netdev - Re: [Patch net] tap: convert a mutex to a spinlock

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Message-ID: <CAM_iQpVbe=iSoAND1hyEmpEcxuTCguXLbPkyHHhTS0s1=9A69Q@mail.gmail.com>
Date:   Thu, 6 Jul 2017 10:56:07 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        borntraeger@...ibm.com, Sainath Grandhi <sainath.grandhi@...el.com>
Subject: Re: [Patch net] tap: convert a mutex to a spinlock

On Thu, Jul 6, 2017 at 1:24 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Wed, 2017-07-05 at 13:50 -0700, Cong Wang wrote:
>> We are not allowed to block on the RCU reader side, so can't
>> just hold the mutex as before. As a quick fix, convert it to
>> a spinlock.
>>
>> Fixes: d9f1f61c0801 ("tap: Extending tap device create/destroy APIs")
>> Reported-by: Christian Borntraeger <borntraeger@...ibm.com>
>> Cc: Sainath Grandhi <sainath.grandhi@...el.com>
>> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
>> ---
>>  drivers/net/tap.c | 18 +++++++++---------
>>  1 file changed, 9 insertions(+), 9 deletions(-)
>>
>> diff --git a/drivers/net/tap.c b/drivers/net/tap.c
>> index 4d4173d..d88ae3c 100644
>> --- a/drivers/net/tap.c
>> +++ b/drivers/net/tap.c
>> @@ -106,7 +106,7 @@ struct major_info {
>>       struct rcu_head rcu;
>>       dev_t major;
>>       struct idr minor_idr;
>> -     struct mutex minor_lock;
>> +     spinlock_t minor_lock;
>>       const char *device_name;
>>       struct list_head next;
>>  };
>> @@ -416,15 +416,15 @@ int tap_get_minor(dev_t major, struct tap_dev *tap)
>>               goto unlock;
>>       }
>>
>> -     mutex_lock(&tap_major->minor_lock);
>> -     retval = idr_alloc(&tap_major->minor_idr, tap, 1, TAP_NUM_DEVS, GFP_KERNEL);
>
>         idr_preload(GFP_KERNEL);


Are you sure we can use GFP_KERNEL? RCU read lock is already taken
at this point, so I am afraid we can't do preload here.