linux-kernel - Re: [PATCH v2] lightnvm: pblk-recovery: lightnvm: pblk: Fix two sleep-in-atomic-context bugs

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Date:   Thu, 6 Sep 2018 13:09:20 +0000
From:   Javier Gonzalez <javier@...xlabs.com>
To:     Hans Holmberg <hans.ml.holmberg@...tronix.com>
CC:     "baijiaju1990@...il.com" <baijiaju1990@...il.com>,
        Matias Bjørling <mb@...htnvm.io>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] lightnvm: pblk-recovery: lightnvm: pblk: Fix two
 sleep-in-atomic-context bugs

> On 6 Sep 2018, at 00.51, Hans Holmberg <hans.ml.holmberg@...tronix.com> wrote:
> 
> On Wed, Sep 5, 2018 at 10:40 AM Jia-Ju Bai <baijiaju1990@...il.com> wrote:
>> The driver may sleep with holding a spinlock.
>> 
>> The function call paths (from bottom to top) in Linux-4.16 are:
>> 
>> [FUNC] nvm_dev_dma_alloc(GFP_KERNEL)
>> drivers/lightnvm/pblk-core.c, 754:
>>        nvm_dev_dma_alloc in pblk_line_submit_smeta_io
>> drivers/lightnvm/pblk-core.c, 1048:
>>        pblk_line_submit_smeta_io in pblk_line_init_bb
>> drivers/lightnvm/pblk-core.c, 1434:
>>        pblk_line_init_bb in pblk_line_replace_data
>> drivers/lightnvm/pblk-recovery.c, 980:
>>        pblk_line_replace_data in pblk_recov_l2p
>> drivers/lightnvm/pblk-recovery.c, 976:
>>        spin_lock in pblk_recov_l2p
>> 
>> [FUNC] bio_map_kern(GFP_KERNEL)
>> drivers/lightnvm/pblk-core.c, 762:
>>        bio_map_kern in pblk_line_submit_smeta_io
>> drivers/lightnvm/pblk-core.c, 1048:
>>        pblk_line_submit_smeta_io in pblk_line_init_bb
>> drivers/lightnvm/pblk-core.c, 1434:
>>        pblk_line_init_bb in pblk_line_replace_data
>> drivers/lightnvm/pblk-recovery.c, 980:
>>        pblk_line_replace_data in pblk_recov_l2p
>> drivers/lightnvm/pblk-recovery.c, 976:
>>        spin_lock in pblk_recov_l2p
>> 
>> To fix these bugs, the call to pblk_line_replace_data()
>> is moved out of the spinlock protection.
>> 
>> These bugs are found by my static analysis tool DSAC.
>> 
>> Signed-off-by: Jia-Ju Bai <baijiaju1990@...il.com>
>> ---
>> v2:
>> * Move the call to pblk_line_replace_data() out of the spinlock
>>  protection, instead of v1 that changes GFP_KERNEL to GFP_ATOMIC in
>>  the calls to bio_map_kern() and nvm_dev_dma_alloc().
>>  Thanks Javier for good advice.
>> ---
>> drivers/lightnvm/pblk-recovery.c | 6 ++++--
>> 1 file changed, 4 insertions(+), 2 deletions(-)
>> 
>> diff --git a/drivers/lightnvm/pblk-recovery.c b/drivers/lightnvm/pblk-recovery.c
>> index 3a5069183859..5fde414d78bb 100644
>> --- a/drivers/lightnvm/pblk-recovery.c
>> +++ b/drivers/lightnvm/pblk-recovery.c
>> @@ -955,12 +955,14 @@ struct pblk_line *pblk_recov_l2p(struct pblk *pblk)
>>                }
>>        }
>> 
>> -       spin_lock(&l_mg->free_lock);
>>        if (!open_lines) {
>> +               spin_lock(&l_mg->free_lock);
>>                WARN_ON_ONCE(!test_and_clear_bit(meta_line,
>>                                                        &l_mg->meta_bitmap));
>> +               spin_unlock(&l_mg->free_lock);
>>                pblk_line_replace_data(pblk);
>>        } else {
>> +               spin_lock(&l_mg->free_lock);
>>                /* Allocate next line for preparation */
>>                l_mg->data_next = pblk_line_get(pblk);
>>                if (l_mg->data_next) {
>> @@ -968,8 +970,8 @@ struct pblk_line *pblk_recov_l2p(struct pblk *pblk)
>>                        l_mg->data_next->type = PBLK_LINETYPE_DATA;
>>                        is_next = 1;
>>                }
>> +               spin_unlock(&l_mg->free_lock);
>>        }
>> -       spin_unlock(&l_mg->free_lock);
> 
> Wouldn't the most straight forward solution here to not take the lock?
> 
> As pblk is doing all the recovery sequentially during initialization,
> I don't see the
> reason for grabbing the lock.
> 

We start doing GC as we recover and, even though right now there is no
interference, the code might change. For such a little change, I would
rather maintain lock consistency than being smart about where we can
avoid locking - specially when this is not the fast path.

Javier


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