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

From: Steve Wise
Date: Mon Jun 15 2009 - 18:03:27 EST

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);

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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