Re: [f2fs-dev] [PATCH] f2fs: do not use mutex lock in atomic context

[Ritesh Harjani]

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@xxxxxxxxxxxxxx>
> > > > ---
> > > >   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?

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@xxxxxxxxxx>
>
> 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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