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

    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@xandros.com>
    >> ---
    >> 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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