Re: [PATCH v2 1/1] rcu/tree: Drop the lock before entering to page allocator

[Joel Fernandes]

On Mon, Jul 27, 2020 at 11:10:12PM +0200, Uladzislau Rezki (Sony) wrote:
> If the kernel is built with CONFIG_PROVE_RAW_LOCK_NESTING
> option, the lockedp will complain about violation of the
> nesting rules:
> 
> <snip>
> [   28.060389] =============================
> [   28.060389] [ BUG: Invalid wait context ]
> [   28.060389] 5.8.0-rc3-rcu #211 Tainted: G            E
> [   28.060389] -----------------------------
> [   28.060390] vmalloc_test/0/523 is trying to lock:
> [   28.060390] ffff96df7ffe0228 (&zone->lock){-.-.}-{3:3}, at: get_page_from_freelist+0xcf0/0x16d0
> [   28.060391] other info that might help us debug this:
> [   28.060391] context-{5:5}
> [   28.060392] 2 locks held by vmalloc_test/0/523:
> [   28.060392]  #0: ffffffffc06750d0 (prepare_for_test_rwsem){++++}-{4:4}, at: test_func+0x76/0x240 [test_vmalloc]
> [   28.060393]  #1: ffff96df5fa1d390 (krc.lock){..-.}-{2:2}, at: kvfree_call_rcu+0x5c/0x230
> [   28.060395] stack backtrace:
> [   28.060395] CPU: 0 PID: 523 Comm: vmalloc_test/0 Tainted: G            E     5.8.0-rc3-rcu #211
> [   28.060395] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
> [   28.060396] Call Trace:
> [   28.060397]  dump_stack+0x96/0xd0
> [   28.060397]  __lock_acquire.cold.65+0x166/0x2d7
> [   28.060398]  ? find_held_lock+0x2d/0x90
> [   28.060399]  lock_acquire+0xad/0x370
> [   28.060400]  ? get_page_from_freelist+0xcf0/0x16d0
> [   28.060401]  ? mark_held_locks+0x48/0x70
> [   28.060402]  _raw_spin_lock+0x25/0x30
> [   28.060403]  ? get_page_from_freelist+0xcf0/0x16d0
> [   28.060404]  get_page_from_freelist+0xcf0/0x16d0
> [   28.060405]  ? __lock_acquire+0x3ee/0x1b90
> [   28.060407]  __alloc_pages_nodemask+0x16a/0x3a0
> [   28.060408]  __get_free_pages+0xd/0x30
> [   28.060409]  kvfree_call_rcu+0x18a/0x230
> <snip>
> 
> Internally the kfree_rcu() uses raw_spinlock_t whereas the
> page allocator internally deals with spinlock_t to access
> to its zones.
> 
> In order to prevent such vialation that is in question we
> can drop the internal raw_spinlock_t before entering to
> the page allocaor.
> 
> Short changelog (v1 -> v2):
>     - rework the commit message;
>     - rework the patch making it smaller;
>     - add more comments.
> 
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
> ---
>  kernel/rcu/tree.c | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 21c2fa5bd8c3..2de112404121 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3287,6 +3287,8 @@ kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr)
>  		return false;
>  
>  	lockdep_assert_held(&krcp->lock);
> +	lockdep_assert_irqs_disabled();
> +
>  	idx = !!is_vmalloc_addr(ptr);
>  
>  	/* Check if a new block is required. */
> @@ -3306,6 +3308,29 @@ kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr)
>  			if (IS_ENABLED(CONFIG_PREEMPT_RT))
>  				return false;
>  
> +			/*
> +			 * If built with CONFIG_PROVE_RAW_LOCK_NESTING option,
> +			 * the lockedp will complain about violation of the
> +			 * nesting rules. It does the raw_spinlock vs. spinlock
> +			 * nesting checks.
> +			 *
> +			 * That is why we drop the raw lock. Please note IRQs are
> +			 * still disabled it guarantees that the "current" stays
> +			 * on the same CPU later on when the raw lock is taken
> +			 * back.
> +			 *
> +			 * It is important because if the page allocator is invoked
> +			 * in fully preemptible context, it can be that we get a page
> +			 * but end up on another CPU. That another CPU might not need
> +			 * a page because of having some extra spots in its internal
> +			 * array for pointer collecting. Staying on same CPU eliminates
> +			 * described issue.
> +			 *
> +			 * Dropping the lock also reduces the critical section by
> +			 * the time taken by the page allocator to obtain a page.
> +			 */
> +			raw_spin_unlock(&krcp->lock);
> +
>  			/*
>  			 * NOTE: For one argument of kvfree_rcu() we can
>  			 * drop the lock and get the page in sleepable
> @@ -3315,6 +3340,8 @@ kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr)
>  			 */
>  			bnode = (struct kvfree_rcu_bulk_data *)
>  				__get_free_page(GFP_NOWAIT | __GFP_NOWARN);
> +
> +			raw_spin_lock(&krcp->lock);

Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

thanks,

 - Joel


>  		}
>  
>  		/* Switch to emergency path. */
> -- 
> 2.20.1
>