Xen devel - Oct 2012 - Re: gnttab: don't use domain lock for serialization

xen/common/grant_table.c 
174 static inline void 
175 double_gt_lock(struct grant_table *lgt, struct grant_table *rgt) 
176 { 
177 if ( lgt < rgt ) 
178 { 
179 spin_lock(&lgt->lock); 
180 spin_lock(&rgt->lock); 
181 } 
182 else 
183 { 
184 if ( lgt != rgt ) 
185     spin_lock(&rgt->lock); 
186 spin_lock(&lgt->lock); //here if the same, we just lock once.
187 } 
188 }

I have a question, when lgt == rgt ? 
remote domain's grant_table is created by itself, so how they get the same?
Thanks a lot!




tupeng212

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>>> On 16.10.12 at 16:38, tupeng212 <tupeng212@gmail.com> wrote:
> xen/common/grant_table.c 
> 174 static inline void 
> 175 double_gt_lock(struct grant_table *lgt, struct grant_table *rgt) 
> 176 { 
> 177 if ( lgt < rgt ) 
> 178 { 
> 179 spin_lock(&lgt->lock); 
> 180 spin_lock(&rgt->lock); 
> 181 } 
> 182 else 
> 183 { 
> 184 if ( lgt != rgt ) 
> 185     spin_lock(&rgt->lock); 
> 186 spin_lock(&lgt->lock); //here if the same, we just lock once.
> 187 } 
> 188 }
> 
> I have a question, when lgt == rgt ? 
> remote domain''s grant_table is created by itself, so how they get
the same?
If backend and frontend are in the same domain (e.g. vbd
attached locally to Dom0), both domains (and hence both grant
tables) involved are the same.

Jan