Recently I receie a report that on Linux v3.0 based kerenl, hot add disk to a md linear device causes kernel crash at linear_congested(). From the crash image analysis, I find in linear_congested(), mddev->raid_disks contains value N, but conf->disks[] only has N-1 pointers available. Then a pointer deference to a NULL pointer crashes the kernel. There is a race between linear_add() and linear_congested(), RCU stuffs used in these two functions cannot avoid the race. Since Linuv v4.0 RCU code is replaced by introducing mddev_suspend(). After checking the upstream code, it seems linear_congested() is not called in generic_make_request() code patch, so mddev_suspend() cannot provent it from being called. The possible race still exists. Here I explain how the race still exists in current code. For a machine has many CPUs, on one CPU, linear_add() is called to add a hard disk to a md linear device; at the same time on other CPU, linear_congested() is called to detect whether this md linear device is congested before issuing an I/O request onto it. Now I use a possible code execution time sequence to demo how the possible race happens, seq linear_add() linear_congested() 0 conf=mddev->private 1 oldconf=mddev->private 2 mddev->raid_disks++ 3 for (i=0; i<mddev->raid_disks;i++) 4 bdev_get_queue(conf->disks[i].rdev->bdev) 5 mddev->private=newconf In linear_add() mddev->raid_disks is increased in time seq 2, and on another CPU in linear_congested() the for-loop iterates conf->disks[i] by the increased mddev->raid_disks in time seq 3,4. But conf with one more element (which is a pointer to struct dev_info type) to conf->disks[] is not updated yet, accessing its structure member in time seq 4 will cause a NULL pointer deference fault. The fix is to update mddev->private with new value before increasing mddev->raid_disks, and to make sure on other CPUs their are seen to be updated in same order as linear_add() does (otherwise the race may still happen), a smp_mb() is necessary. A question is, by this fix, if mddev->private is update to new value in linear_add(), but in linear_congested() the for-loop still tests old value of mddev->raid_disks, then the iteration will miss the last element of conf->disks[]. My answer is don't worry it, it's OK. the reasons are, - When updating mddev->private, the md linear device is suspend, no I/O may happen, it is safe to missing congestion status of the last new-added hard disk. - In the worst case linear_congested() returns 0 and I/O sent to this md linear device, but the new added hard disk is congested, then the I/O request will be blocked for a while if it just happenly hits the new added hard disk. linear_congested() is in code path of wb_congested(), which is quite hot in write back code path. Comparing to add locking code in linear_congested(), the cost of the worst case is acceptable. The bug is reported on Linux v3.0 based kernel, it can and should be applied to all kernels since Linux v3.0. I see linear_add() is merged into mainline since Linux v2.6.18, maybe stable kernel maintainers after this version may consider to pick this fix as well. Signed-off-by: Coly Li <colyli@xxxxxxx> Cc: Shaohua Li <shli@xxxxxx> Cc: Neil Brown <neilb@xxxxxxxx> Cc: stable@xxxxxxxxxxxxxxx --- drivers/md/linear.c | 14 +++++++++++++- 1 file changed, 13 insertions(+), 1 deletion(-) diff --git a/drivers/md/linear.c b/drivers/md/linear.c index 5975c99..48ccfad 100644 --- a/drivers/md/linear.c +++ b/drivers/md/linear.c @@ -196,10 +196,22 @@ static int linear_add(struct mddev *mddev, struct md_rdev *rdev) if (!newconf) return -ENOMEM; + /* In linear_congested(), mddev->raid_disks and mddev->private + * are accessed without protection by mddev_suspend(). If on + * another CPU, in linear_congested() mddev->private is still seen + * to contains old value but mddev->raid_disks is seen to have the + * increased value, the last iteration to conf->disks[i].rdev will + * trigger a NULL pointer deference. To avoid this race, here + * mddev->private must be updated before increasing + * mddev->raid_disks, and a smp_mb() is required between them. Then + * in linear_congested(), we are sure the updated mddev->private is + * seen when iterating conf->disks[i]. + */ mddev_suspend(mddev); oldconf = mddev->private; - mddev->raid_disks++; mddev->private = newconf; + smp_mb(); + mddev->raid_disks++; md_set_array_sectors(mddev, linear_size(mddev, 0, 0)); set_capacity(mddev->gendisk, mddev->array_sectors); mddev_resume(mddev); -- 2.6.6 -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
