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Date: Sun, 23 Nov 2008 09:00:38 -0500 From: Theodore Tso <tytso@....EDU> To: "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com> Cc: 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 Fri, Nov 21, 2008 at 10:14:33PM +0530, Aneesh Kumar K.V wrote: > We need to make sure we update the block bitmap and clear > EXT4_BG_BLOCK_UNINIT flag with sb_bgl_lock held. We look > at EXT4_BG_BLOCK_UNINIT and reinit the block bitmap each > time in ext4_read_block_bitmap (introduced by > c806e68f5647109350ec546fee5b526962970fd2 ) You are changing mb_clear_bits() and and mb_set_bits() so they take the spinlock over the entire operaiton, instead of over each particular bit. These function are used in a largish number of places, not just for updating the block bitmap, but also the mb buddy bitmaps, etc. So there may be a scalability impact here, although taking the spinlock once instead of multiple times is probably a win. 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; } 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?!? Andreas, if you are using mb_clear_bits() and mb_set_bits() in Lustre's mballoc.c with this in production, you may want to take a look at this patch. - Ted -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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