[PATCH v3 04/13] coresight: perf: Avoid unncessary CPU hotplug read lock

Suzuki K Poulose
2018-07-26T12:55:08Z

We hold the read lock on CPU hotplug to simply copy the
online mask, which is not really needed. And this can
cause a lockdep warning, like :

[ 54.632093] ======================================================
[ 54.638207] WARNING: possible circular locking dependency detected
[ 54.644322] 4.18.0-rc3-00042-g2d39e6356bb7-dirty #309 Not tainted
[ 54.650350] ------------------------------------------------------
[ 54.656464] perf/2862 is trying to acquire lock:
[ 54.661031] 000000007e21d170 (&event->mmap_mutex){+.+.}, at: perf_event_set_output+0x98/0x138
[ 54.669486]
[ 54.669486] but task is already holding lock:
[ 54.675256] 000000001080eb1b (&cpuctx_mutex){+.+.}, at: perf_event_ctx_lock_nested+0xf8/0x1f0
[ 54.683704]
[ 54.683704] which lock already depends on the new lock.
[ 54.683704]
[ 54.691797]
[ 54.691797] the existing dependency chain (in reverse order) is:
[ 54.699201]
[ 54.699201] -> #3 (&cpuctx_mutex){+.+.}:
[ 54.704556] __mutex_lock+0x70/0x808
[ 54.708608] mutex_lock_nested+0x1c/0x28
[ 54.713005] perf_event_init_cpu+0x8c/0xd8
[ 54.717574] perf_event_init+0x194/0x1d4
[ 54.721971] start_kernel+0x2b8/0x42c
[ 54.726107]
[ 54.726107] -> #2 (pmus_lock){+.+.}:
[ 54.731114] __mutex_lock+0x70/0x808
[ 54.735165] mutex_lock_nested+0x1c/0x28
[ 54.739560] perf_event_init_cpu+0x30/0xd8
[ 54.744129] cpuhp_invoke_callback+0x84/0x248
[ 54.748954] _cpu_up+0xe8/0x1c8
[ 54.752576] do_cpu_up+0xa8/0xc8
[ 54.756283] cpu_up+0x10/0x18
[ 54.759731] smp_init+0xa0/0x114
[ 54.763438] kernel_init_freeable+0x120/0x288
[ 54.768264] kernel_init+0x10/0x108
[ 54.772230] ret_from_fork+0x10/0x18
[ 54.776279]
[ 54.776279] -> #1 (cpu_hotplug_lock.rw_sem){++++}:
[ 54.782492] cpus_read_lock+0x34/0xb0
[ 54.786631] etm_setup_aux+0x5c/0x308
[ 54.790769] rb_alloc_aux+0x1ec/0x300
[ 54.794906] perf_mmap+0x284/0x610
[ 54.798787] mmap_region+0x388/0x570
[ 54.802838] do_mmap+0x344/0x4f8
[ 54.806544] vm_mmap_pgoff+0xe4/0x110
[ 54.810682] ksys_mmap_pgoff+0xa8/0x240
[ 54.814992] sys_mmap+0x18/0x28
[ 54.818613] el0_svc_naked+0x30/0x34
[ 54.822661]
[ 54.822661] -> #0 (&event->mmap_mutex){+.+.}:
[ 54.828445] lock_acquire+0x48/0x68
[ 54.832409] __mutex_lock+0x70/0x808
[ 54.836459] mutex_lock_nested+0x1c/0x28
[ 54.840855] perf_event_set_output+0x98/0x138
[ 54.845680] _perf_ioctl+0x2a0/0x6a0
[ 54.849731] perf_ioctl+0x3c/0x68
[ 54.853526] do_vfs_ioctl+0xb8/0xa20
[ 54.857577] ksys_ioctl+0x80/0xb8
[ 54.861370] sys_ioctl+0xc/0x18
[ 54.864990] el0_svc_naked+0x30/0x34
[ 54.869039]
[ 54.869039] other info that might help us debug this:
[ 54.869039]
[ 54.876960] Chain exists of:
[ 54.876960] &event->mmap_mutex --> pmus_lock --> &cpuctx_mutex
[ 54.876960]
[ 54.887217] Possible unsafe locking scenario:
[ 54.887217]
[ 54.893073] CPU0 CPU1
[ 54.897552] ---- ----
[ 54.902030] lock(&cpuctx_mutex);
[ 54.905396] lock(pmus_lock);
[ 54.910911] lock(&cpuctx_mutex);
[ 54.916770] lock(&event->mmap_mutex);
[ 54.920566]
[ 54.920566] *** DEADLOCK ***
[ 54.920566]
[ 54.926424] 1 lock held by perf/2862:
[ 54.930042] #0: 000000001080eb1b (&cpuctx_mutex){+.+.}, at: perf_event_ctx_lock_nested+0xf8/0x1f0

Since we have per-cpu array for the paths, we simply don't care about
the number of online CPUs. This patch gets rid of the
{get/put}_online_cpus().

Reported-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
---
Changes since v2:
- Splitted from the original patch (per-cpu path management)
- Added splat reported by Mathieu in the commit description.
---
drivers/hwtracing/coresight/coresight-etm-perf.c | 3 ---
1 file changed, 3 deletions(-)

diff --git a/drivers/hwtracing/coresight/coresight-etm-perf.c b/drivers/hwtracing/coresight/coresight-etm-perf.c
index 6338dd1..6beb662 100644
--- a/drivers/hwtracing/coresight/coresight-etm-perf.c
+++ b/drivers/hwtracing/coresight/coresight-etm-perf.c
@@ -161,15 +161,12 @@ static void *alloc_event_data(int cpu)
if (!event_data)
return NULL;

- /* Make sure nothing disappears under us */
- get_online_cpus();

mask = &event_data->mask;
if (cpu != -1)
cpumask_set_cpu(cpu, mask);
else
cpumask_copy(mask, cpu_online_mask);
- put_online_cpus();

/*
* Each CPU has a single path between source and destination. As such
--
2.7.4