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SubjectRe: [PATCH v2 1/1] rcu/tree: Drop the lock before entering to page allocator
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@gmail.com>
> ---
> 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@joelfernandes.org>

thanks,

- Joel


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

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