On 11 February 2013 20:07, Andy Shevchenko
<andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
> Commit e1ee7c86 ("cpufreq: Convert the cpufreq_driver_lock to use RCU")
> introduces an RCU storage for cpufreq_driver. On Intel Medfield board we are
> getting a warning during kernel boot.
>
> [ 6.074511] ===============================
> [ 6.078634] [ INFO: suspicious RCU usage. ]
> [ 6.082930] 3.8.0-rc6-next-20130208-00036-g83eb4b6-dirty #1058 Not tainted
> [ 6.089748] -------------------------------
> [ 6.093926] drivers/cpufreq/cpufreq.c:1766 suspicious rcu_dereference_check() usage!
> [ 6.101651]
> [ 6.101651] other info that might help us debug this:
> [ 6.101651]
> [ 6.109659]
> [ 6.109659] rcu_scheduler_active = 1, debug_locks = 0
> [ 6.116215] no locks held by swapper/0/1.
> [ 6.120167]
> [ 6.120167] stack backtrace:
> [ 6.124516] Pid: 1, comm: swapper/0 Not tainted 3.8.0-rc6-next-20130208-00036-g83eb4b6-dirty #1058
> [ 6.133470] Call Trace:
> [ 6.135917] [<c1067669>] lockdep_rcu_suspicious+0xd4/0xdd
> [ 6.141390] [<c181d642>] ? cpufreq_core_init+0x8e/0x8e
> [ 6.146604] [<c1344879>] cpufreq_update_policy+0x71/0x1d7
> [ 6.152081] [<c1069d2f>] ? __lock_acquire+0xca4/0xcd4
> [ 6.157210] [<c1067ef0>] ? mark_lock+0x1f/0x209
> [ 6.161820] [<c1068137>] ? mark_held_locks+0x5d/0x7d
> [ 6.166862] [<c150c209>] ? __mutex_lock_common+0x2e0/0x344
> [ 6.172425] [<c106825c>] ? trace_hardirqs_on_caller+0x105/0x195
> [ 6.178425] [<c150c3ae>] ? __mutex_unlock_slowpath+0xe6/0x100
> [ 6.184250] [<c106825c>] ? trace_hardirqs_on_caller+0x105/0x195
> [ 6.190248] [<c10682f7>] ? trace_hardirqs_on+0xb/0xd
> [ 6.195291] [<c181d642>] ? cpufreq_core_init+0x8e/0x8e
> [ 6.200506] [<c150c3d0>] ? mutex_unlock+0x8/0xa
> [ 6.205114] [<c181d642>] ? cpufreq_core_init+0x8e/0x8e
> [ 6.210331] [<c181d68a>] cpufreq_stats_init+0x48/0xbd
> [ 6.215461] [<c181d68a>] ? cpufreq_stats_init+0x48/0xbd
> [ 6.220763] [<c181d642>] ? cpufreq_core_init+0x8e/0x8e
> [ 6.225981] [<c1001085>] do_one_initcall+0x6c/0x10b
> [ 6.230938] [<c17faa52>] ? kernel_init_freeable+0xf3/0x18e
> [ 6.236501] [<c17faa68>] kernel_init_freeable+0x109/0x18e
> [ 6.241978] [<c14fa2b4>] kernel_init+0x8/0xb4
> [ 6.246412] [<c150e937>] ret_from_kernel_thread+0x1b/0x28
> [ 6.251889] [<c14fa2ac>] ? rest_init+0x108/0x108
>
> The patch moves dereferencing part after policy check.
>
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> ---
> drivers/cpufreq/cpufreq.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 79511ab..7d84b205 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -1763,13 +1763,15 @@ int cpufreq_update_policy(unsigned int cpu)
> struct cpufreq_policy *data = cpufreq_cpu_get(cpu);
> struct cpufreq_policy policy;
> int ret;
> - struct cpufreq_driver *driver = rcu_dereference(cpufreq_driver);
> + struct cpufreq_driver *driver;
>
> if (!data) {
> ret = -ENODEV;
> goto no_policy;
> }
>
> + driver = rcu_dereference(cpufreq_driver);
> +
Hi Andy,
I don't quite understand why this warning came at the first place and how it
got fixed :(
Another thing, rcu patch is already dropped by Rafael for 3.9 and isn't there in
linux-next.
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