linux-kernel - [PATCHSET 2.6.22-rc4] sysfs: fix race conditions

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Date:	Mon, 11 Jun 2007 14:01:07 +0900
From:	Tejun Heo <htejun@...il.com>
To:	linux-kernel@...r.kernel.org, greg@...ah.com,
	akpm@...ux-foundation.org, cebbert@...hat.com, sandeen@...hat.com,
	maneesh@...ibm.com, cs@...uila.co.jp
Subject: [PATCHSET 2.6.22-rc4] sysfs: fix race conditions

Hello, all.

Currently, there are several race conditions around dentry/inode
reclamation.

a. sysfs_dirent->s_dentry dereferencing in sysfs_readdir()

b. sysfs_dirent->s_dentry dereferencing in sysfs_drop_dentry()

c. sysfs_dirent->s_dentry clearing in sysfs_d_iput()

All aboves are done without synchronization and can cause oops if the
timing is right (or wrong).

These race conditions are difficult to trigger but with the attached
patch (sysfs-races.patch) and the following commands running
parallelly, all three are reliably reproducible (you may have to
change timings or disable others to trigger specific one).

1. while true; do insmod drivers/ata/libata.ko; insmod drivers/ata/ata_piix.ko; sleep 1; rmmod ata_piix; rmmod libata; sleep 1; echo -n . ; done
2. while true; do find /sys -type f | xargs cat > /dev/null; echo -n .; sleep 1; done
3. while true; do find /sys/class/scsi_disk -type f | sort | xargs cat > /dev/null; echo -n .; sleep 1; done
4. while true; do umount /sys; sleep 1; mount /sys; sleep 1; echo -n .; done

#1 assumes there are several devices attached to ata_piix controller.
Change #1 to any module or command which creates and removes sysfs
nodes repeatedly and adjust #3 to cat those sysfs nodes.

All known race conditions are fixed in the current -mm.  #a is
replaced by adding sd->s_ino and allocating unique ino with ida.  #b
and #c are fixed during sysfs_drop_dentry() rewrite.  However, those
changes were too big to apply to 2.6.22-rcX or any stable branches.

This patchset contains three minimal backports of fixes in -mm.  With
all patches in the patchset and sysfs-races.patch applied, kernel
survived ~20 hours of stress test without any problem.

Thanks.

-- 
tejun

View attachment "sysfs-races.patch" of type "text/x-diff" (1188 bytes)