linux-ext4 - Re: [PATCH -V2 3/5] ext4: Fix the race between read_block_bitmap and mark_diskspace

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Date:	Mon, 24 Nov 2008 17:03:23 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Alex Zhuravlev <Alex.Zhuravlev@....COM>
Cc:	Theodore Tso <tytso@....EDU>, cmm@...ibm.com, sandeen@...hat.com,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH -V2 3/5] ext4: Fix the race between read_block_bitmap
	and mark_diskspace_used

On Mon, Nov 24, 2008 at 10:14:27AM +0300, Alex Zhuravlev wrote:
> Theodore Tso wrote:
>> My bigger concern is given that we are playing games like *this*:
>>
>> 		if ((cur & 31) == 0 && (len - cur) >= 32) {
>> 			/* fast path: set whole word at once */
>> 			addr = bm + (cur >> 3);
>> 			*addr = 0xffffffff;
>> 			cur += 32;
>> 			continue;
>> 		}
>
> this is to avoid expensive LOCK prefix in some cases.
>
>> without taking a lock, I'm a little surprised we haven't been
>> seriously burned by other race conditions.  What's the point of
>> calling mb_set_bit_atomic() and passing in a spinlock if we are doing
>> this kind of check without the protection of the same spinlock?!?
>
> why would we need a lock for a whole word bitop ?

Ok the changes was not done for this purpose. I need to make sure we
update bitmap and clear group_desc uninit flag after taking sb_bgl_lock
That means when we claim blocks we can't use mb_set_bits with
sb_bgl_lock because we would already be holding it. How about the below
change

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 444ad99..53180b1 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -1031,7 +1031,10 @@ static void mb_clear_bits(spinlock_t *lock, void *bm, int cur, int len)
 			cur += 32;
 			continue;
 		}
-		mb_clear_bit_atomic(lock, cur, bm);
+		if (lock)
+			mb_clear_bit_atomic(lock, cur, bm);
+		else
+			mb_clear_bit(cur, bm);
 		cur++;
 	}
 }
@@ -1049,7 +1052,10 @@ static void mb_set_bits(spinlock_t *lock, void *bm, int cur, int len)
 			cur += 32;
 			continue;
 		}
-		mb_set_bit_atomic(lock, cur, bm);
+		if (lock)
+			mb_set_bit_atomic(lock, cur, bm);
+		else
+			mb_set_bit(cur, bm);
 		cur++;
 	}
 }
--
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