Date:   Tue, 31 Jan 2023 14:31:20 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     James Clark <james.clark@arm.com>
Cc:     linux-perf-users@vger.kernel.org, ravi.bangoria@amd.com,
        syzbot+697196bc0265049822bd@syzkaller.appspotmail.com,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] perf: Fix warning from concurrent read/write of
 perf_event_pmu_context
Message-ID: <Y9kYKBMBEa7j+O3n@hirez.programming.kicks-ass.net>
References: <20230127143141.1782804-1-james.clark@arm.com>
 <20230127143141.1782804-2-james.clark@arm.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
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In-Reply-To: <20230127143141.1782804-2-james.clark@arm.com>
Precedence: bulk

On Fri, Jan 27, 2023 at 02:31:41PM +0000, James Clark wrote:

> @@ -4897,32 +4895,32 @@ static void free_epc_rcu(struct rcu_head *head)
>  static void put_pmu_ctx(struct perf_event_pmu_context *epc)
>  {
>  	unsigned long flags;
> +	struct perf_event_context *ctx = epc->ctx;
>  
> -	if (!atomic_dec_and_test(&epc->refcount))
> +	/*
> +	 * XXX
> +	 *
> +	 * lockdep_assert_held(&ctx->mutex);
> +	 *
> +	 * can't because of the call-site in _free_event()/put_event()
> +	 * which isn't always called under ctx->mutex.
> +	 */
> +	raw_spin_lock_irqsave(&ctx->lock, flags);
> +	if (!atomic_dec_and_test(&epc->refcount)) {
> +		raw_spin_unlock_irqrestore(&ctx->lock, flags);
>  		return;
> +	}

This is a bit of an anti-pattern and better donw using dec_and_lock. As
such, I'll fold the below into it.

--- a/include/linux/spinlock.h
+++ b/include/linux/spinlock.h
@@ -476,6 +476,15 @@ extern int _atomic_dec_and_lock_irqsave(
 #define atomic_dec_and_lock_irqsave(atomic, lock, flags) \
 		__cond_lock(lock, _atomic_dec_and_lock_irqsave(atomic, lock, &(flags)))
 
+extern int _atomic_dec_and_raw_lock(atomic_t *atomic, raw_spinlock_t *lock);
+#define atomic_dec_and_raw_lock(atomic, lock) \
+		__cond_lock(lock, _atomic_dec_and_raw_lock(atomic, lock))
+
+extern int _atomic_dec_and_raw_lock_irqsave(atomic_t *atomic, raw_spinlock_t *lock,
+					unsigned long *flags);
+#define atomic_dec_and_raw_lock_irqsave(atomic, lock, flags) \
+		__cond_lock(lock, _atomic_dec_and_raw_lock_irqsave(atomic, lock, &(flags)))
+
 int __alloc_bucket_spinlocks(spinlock_t **locks, unsigned int *lock_mask,
 			     size_t max_size, unsigned int cpu_mult,
 			     gfp_t gfp, const char *name,
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4901,11 +4901,8 @@ static void put_pmu_ctx(struct perf_even
 	 * can't because of the call-site in _free_event()/put_event()
 	 * which isn't always called under ctx->mutex.
 	 */
-	raw_spin_lock_irqsave(&ctx->lock, flags);
-	if (!atomic_dec_and_test(&epc->refcount)) {
-		raw_spin_unlock_irqrestore(&ctx->lock, flags);
+	if (!atomic_dec_and_raw_lock_irqsave(&epc->refcount, &ctx->lock, flags))
 		return;
-	}
 
 	WARN_ON_ONCE(list_empty(&epc->pmu_ctx_entry));
 
--- a/lib/dec_and_lock.c
+++ b/lib/dec_and_lock.c
@@ -49,3 +49,34 @@ int _atomic_dec_and_lock_irqsave(atomic_
 	return 0;
 }
 EXPORT_SYMBOL(_atomic_dec_and_lock_irqsave);
+
+int _atomic_dec_and_raw_lock(atomic_t *atomic, raw_spinlock_t *lock)
+{
+	/* Subtract 1 from counter unless that drops it to 0 (ie. it was 1) */
+	if (atomic_add_unless(atomic, -1, 1))
+		return 0;
+
+	/* Otherwise do it the slow way */
+	raw_spin_lock(lock);
+	if (atomic_dec_and_test(atomic))
+		return 1;
+	raw_spin_unlock(lock);
+	return 0;
+}
+EXPORT_SYMBOL(_atomic_dec_and_raw_lock);
+
+int _atomic_dec_and_raw_lock_irqsave(atomic_t *atomic, raw_spinlock_t *lock,
+				     unsigned long *flags)
+{
+	/* Subtract 1 from counter unless that drops it to 0 (ie. it was 1) */
+	if (atomic_add_unless(atomic, -1, 1))
+		return 0;
+
+	/* Otherwise do it the slow way */
+	raw_spin_lock_irqsave(lock, *flags);
+	if (atomic_dec_and_test(atomic))
+		return 1;
+	raw_spin_unlock_irqrestore(lock, *flags);
+	return 0;
+}
+EXPORT_SYMBOL(_atomic_dec_and_raw_lock_irqsave);