Re: [PATCH] LIB: remove unmatched write_lock() in gen_pool_destroy

[Steve Wise]

Jiri Kosina wrote:
> On Fri, 12 Jun 2009, Zygo Blaxell wrote:
>
>   
> > Fix mismatch between calls to write_lock() and write_unlock() in
> > gen_pool_destroy by removing the write_lock().
> >
> > Signed-off-by: Zygo Blaxell <zygo.blaxell@xxxxxxxxxxx>
> > ---
> > There is a call to write_lock() in gen_pool_destroy which is not balanced
> > by any corresponding write_unlock().  This causes problems with preemption
> > because the preemption-disable counter is incremented in the write_lock()
> > call, but never decremented by any call to write_unlock().  This bug is
> > difficult to observe in the field because only two in-tree drivers call
> > gen_pool_destroy, and one of them is non-x86 arch-specific code.
> >
> > To fix this, I have chosen removing the write_lock() over adding a
> > write_unlock() because the lock in question is inside a structure which
> > is being freed.  Any other thread that waited to acquire such a lock
> > while gen_pool_destroy was running would find itself holding a lock
> > in recently-freed or about-to-be-freed memory.  This would result in
> > memory corruption or a crash whether &pool->lock is held or not.
> >
> > Using a pool while it is in the process of being destroyed is a bug that
> > must be resolved outside of the gen_pool_destroy function.
> >
> >  lib/genalloc.c |    1 -
> >  1 files changed, 0 insertions(+), 1 deletions(-)
> >
> > diff --git a/lib/genalloc.c b/lib/genalloc.c
> > index f6d276d..eed2bdb 100644
> > --- a/lib/genalloc.c
> > +++ b/lib/genalloc.c
> > @@ -85,7 +85,6 @@ void gen_pool_destroy(struct gen_pool *pool)
> >  	int bit, end_bit;
> >  
> >  
> > -	write_lock(&pool->lock);
> >  	list_for_each_safe(_chunk, _next_chunk, &pool->chunks) {
> >  		chunk = list_entry(_chunk, struct gen_pool_chunk, next_chunk);
> >  		list_del(&chunk->next_chunk);
> > --
> > 1.5.6.5
> >
> >     
>
> Hi Zygo,
>
> this doesn't really qualify for trivial tree, as it introduces a 
> significant code change. Adding some CCs.
>
>   

Looks ok to me.  Its dumb to aquire the lock you're gonna free anyway.  
Maybe some BUG_ON() that sez nobody better be holding this lock?



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