Messages in this thread | | | Date | Tue, 19 Mar 2019 16:37:37 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] sched: Do not re-read h_load_next during hierarchical load calculation v2 |
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On Tue, Mar 19, 2019 at 12:36:10PM +0000, Mel Gorman wrote: > Changelog since v1 > o Use WRITE_ONCE > o Add Fixes: > o Add reviewed-by for the READ_ONCE part as I considered it to still be > ok even after the WRITE_ONCE > > A NULL pointer dereference bug was reported on a distribution kernel but > the same issue should be present on mainline kernel. It occured on s390 > but should not be arch-specific. A partial oops looks like > > [775277.408564] Unable to handle kernel pointer dereference in virtual kernel address space > ... > [775277.408759] Call Trace: > [775277.408763] ([<0002c11c56899c61>] 0x2c11c56899c61) > [775277.408766] [<0000000000177bb4>] try_to_wake_up+0xfc/0x450 > [775277.408773] [<000003ff81ede872>] vhost_poll_wakeup+0x3a/0x50 [vhost] > [775277.408777] [<0000000000194ae4>] __wake_up_common+0xbc/0x178 > [775277.408779] [<0000000000194f86>] __wake_up_common_lock+0x9e/0x160 > [775277.408780] [<00000000001950de>] __wake_up_sync_key+0x4e/0x60 > [775277.408785] [<00000000005d911e>] sock_def_readable+0x5e/0x98 > > The bug hits any time between 1 hour to 3 days. The dereference occurs > in update_cfs_rq_h_load when accumulating h_load. The problem is that > cfq_rq->h_load_next is not protected by any locking and can be updated > by parallel calls to task_h_load. Depending on the compiler, code may be > generated that re-reads cfq_rq->h_load_next after the check for NULL and > then oops when reading se->avg.load_avg. The dissassembly showed that it > was possible to reread h_load_next after the check for NULL. > > While this does not appear to be an issue for later compilers, it's still > an accident if the correct code is generated. Full locking in this path > would have high overhead so this patch uses READ_ONCE to read h_load_next > only once and check for NULL before dereferencing. It was confirmed that > there were no further oops after 10 days of testing. > > As Peter pointed out, it is also necessary to use WRITE_ONCE to avoid any > potential problems with store tearing. > > Fixes: 685207963be9 ("sched: Move h_load calculation to task_h_load()") > [peterz@infradead.org: Use WRITE_ONCE to protect against store tearing] > Signed-off-by: Mel Gorman <mgorman@techsingularity.net> > Reviewed-by: Valentin Schneider <valentin.schneider@arm.com> > Cc: stable@vger.kernel.org
Thanks!
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