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perf/x86/intel/bts: Fix confused ordering of PMU callbacks
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The intel_bts driver is using a CPU-local 'started' variable to order
callbacks and PMIs and make sure that AUX transactions don't get messed
up. However, the ordering rules in regard to this variable is a complete
mess, which recently resulted in perf_fuzzer-triggered warnings and
panics.

The general ordering rule that is patch is enforcing is that this
cpu-local variable be set only when the cpu-local AUX transaction is
active; consequently, this variable is to be checked before the AUX
related bits can be touched.

Reported-by: Vince Weaver <vincent.weaver@maine.edu>
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: vince@deater.net
Link: http://lkml.kernel.org/r/20160906132353.19887-4-alexander.shishkin@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
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@virtuoso
virtuoso authored and Ingo Molnar committed Sep 10, 2016
1 parent b79ccad commit a9a9440
Showing 1 changed file with 80 additions and 24 deletions.
104 changes: 80 additions & 24 deletions arch/x86/events/intel/bts.c
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Original file line number Diff line number Diff line change
Expand Up @@ -31,7 +31,17 @@
struct bts_ctx {
struct perf_output_handle handle;
struct debug_store ds_back;
int started;
int state;
};

/* BTS context states: */
enum {
/* no ongoing AUX transactions */
BTS_STATE_STOPPED = 0,
/* AUX transaction is on, BTS tracing is disabled */
BTS_STATE_INACTIVE,
/* AUX transaction is on, BTS tracing is running */
BTS_STATE_ACTIVE,
};

static DEFINE_PER_CPU(struct bts_ctx, bts_ctx);
Expand Down Expand Up @@ -204,6 +214,15 @@ static void bts_update(struct bts_ctx *bts)
static int
bts_buffer_reset(struct bts_buffer *buf, struct perf_output_handle *handle);

/*
* Ordering PMU callbacks wrt themselves and the PMI is done by means
* of bts::state, which:
* - is set when bts::handle::event is valid, that is, between
* perf_aux_output_begin() and perf_aux_output_end();
* - is zero otherwise;
* - is ordered against bts::handle::event with a compiler barrier.
*/

static void __bts_event_start(struct perf_event *event)
{
struct bts_ctx *bts = this_cpu_ptr(&bts_ctx);
Expand All @@ -221,10 +240,13 @@ static void __bts_event_start(struct perf_event *event)

/*
* local barrier to make sure that ds configuration made it
* before we enable BTS
* before we enable BTS and bts::state goes ACTIVE
*/
wmb();

/* INACTIVE/STOPPED -> ACTIVE */
WRITE_ONCE(bts->state, BTS_STATE_ACTIVE);

intel_pmu_enable_bts(config);

}
Expand All @@ -251,9 +273,6 @@ static void bts_event_start(struct perf_event *event, int flags)

__bts_event_start(event);

/* PMI handler: this counter is running and likely generating PMIs */
ACCESS_ONCE(bts->started) = 1;

return;

fail_end_stop:
Expand All @@ -263,30 +282,34 @@ static void bts_event_start(struct perf_event *event, int flags)
event->hw.state = PERF_HES_STOPPED;
}

static void __bts_event_stop(struct perf_event *event)
static void __bts_event_stop(struct perf_event *event, int state)
{
struct bts_ctx *bts = this_cpu_ptr(&bts_ctx);

/* ACTIVE -> INACTIVE(PMI)/STOPPED(->stop()) */
WRITE_ONCE(bts->state, state);

/*
* No extra synchronization is mandated by the documentation to have
* BTS data stores globally visible.
*/
intel_pmu_disable_bts();

if (event->hw.state & PERF_HES_STOPPED)
return;

ACCESS_ONCE(event->hw.state) |= PERF_HES_STOPPED;
}

static void bts_event_stop(struct perf_event *event, int flags)
{
struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
struct bts_ctx *bts = this_cpu_ptr(&bts_ctx);
struct bts_buffer *buf = perf_get_aux(&bts->handle);
struct bts_buffer *buf = NULL;
int state = READ_ONCE(bts->state);

if (state == BTS_STATE_ACTIVE)
__bts_event_stop(event, BTS_STATE_STOPPED);

/* PMI handler: don't restart this counter */
ACCESS_ONCE(bts->started) = 0;
if (state != BTS_STATE_STOPPED)
buf = perf_get_aux(&bts->handle);

__bts_event_stop(event);
event->hw.state |= PERF_HES_STOPPED;

if (flags & PERF_EF_UPDATE) {
bts_update(bts);
Expand All @@ -296,6 +319,7 @@ static void bts_event_stop(struct perf_event *event, int flags)
bts->handle.head =
local_xchg(&buf->data_size,
buf->nr_pages << PAGE_SHIFT);

perf_aux_output_end(&bts->handle, local_xchg(&buf->data_size, 0),
!!local_xchg(&buf->lost, 0));
}
Expand All @@ -310,17 +334,36 @@ static void bts_event_stop(struct perf_event *event, int flags)
void intel_bts_enable_local(void)
{
struct bts_ctx *bts = this_cpu_ptr(&bts_ctx);
int state = READ_ONCE(bts->state);

/*
* Here we transition from INACTIVE to ACTIVE;
* if we instead are STOPPED from the interrupt handler,
* stay that way. Can't be ACTIVE here though.
*/
if (WARN_ON_ONCE(state == BTS_STATE_ACTIVE))
return;

if (state == BTS_STATE_STOPPED)
return;

if (bts->handle.event && bts->started)
if (bts->handle.event)
__bts_event_start(bts->handle.event);
}

void intel_bts_disable_local(void)
{
struct bts_ctx *bts = this_cpu_ptr(&bts_ctx);

/*
* Here we transition from ACTIVE to INACTIVE;
* do nothing for STOPPED or INACTIVE.
*/
if (READ_ONCE(bts->state) != BTS_STATE_ACTIVE)
return;

if (bts->handle.event)
__bts_event_stop(bts->handle.event);
__bts_event_stop(bts->handle.event, BTS_STATE_INACTIVE);
}

static int
Expand Down Expand Up @@ -407,9 +450,13 @@ int intel_bts_interrupt(void)
struct perf_event *event = bts->handle.event;
struct bts_buffer *buf;
s64 old_head;
int err;
int err = -ENOSPC;

if (!event || !bts->started)
/*
* this is wrapped in intel_bts_enable_local/intel_bts_disable_local,
* so we can only be INACTIVE or STOPPED
*/
if (READ_ONCE(bts->state) == BTS_STATE_STOPPED)
return 0;

buf = perf_get_aux(&bts->handle);
Expand All @@ -432,12 +479,21 @@ int intel_bts_interrupt(void)
!!local_xchg(&buf->lost, 0));

buf = perf_aux_output_begin(&bts->handle, event);
if (!buf)
return 1;
if (buf)
err = bts_buffer_reset(buf, &bts->handle);

err = bts_buffer_reset(buf, &bts->handle);
if (err)
perf_aux_output_end(&bts->handle, 0, false);
if (err) {
WRITE_ONCE(bts->state, BTS_STATE_STOPPED);

if (buf) {
/*
* BTS_STATE_STOPPED should be visible before
* cleared handle::event
*/
barrier();
perf_aux_output_end(&bts->handle, 0, false);
}
}

return 1;
}
Expand Down

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