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 10:14:27 +0300
From:	Alex Zhuravlev <Alex.Zhuravlev@....COM>
To:	Theodore Tso <tytso@....EDU>
Cc:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	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

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 ?

thanks, Alex

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