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Subject: Re: [f2fs-dev] [PATCH] f2fs: do not use mutex lock in atomic context
To:     Ritesh Harjani <riteshh@codeaurora.org>, Chao Yu <chao@kernel.org>,
        Sahitya Tummala <stummala@codeaurora.org>
CC:     Jaegeuk Kim <jaegeuk@kernel.org>, <linux-kernel@vger.kernel.org>,
        <linux-f2fs-devel@lists.sourceforge.net>
References: <1549267613-9288-1-git-send-email-stummala@codeaurora.org>
 <c98cbfcb-f45d-e9a9-5c2e-6730d0448e82@huawei.com>
 <20190214074628.GD7308@codeaurora.org>
 <5650af07-c55d-fcb4-ca98-eca45248392d@kernel.org>
 <77c63bf6-069c-704c-220a-b50d997d2463@codeaurora.org>
From:   Chao Yu <yuchao0@huawei.com>
Message-ID: <70630a8c-4f7d-7d97-54a2-b1e4bceca492@huawei.com>
Date:   Fri, 15 Feb 2019 17:10:57 +0800
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On 2019/2/15 12:28, Ritesh Harjani wrote:
> 
> On 2/14/2019 9:40 PM, Chao Yu wrote:
>> On 2019-2-14 15:46, Sahitya Tummala wrote:
>>> On Wed, Feb 13, 2019 at 11:25:31AM +0800, Chao Yu wrote:
>>>> On 2019/2/4 16:06, Sahitya Tummala wrote:
>>>>> Fix below warning coming because of using mutex lock in atomic context.
>>>>>
>>>>> BUG: sleeping function called from invalid context at kernel/locking/mutex.c:98
>>>>> in_atomic(): 1, irqs_disabled(): 0, pid: 585, name: sh
>>>>> Preemption disabled at: __radix_tree_preload+0x28/0x130
>>>>> Call trace:
>>>>>   dump_backtrace+0x0/0x2b4
>>>>>   show_stack+0x20/0x28
>>>>>   dump_stack+0xa8/0xe0
>>>>>   ___might_sleep+0x144/0x194
>>>>>   __might_sleep+0x58/0x8c
>>>>>   mutex_lock+0x2c/0x48
>>>>>   f2fs_trace_pid+0x88/0x14c
>>>>>   f2fs_set_node_page_dirty+0xd0/0x184
>>>>>
>>>>> Do not use f2fs_radix_tree_insert() to avoid doing cond_resched() with
>>>>> spin_lock() acquired.
>>>>>
>>>>> Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
>>>>> ---
>>>>>   fs/f2fs/trace.c | 20 +++++++++++++-------
>>>>>   1 file changed, 13 insertions(+), 7 deletions(-)
>>>>>
>>>>> diff --git a/fs/f2fs/trace.c b/fs/f2fs/trace.c
>>>>> index ce2a5eb..d0ab533 100644
>>>>> --- a/fs/f2fs/trace.c
>>>>> +++ b/fs/f2fs/trace.c
>>>>> @@ -14,7 +14,7 @@
>>>>>   #include "trace.h"
>>>>>   
>>>>>   static RADIX_TREE(pids, GFP_ATOMIC);
>>>>> -static struct mutex pids_lock;
>>>>> +static spinlock_t pids_lock;
>>>>>   static struct last_io_info last_io;
>>>>>   
>>>>>   static inline void __print_last_io(void)
>>>>> @@ -58,23 +58,29 @@ void f2fs_trace_pid(struct page *page)
>>>>>   
>>>>>   	set_page_private(page, (unsigned long)pid);
>>>>>   
>>>>> +retry:
>>>>>   	if (radix_tree_preload(GFP_NOFS))
>>>>>   		return;
>>>>>   
>>>>> -	mutex_lock(&pids_lock);
>>>>> +	spin_lock(&pids_lock);
>>>>>   	p = radix_tree_lookup(&pids, pid);
>>>>>   	if (p == current)
>>>>>   		goto out;
>>>>>   	if (p)
>>>>>   		radix_tree_delete(&pids, pid);
>>>>>   
>>>>> -	f2fs_radix_tree_insert(&pids, pid, current);
> 
> Do you know why do we have a retry logic here? When anyways we have 
> called for radix_tree_delete with pid key?
> Which should ensure the slot is empty, no?
> Then why in the original code (f2fs_radix_tree_insert), we were 
> retrying. For what condition a retry was needed?

Hi,

f2fs_radix_tree_insert is used in many places, it was introduced to used in
some paths we should not failed.

And here, I guess we used it for the same purpose, if we failed to insert
@current pointer into radix, next time, we may not skip calling
trace_printk, actually it will print the same current->comm info as
previous one, it's redundant.

Thanks,

> 
> Regards
> Ritesh
> 
> 
>>>>> +	if (radix_tree_insert(&pids, pid, current)) {
>>>>> +		spin_unlock(&pids_lock);
>>>>> +		radix_tree_preload_end();
>>>>> +		cond_resched();
>>>>> +		goto retry;
>>>>> +	}
>>>>>   
>>>>>   	trace_printk("%3x:%3x %4x %-16s\n",
>>>>>   			MAJOR(inode->i_sb->s_dev), MINOR(inode->i_sb->s_dev),
>>>>>   			pid, current->comm);
>>>> Hi Sahitya,
>>>>
>>>> Can trace_printk sleep? For safety, how about moving it out of spinlock?
>>>>
>>> Hi Chao,
>>>
>>> Yes, trace_printk() is safe to use in atomic context (unlike printk).
>> Hi Sahitya,
>>
>> Thanks for your confirmation. :)
>>
>> Reviewed-by: Chao Yu <yuchao0@huawei.com>
>>
>> Thanks,
>>
>>> Thanks,
>>> Sahitya.
>>>
>>>> Thanks,
>>>>
>>>>>   out:
>>>>> -	mutex_unlock(&pids_lock);
>>>>> +	spin_unlock(&pids_lock);
>>>>>   	radix_tree_preload_end();
>>>>>   }
>>>>>   
>>>>> @@ -119,7 +125,7 @@ void f2fs_trace_ios(struct f2fs_io_info *fio, int flush)
>>>>>   
>>>>>   void f2fs_build_trace_ios(void)
>>>>>   {
>>>>> -	mutex_init(&pids_lock);
>>>>> +	spin_lock_init(&pids_lock);
>>>>>   }
>>>>>   
>>>>>   #define PIDVEC_SIZE	128
>>>>> @@ -147,7 +153,7 @@ void f2fs_destroy_trace_ios(void)
>>>>>   	pid_t next_pid = 0;
>>>>>   	unsigned int found;
>>>>>   
>>>>> -	mutex_lock(&pids_lock);
>>>>> +	spin_lock(&pids_lock);
>>>>>   	while ((found = gang_lookup_pids(pid, next_pid, PIDVEC_SIZE))) {
>>>>>   		unsigned idx;
>>>>>   
>>>>> @@ -155,5 +161,5 @@ void f2fs_destroy_trace_ios(void)
>>>>>   		for (idx = 0; idx < found; idx++)
>>>>>   			radix_tree_delete(&pids, pid[idx]);
>>>>>   	}
>>>>> -	mutex_unlock(&pids_lock);
>>>>> +	spin_unlock(&pids_lock);
>>>>>   }
>>>>>
>>
>> _______________________________________________
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>> Linux-f2fs-devel@lists.sourceforge.net
>> https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel
> 
> .
>