int perf_output_begin(struct perf_output_handle *handle,
return 0;
fail:
__perf_output_wakeup(handle);
perf_output_wakeup(handle);
out:
rcu_read_unlock();
@ -1809,16 +1877,20 @@ static void perf_output_copy(struct perf_output_handle *handle,
static void perf_output_end(struct perf_output_handle *handle)
{
int wakeup_events = handle->counter->hw_event.wakeup_events;
struct perf_counter *counter = handle->counter;
struct perf_mmap_data *data = handle->data;
int wakeup_events = counter->hw_event.wakeup_events;
if (handle->overflow && wakeup_events) {
int events = atomic_inc_return(&handle->data->events);
int events = atomic_inc_return(&data->events);
if (events >= wakeup_events) {
atomic_sub(wakeup_events, &handle->data->events);
__perf_output_wakeup(handle);
atomic_sub(wakeup_events, &data->events);
handle->wakeup = 1;
}
} else if (handle->wakeup)
__perf_output_wakeup(handle);
}
perf_output_unlock(handle);
rcu_read_unlock();
}