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Date: Fri, 17 Sep 2021 16:58:21 +0200
From: Dmitry Vyukov <dvyukov@...gle.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org,
Andrey Konovalov <andreyknvl@...il.com>,
Thomas Gleixner <tglx@...utronix.de>,
Steven Rostedt <rostedt@...dmis.org>,
Marco Elver <elver@...gle.com>,
Clark Williams <williams@...hat.com>
Subject: Re: [PATCH 5/5] kcov: Replace local_irq_save() with a local_lock_t.
/On Mon, 30 Aug 2021 at 19:26, Sebastian Andrzej Siewior
<bigeasy@...utronix.de> wrote:
>
> The kcov code mixes local_irq_save() and spin_lock() in
> kcov_remote_{start|end}(). This creates a warning on PREEMPT_RT because
> local_irq_save() disables interrupts and spin_lock_t is turned into a
> sleeping lock which can not be acquired in a section with disabled
> interrupts.
>
> The kcov_remote_lock is used to synchronize the access to the hash-list
> kcov_remote_map. The local_irq_save() block protects access to the
> per-CPU data kcov_percpu_data.
>
> There no compelling reason to change the lock type to raw_spin_lock_t to
> make it work with local_irq_save(). Changing it would require to move
> memory allocation (in kcov_remote_add()) and deallocation outside of the
> locked section.
> Adding an unlimited amount of entries to the hashlist will increase the
> IRQ-off time during lookup. It could be argued that this is debug code
> and the latency does not matter. There is however no need to do so and
> it would allow to use this facility in an RT enabled build.
>
> Using a local_lock_t instead of local_irq_save() has the befit of adding
> a protection scope within the source which makes it obvious what is
> protected. On a !PREEMPT_RT && !LOCKDEP build the local_lock_irqsave()
> maps directly to local_irq_save() so there is overhead at runtime.
s/befit/benefit/
s/overhead/no overhead/
but otherwise
Acked-by: Dmitry Vyukov <dvyukov@...gle.com>
> Replace the local_irq_save() section with a local_lock_t.
>
> Reported-by: Clark Williams <williams@...hat.com>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
> kernel/kcov.c | 30 +++++++++++++++++-------------
> 1 file changed, 17 insertions(+), 13 deletions(-)
>
> diff --git a/kernel/kcov.c b/kernel/kcov.c
> index 620dc4ffeb685..36ca640c4f8e7 100644
> --- a/kernel/kcov.c
> +++ b/kernel/kcov.c
> @@ -88,6 +88,7 @@ static struct list_head kcov_remote_areas = LIST_HEAD_INIT(kcov_remote_areas);
>
> struct kcov_percpu_data {
> void *irq_area;
> + local_lock_t lock;
>
> unsigned int saved_mode;
> unsigned int saved_size;
> @@ -96,7 +97,9 @@ struct kcov_percpu_data {
> int saved_sequence;
> };
>
> -static DEFINE_PER_CPU(struct kcov_percpu_data, kcov_percpu_data);
> +static DEFINE_PER_CPU(struct kcov_percpu_data, kcov_percpu_data) = {
> + .lock = INIT_LOCAL_LOCK(lock),
> +};
>
> /* Must be called with kcov_remote_lock locked. */
> static struct kcov_remote *kcov_remote_find(u64 handle)
> @@ -824,7 +827,7 @@ void kcov_remote_start(u64 handle)
> if (!in_task() && !in_serving_softirq())
> return;
>
> - local_irq_save(flags);
> + local_lock_irqsave(&kcov_percpu_data.lock, flags);
>
> /*
> * Check that kcov_remote_start() is not called twice in background
> @@ -832,7 +835,7 @@ void kcov_remote_start(u64 handle)
> */
> mode = READ_ONCE(t->kcov_mode);
> if (WARN_ON(in_task() && kcov_mode_enabled(mode))) {
> - local_irq_restore(flags);
> + local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
> return;
> }
> /*
> @@ -841,14 +844,15 @@ void kcov_remote_start(u64 handle)
> * happened while collecting coverage from a background thread.
> */
> if (WARN_ON(in_serving_softirq() && t->kcov_softirq)) {
> - local_irq_restore(flags);
> + local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
> return;
> }
>
> spin_lock(&kcov_remote_lock);
> remote = kcov_remote_find(handle);
> if (!remote) {
> - spin_unlock_irqrestore(&kcov_remote_lock, flags);
> + spin_unlock(&kcov_remote_lock);
> + local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
> return;
> }
> kcov_debug("handle = %llx, context: %s\n", handle,
> @@ -873,13 +877,13 @@ void kcov_remote_start(u64 handle)
>
> /* Can only happen when in_task(). */
> if (!area) {
> - local_irqrestore(flags);
> + local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
> area = vmalloc(size * sizeof(unsigned long));
> if (!area) {
> kcov_put(kcov);
> return;
> }
> - local_irq_save(flags);
> + local_lock_irqsave(&kcov_percpu_data.lock, flags);
> }
>
> /* Reset coverage size. */
> @@ -891,7 +895,7 @@ void kcov_remote_start(u64 handle)
> }
> kcov_start(t, kcov, size, area, mode, sequence);
>
> - local_irq_restore(flags);
> + local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
>
> }
> EXPORT_SYMBOL(kcov_remote_start);
> @@ -965,12 +969,12 @@ void kcov_remote_stop(void)
> if (!in_task() && !in_serving_softirq())
> return;
>
> - local_irq_save(flags);
> + local_lock_irqsave(&kcov_percpu_data.lock, flags);
>
> mode = READ_ONCE(t->kcov_mode);
> barrier();
> if (!kcov_mode_enabled(mode)) {
> - local_irq_restore(flags);
> + local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
> return;
> }
> /*
> @@ -978,12 +982,12 @@ void kcov_remote_stop(void)
> * actually found the remote handle and started collecting coverage.
> */
> if (in_serving_softirq() && !t->kcov_softirq) {
> - local_irq_restore(flags);
> + local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
> return;
> }
> /* Make sure that kcov_softirq is only set when in softirq. */
> if (WARN_ON(!in_serving_softirq() && t->kcov_softirq)) {
> - local_irq_restore(flags);
> + local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
> return;
> }
>
> @@ -1013,7 +1017,7 @@ void kcov_remote_stop(void)
> spin_unlock(&kcov_remote_lock);
> }
>
> - local_irq_restore(flags);
> + local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
>
> /* Get in kcov_remote_start(). */
> kcov_put(kcov);
> --
> 2.33.0
>
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