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McKenney" , Andrew Morton , "Theodore Y . Ts'o" , Matthew Wilcox , Sebastian Andrzej Siewior , Oleksiy Avramchenko Subject: Re: [PATCH v2 1/1] rcu/tree: Drop the lock before entering to page allocator Message-ID: <20200730125348.GA2074029@google.com> References: <20200727211012.30948-1-urezki@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200727211012.30948-1-urezki@gmail.com> X-Rspamd-Queue-Id: 9146716A1B3 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam04 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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: > > > [ 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 > > > 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) > --- > 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) thanks, - Joel > } > > /* Switch to emergency path. */ > -- > 2.20.1 >